:mod:`os` --- Miscellaneous operating system interfaces
=======================================================

.. module:: os
   :synopsis: Miscellaneous operating system interfaces.

**Source code:** :source:`Lib/os.py`

--------------

This module provides a portable way of using operating system dependent
functionality.  If you just want to read or write a file see :func:`open`, if
you want to manipulate paths, see the :mod:`os.path` module, and if you want to
read all the lines in all the files on the command line see the :mod:`fileinput`
module.  For creating temporary files and directories see the :mod:`tempfile`
module, and for high-level file and directory handling see the :mod:`shutil`
module.

Notes on the availability of these functions:

* The design of all built-in operating system dependent modules of Python is
  such that as long as the same functionality is available, it uses the same
  interface; for example, the function ``os.stat(path)`` returns stat
  information about *path* in the same format (which happens to have originated
  with the POSIX interface).

* Extensions peculiar to a particular operating system are also available
  through the :mod:`os` module, but using them is of course a threat to
  portability.

* All functions accepting path or file names accept both bytes and string
  objects, and result in an object of the same type, if a path or file name is
  returned.

* An "Availability: Unix" note means that this function is commonly found on
  Unix systems.  It does not make any claims about its existence on a specific
  operating system.

* If not separately noted, all functions that claim "Availability: Unix" are
  supported on Mac OS X, which builds on a Unix core.

.. Availability notes get their own line and occur at the end of the function
.. documentation.

.. note::

   All functions in this module raise :exc:`OSError` in the case of invalid or
   inaccessible file names and paths, or other arguments that have the correct
   type, but are not accepted by the operating system.

.. exception:: error

   An alias for the built-in :exc:`OSError` exception.


.. data:: name

   The name of the operating system dependent module imported.  The following
   names have currently been registered: ``'posix'``, ``'nt'``,
   ``'java'``.

   .. seealso::
      :attr:`sys.platform` has a finer granularity.  :func:`os.uname` gives
      system-dependent version information.

      The :mod:`platform` module provides detailed checks for the
      system's identity.


.. _os-filenames:
.. _filesystem-encoding:

File Names, Command Line Arguments, and Environment Variables
-------------------------------------------------------------

In Python, file names, command line arguments, and environment variables are
represented using the string type. On some systems, decoding these strings to
and from bytes is necessary before passing them to the operating system. Python
uses the file system encoding to perform this conversion (see
:func:`sys.getfilesystemencoding`).

.. versionchanged:: 3.1
   On some systems, conversion using the file system encoding may fail. In this
   case, Python uses the :ref:`surrogateescape encoding error handler
   <surrogateescape>`, which means that undecodable bytes are replaced by a
   Unicode character U+DCxx on decoding, and these are again translated to the
   original byte on encoding.


The file system encoding must guarantee to successfully decode all bytes
below 128. If the file system encoding fails to provide this guarantee, API
functions may raise UnicodeErrors.


.. _os-procinfo:

Process Parameters
------------------

These functions and data items provide information and operate on the current
process and user.


.. function:: ctermid()

   Return the filename corresponding to the controlling terminal of the process.

   Availability: Unix.


.. data:: environ

   A :term:`mapping` object representing the string environment. For example,
   ``environ['HOME']`` is the pathname of your home directory (on some platforms),
   and is equivalent to ``getenv("HOME")`` in C.

   This mapping is captured the first time the :mod:`os` module is imported,
   typically during Python startup as part of processing :file:`site.py`.  Changes
   to the environment made after this time are not reflected in ``os.environ``,
   except for changes made by modifying ``os.environ`` directly.

   If the platform supports the :func:`putenv` function, this mapping may be used
   to modify the environment as well as query the environment.  :func:`putenv` will
   be called automatically when the mapping is modified.

   On Unix, keys and values use :func:`sys.getfilesystemencoding` and
   ``'surrogateescape'`` error handler. Use :data:`environb` if you would like
   to use a different encoding.

   .. note::

      Calling :func:`putenv` directly does not change ``os.environ``, so it's better
      to modify ``os.environ``.

   .. note::

      On some platforms, including FreeBSD and Mac OS X, setting ``environ`` may
      cause memory leaks.  Refer to the system documentation for
      :c:func:`putenv`.

   If :func:`putenv` is not provided, a modified copy of this mapping  may be
   passed to the appropriate process-creation functions to cause  child processes
   to use a modified environment.

   If the platform supports the :func:`unsetenv` function, you can delete items in
   this mapping to unset environment variables. :func:`unsetenv` will be called
   automatically when an item is deleted from ``os.environ``, and when
   one of the :meth:`pop` or :meth:`clear` methods is called.


.. data:: environb

   Bytes version of :data:`environ`: a :term:`mapping` object representing the
   environment as byte strings. :data:`environ` and :data:`environb` are
   synchronized (modify :data:`environb` updates :data:`environ`, and vice
   versa).

   :data:`environb` is only available if :data:`supports_bytes_environ` is
   True.

   .. versionadded:: 3.2


.. function:: chdir(path)
              fchdir(fd)
              getcwd()
   :noindex:

   These functions are described in :ref:`os-file-dir`.


.. function:: fsencode(filename)

   Encode :term:`path-like <path-like object>` *filename* to the filesystem
   encoding with ``'surrogateescape'`` error handler, or ``'strict'`` on
   Windows; return :class:`bytes` unchanged.

   :func:`fsdecode` is the reverse function.

   .. versionadded:: 3.2

   .. versionchanged:: 3.6
      Support added to accept objects implementing the :class:`os.PathLike`
      interface.


.. function:: fsdecode(filename)

   Decode the :term:`path-like <path-like object>` *filename* from the
   filesystem encoding with ``'surrogateescape'`` error handler, or ``'strict'``
   on Windows; return :class:`str` unchanged.

   :func:`fsencode` is the reverse function.

   .. versionadded:: 3.2

   .. versionchanged:: 3.6
      Support added to accept objects implementing the :class:`os.PathLike`
      interface.


.. function:: fspath(path)

   Return the file system representation of the path.

   If :class:`str` or :class:`bytes` is passed in, it is returned unchanged.
   Otherwise :meth:`~os.PathLike.__fspath__` is called and its value is
   returned as long as it is a :class:`str` or :class:`bytes` object.
   In all other cases, :exc:`TypeError` is raised.

   .. versionadded:: 3.6


.. class:: PathLike

   An :term:`abstract base class` for objects representing a file system path,
   e.g. :class:`pathlib.PurePath`.

   .. versionadded:: 3.6

   .. abstractmethod:: __fspath__()

      Return the file system path representation of the object.

      The method should only return a :class:`str` or :class:`bytes` object,
      with the preference being for :class:`str`.


.. function:: getenv(key, default=None)

   Return the value of the environment variable *key* if it exists, or
   *default* if it doesn't. *key*, *default* and the result are str.

   On Unix, keys and values are decoded with :func:`sys.getfilesystemencoding`
   and ``'surrogateescape'`` error handler. Use :func:`os.getenvb` if you
   would like to use a different encoding.

   Availability: most flavors of Unix, Windows.


.. function:: getenvb(key, default=None)

   Return the value of the environment variable *key* if it exists, or
   *default* if it doesn't. *key*, *default* and the result are bytes.

   :func:`getenvb` is only available if :data:`supports_bytes_environ`
   is True.

   Availability: most flavors of Unix.

   .. versionadded:: 3.2


.. function:: get_exec_path(env=None)

   Returns the list of directories that will be searched for a named
   executable, similar to a shell, when launching a process.
   *env*, when specified, should be an environment variable dictionary
   to lookup the PATH in.
   By default, when *env* is ``None``, :data:`environ` is used.

   .. versionadded:: 3.2


.. function:: getegid()

   Return the effective group id of the current process.  This corresponds to the
   "set id" bit on the file being executed in the current process.

   Availability: Unix.


.. function:: geteuid()

   .. index:: single: user; effective id

   Return the current process's effective user id.

   Availability: Unix.


.. function:: getgid()

   .. index:: single: process; group

   Return the real group id of the current process.

   Availability: Unix.


.. function:: getgrouplist(user, group)

   Return list of group ids that *user* belongs to. If *group* is not in the
   list, it is included; typically, *group* is specified as the group ID
   field from the password record for *user*.

   Availability: Unix.

   .. versionadded:: 3.3


.. function:: getgroups()

   Return list of supplemental group ids associated with the current process.

   Availability: Unix.

   .. note::

      On Mac OS X, :func:`getgroups` behavior differs somewhat from
      other Unix platforms. If the Python interpreter was built with a
      deployment target of :const:`10.5` or earlier, :func:`getgroups` returns
      the list of effective group ids associated with the current user process;
      this list is limited to a system-defined number of entries, typically 16,
      and may be modified by calls to :func:`setgroups` if suitably privileged.
      If built with a deployment target greater than :const:`10.5`,
      :func:`getgroups` returns the current group access list for the user
      associated with the effective user id of the process; the group access
      list may change over the lifetime of the process, it is not affected by
      calls to :func:`setgroups`, and its length is not limited to 16.  The
      deployment target value, :const:`MACOSX_DEPLOYMENT_TARGET`, can be
      obtained with :func:`sysconfig.get_config_var`.


.. function:: getlogin()

   Return the name of the user logged in on the controlling terminal of the
   process.  For most purposes, it is more useful to use
   :func:`getpass.getuser` since the latter checks the environment variables
   :envvar:`LOGNAME` or :envvar:`USERNAME` to find out who the user is, and
   falls back to ``pwd.getpwuid(os.getuid())[0]`` to get the login name of the
   current real user id.

   Availability: Unix, Windows.


.. function:: getpgid(pid)

   Return the process group id of the process with process id *pid*. If *pid* is 0,
   the process group id of the current process is returned.

   Availability: Unix.

.. function:: getpgrp()

   .. index:: single: process; group

   Return the id of the current process group.

   Availability: Unix.


.. function:: getpid()

   .. index:: single: process; id

   Return the current process id.


.. function:: getppid()

   .. index:: single: process; id of parent

   Return the parent's process id.  When the parent process has exited, on Unix
   the id returned is the one of the init process (1), on Windows it is still
   the same id, which may be already reused by another process.

   Availability: Unix, Windows.

   .. versionchanged:: 3.2
      Added support for Windows.


.. function:: getpriority(which, who)

   .. index:: single: process; scheduling priority

   Get program scheduling priority.  The value *which* is one of
   :const:`PRIO_PROCESS`, :const:`PRIO_PGRP`, or :const:`PRIO_USER`, and *who*
   is interpreted relative to *which* (a process identifier for
   :const:`PRIO_PROCESS`, process group identifier for :const:`PRIO_PGRP`, and a
   user ID for :const:`PRIO_USER`).  A zero value for *who* denotes
   (respectively) the calling process, the process group of the calling process,
   or the real user ID of the calling process.

   Availability: Unix.

   .. versionadded:: 3.3


.. data:: PRIO_PROCESS
          PRIO_PGRP
          PRIO_USER

   Parameters for the :func:`getpriority` and :func:`setpriority` functions.

   Availability: Unix.

   .. versionadded:: 3.3


.. function:: getresuid()

   Return a tuple (ruid, euid, suid) denoting the current process's
   real, effective, and saved user ids.

   Availability: Unix.

   .. versionadded:: 3.2


.. function:: getresgid()

   Return a tuple (rgid, egid, sgid) denoting the current process's
   real, effective, and saved group ids.

   Availability: Unix.

   .. versionadded:: 3.2


.. function:: getuid()

   .. index:: single: user; id

   Return the current process's real user id.

   Availability: Unix.


.. function:: initgroups(username, gid)

   Call the system initgroups() to initialize the group access list with all of
   the groups of which the specified username is a member, plus the specified
   group id.

   Availability: Unix.

   .. versionadded:: 3.2


.. function:: putenv(key, value)

   .. index:: single: environment variables; setting

   Set the environment variable named *key* to the string *value*.  Such
   changes to the environment affect subprocesses started with :func:`os.system`,
   :func:`popen` or :func:`fork` and :func:`execv`.

   Availability: most flavors of Unix, Windows.

   .. note::

      On some platforms, including FreeBSD and Mac OS X, setting ``environ`` may
      cause memory leaks. Refer to the system documentation for putenv.

   When :func:`putenv` is supported, assignments to items in ``os.environ`` are
   automatically translated into corresponding calls to :func:`putenv`; however,
   calls to :func:`putenv` don't update ``os.environ``, so it is actually
   preferable to assign to items of ``os.environ``.


.. function:: setegid(egid)

   Set the current process's effective group id.

   Availability: Unix.


.. function:: seteuid(euid)

   Set the current process's effective user id.

   Availability: Unix.


.. function:: setgid(gid)

   Set the current process' group id.

   Availability: Unix.


.. function:: setgroups(groups)

   Set the list of supplemental group ids associated with the current process to
   *groups*. *groups* must be a sequence, and each element must be an integer
   identifying a group. This operation is typically available only to the superuser.

   Availability: Unix.

   .. note:: On Mac OS X, the length of *groups* may not exceed the
      system-defined maximum number of effective group ids, typically 16.
      See the documentation for :func:`getgroups` for cases where it may not
      return the same group list set by calling setgroups().

.. function:: setpgrp()

   Call the system call :c:func:`setpgrp` or ``setpgrp(0, 0)`` depending on
   which version is implemented (if any).  See the Unix manual for the semantics.

   Availability: Unix.


.. function:: setpgid(pid, pgrp)

   Call the system call :c:func:`setpgid` to set the process group id of the
   process with id *pid* to the process group with id *pgrp*.  See the Unix manual
   for the semantics.

   Availability: Unix.


.. function:: setpriority(which, who, priority)

   .. index:: single: process; scheduling priority

   Set program scheduling priority. The value *which* is one of
   :const:`PRIO_PROCESS`, :const:`PRIO_PGRP`, or :const:`PRIO_USER`, and *who*
   is interpreted relative to *which* (a process identifier for
   :const:`PRIO_PROCESS`, process group identifier for :const:`PRIO_PGRP`, and a
   user ID for :const:`PRIO_USER`). A zero value for *who* denotes
   (respectively) the calling process, the process group of the calling process,
   or the real user ID of the calling process.
   *priority* is a value in the range -20 to 19. The default priority is 0;
   lower priorities cause more favorable scheduling.

   Availability: Unix

   .. versionadded:: 3.3


.. function:: setregid(rgid, egid)

   Set the current process's real and effective group ids.

   Availability: Unix.


.. function:: setresgid(rgid, egid, sgid)

   Set the current process's real, effective, and saved group ids.

   Availability: Unix.

   .. versionadded:: 3.2


.. function:: setresuid(ruid, euid, suid)

   Set the current process's real, effective, and saved user ids.

   Availability: Unix.

   .. versionadded:: 3.2


.. function:: setreuid(ruid, euid)

   Set the current process's real and effective user ids.

   Availability: Unix.


.. function:: getsid(pid)

   Call the system call :c:func:`getsid`.  See the Unix manual for the semantics.

   Availability: Unix.


.. function:: setsid()

   Call the system call :c:func:`setsid`.  See the Unix manual for the semantics.

   Availability: Unix.


.. function:: setuid(uid)

   .. index:: single: user; id, setting

   Set the current process's user id.

   Availability: Unix.


.. placed in this section since it relates to errno.... a little weak
.. function:: strerror(code)

   Return the error message corresponding to the error code in *code*.
   On platforms where :c:func:`strerror` returns ``NULL`` when given an unknown
   error number, :exc:`ValueError` is raised.


.. data:: supports_bytes_environ

   ``True`` if the native OS type of the environment is bytes (eg. ``False`` on
   Windows).

   .. versionadded:: 3.2


.. function:: umask(mask)

   Set the current numeric umask and return the previous umask.


.. function:: uname()

   .. index::
      single: gethostname() (in module socket)
      single: gethostbyaddr() (in module socket)

   Returns information identifying the current operating system.
   The return value is an object with five attributes:

   * :attr:`sysname` - operating system name
   * :attr:`nodename` - name of machine on network (implementation-defined)
   * :attr:`release` - operating system release
   * :attr:`version` - operating system version
   * :attr:`machine` - hardware identifier

   For backwards compatibility, this object is also iterable, behaving
   like a five-tuple containing :attr:`sysname`, :attr:`nodename`,
   :attr:`release`, :attr:`version`, and :attr:`machine`
   in that order.

   Some systems truncate :attr:`nodename` to 8 characters or to the
   leading component; a better way to get the hostname is
   :func:`socket.gethostname`  or even
   ``socket.gethostbyaddr(socket.gethostname())``.

   Availability: recent flavors of Unix.

   .. versionchanged:: 3.3
      Return type changed from a tuple to a tuple-like object
      with named attributes.


.. function:: unsetenv(key)

   .. index:: single: environment variables; deleting

   Unset (delete) the environment variable named *key*. Such changes to the
   environment affect subprocesses started with :func:`os.system`, :func:`popen` or
   :func:`fork` and :func:`execv`.

   When :func:`unsetenv` is supported, deletion of items in ``os.environ`` is
   automatically translated into a corresponding call to :func:`unsetenv`; however,
   calls to :func:`unsetenv` don't update ``os.environ``, so it is actually
   preferable to delete items of ``os.environ``.

   Availability: most flavors of Unix, Windows.


.. _os-newstreams:

File Object Creation
--------------------

This function creates new :term:`file objects <file object>`.  (See also
:func:`~os.open` for opening file descriptors.)


.. function:: fdopen(fd, *args, **kwargs)

   Return an open file object connected to the file descriptor *fd*.  This is an
   alias of the :func:`open` built-in function and accepts the same arguments.
   The only difference is that the first argument of :func:`fdopen` must always
   be an integer.


.. _os-fd-ops:

File Descriptor Operations
--------------------------

These functions operate on I/O streams referenced using file descriptors.

File descriptors are small integers corresponding to a file that has been opened
by the current process.  For example, standard input is usually file descriptor
0, standard output is 1, and standard error is 2.  Further files opened by a
process will then be assigned 3, 4, 5, and so forth.  The name "file descriptor"
is slightly deceptive; on Unix platforms, sockets and pipes are also referenced
by file descriptors.

The :meth:`~io.IOBase.fileno` method can be used to obtain the file descriptor
associated with a :term:`file object` when required.  Note that using the file
descriptor directly will bypass the file object methods, ignoring aspects such
as internal buffering of data.


.. function:: close(fd)

   Close file descriptor *fd*.

   .. note::

      This function is intended for low-level I/O and must be applied to a file
      descriptor as returned by :func:`os.open` or :func:`pipe`.  To close a "file
      object" returned by the built-in function :func:`open` or by :func:`popen` or
      :func:`fdopen`, use its :meth:`~io.IOBase.close` method.


.. function:: closerange(fd_low, fd_high)

   Close all file descriptors from *fd_low* (inclusive) to *fd_high* (exclusive),
   ignoring errors. Equivalent to (but much faster than)::

      for fd in range(fd_low, fd_high):
          try:
              os.close(fd)
          except OSError:
              pass


.. function:: device_encoding(fd)

   Return a string describing the encoding of the device associated with *fd*
   if it is connected to a terminal; else return :const:`None`.


.. function:: dup(fd)

   Return a duplicate of file descriptor *fd*. The new file descriptor is
   :ref:`non-inheritable <fd_inheritance>`.

   On Windows, when duplicating a standard stream (0: stdin, 1: stdout,
   2: stderr), the new file descriptor is :ref:`inheritable
   <fd_inheritance>`.

   .. versionchanged:: 3.4
      The new file descriptor is now non-inheritable.


.. function:: dup2(fd, fd2, inheritable=True)

   Duplicate file descriptor *fd* to *fd2*, closing the latter first if
   necessary. Return *fd2*. The new file descriptor is :ref:`inheritable
   <fd_inheritance>` by default or non-inheritable if *inheritable*
   is ``False``.

   .. versionchanged:: 3.4
      Add the optional *inheritable* parameter.

   .. versionchanged:: 3.7
      Return *fd2* on success. Previously, ``None`` was always returned.


.. function:: fchmod(fd, mode)

   Change the mode of the file given by *fd* to the numeric *mode*.  See the
   docs for :func:`chmod` for possible values of *mode*.  As of Python 3.3, this
   is equivalent to ``os.chmod(fd, mode)``.

   Availability: Unix.


.. function:: fchown(fd, uid, gid)

   Change the owner and group id of the file given by *fd* to the numeric *uid*
   and *gid*.  To leave one of the ids unchanged, set it to -1.  See
   :func:`chown`.  As of Python 3.3, this is equivalent to ``os.chown(fd, uid,
   gid)``.

   Availability: Unix.


.. function:: fdatasync(fd)

   Force write of file with filedescriptor *fd* to disk. Does not force update of
   metadata.

   Availability: Unix.

   .. note::
      This function is not available on MacOS.


.. function:: fpathconf(fd, name)

   Return system configuration information relevant to an open file. *name*
   specifies the configuration value to retrieve; it may be a string which is the
   name of a defined system value; these names are specified in a number of
   standards (POSIX.1, Unix 95, Unix 98, and others).  Some platforms define
   additional names as well.  The names known to the host operating system are
   given in the ``pathconf_names`` dictionary.  For configuration variables not
   included in that mapping, passing an integer for *name* is also accepted.

   If *name* is a string and is not known, :exc:`ValueError` is raised.  If a
   specific value for *name* is not supported by the host system, even if it is
   included in ``pathconf_names``, an :exc:`OSError` is raised with
   :const:`errno.EINVAL` for the error number.

   As of Python 3.3, this is equivalent to ``os.pathconf(fd, name)``.

   Availability: Unix.


.. function:: fstat(fd)

   Get the status of the file descriptor *fd*. Return a :class:`stat_result`
   object.

   As of Python 3.3, this is equivalent to ``os.stat(fd)``.

   .. seealso::

      The :func:`.stat` function.


.. function:: fstatvfs(fd)

   Return information about the filesystem containing the file associated with
   file descriptor *fd*, like :func:`statvfs`.  As of Python 3.3, this is
   equivalent to ``os.statvfs(fd)``.

   Availability: Unix.


.. function:: fsync(fd)

   Force write of file with filedescriptor *fd* to disk.  On Unix, this calls the
   native :c:func:`fsync` function; on Windows, the MS :c:func:`_commit` function.

   If you're starting with a buffered Python :term:`file object` *f*, first do
   ``f.flush()``, and then do ``os.fsync(f.fileno())``, to ensure that all internal
   buffers associated with *f* are written to disk.

   Availability: Unix, Windows.


.. function:: ftruncate(fd, length)

   Truncate the file corresponding to file descriptor *fd*, so that it is at
   most *length* bytes in size.  As of Python 3.3, this is equivalent to
   ``os.truncate(fd, length)``.

   Availability: Unix, Windows.

   .. versionchanged:: 3.5
      Added support for Windows

.. function:: get_blocking(fd)

   Get the blocking mode of the file descriptor: ``False`` if the
   :data:`O_NONBLOCK` flag is set, ``True`` if the flag is cleared.

   See also :func:`set_blocking` and :meth:`socket.socket.setblocking`.

   Availability: Unix.

   .. versionadded:: 3.5

.. function:: isatty(fd)

   Return ``True`` if the file descriptor *fd* is open and connected to a
   tty(-like) device, else ``False``.


.. function:: lockf(fd, cmd, len)

   Apply, test or remove a POSIX lock on an open file descriptor.
   *fd* is an open file descriptor.
   *cmd* specifies the command to use - one of :data:`F_LOCK`, :data:`F_TLOCK`,
   :data:`F_ULOCK` or :data:`F_TEST`.
   *len* specifies the section of the file to lock.

   Availability: Unix.

   .. versionadded:: 3.3


.. data:: F_LOCK
          F_TLOCK
          F_ULOCK
          F_TEST

   Flags that specify what action :func:`lockf` will take.

   Availability: Unix.

   .. versionadded:: 3.3


.. function:: lseek(fd, pos, how)

   Set the current position of file descriptor *fd* to position *pos*, modified
   by *how*: :const:`SEEK_SET` or ``0`` to set the position relative to the
   beginning of the file; :const:`SEEK_CUR` or ``1`` to set it relative to the
   current position; :const:`SEEK_END` or ``2`` to set it relative to the end of
   the file. Return the new cursor position in bytes, starting from the beginning.


.. data:: SEEK_SET
          SEEK_CUR
          SEEK_END

   Parameters to the :func:`lseek` function. Their values are 0, 1, and 2,
   respectively.

   .. versionadded:: 3.3
      Some operating systems could support additional values, like
      :data:`os.SEEK_HOLE` or :data:`os.SEEK_DATA`.


.. function:: open(path, flags, mode=0o777, *, dir_fd=None)

   Open the file *path* and set various flags according to *flags* and possibly
   its mode according to *mode*.  When computing *mode*, the current umask value
   is first masked out.  Return the file descriptor for the newly opened file.
   The new file descriptor is :ref:`non-inheritable <fd_inheritance>`.

   For a description of the flag and mode values, see the C run-time documentation;
   flag constants (like :const:`O_RDONLY` and :const:`O_WRONLY`) are defined in
   the :mod:`os` module.  In particular, on Windows adding
   :const:`O_BINARY` is needed to open files in binary mode.

   This function can support :ref:`paths relative to directory descriptors
   <dir_fd>` with the *dir_fd* parameter.

   .. versionchanged:: 3.4
      The new file descriptor is now non-inheritable.

   .. note::

      This function is intended for low-level I/O.  For normal usage, use the
      built-in function :func:`open`, which returns a :term:`file object` with
      :meth:`~file.read` and :meth:`~file.write` methods (and many more).  To
      wrap a file descriptor in a file object, use :func:`fdopen`.

   .. versionadded:: 3.3
      The *dir_fd* argument.

   .. versionchanged:: 3.5
      If the system call is interrupted and the signal handler does not raise an
      exception, the function now retries the system call instead of raising an
      :exc:`InterruptedError` exception (see :pep:`475` for the rationale).

   .. versionchanged:: 3.6
      Accepts a :term:`path-like object`.

The following constants are options for the *flags* parameter to the
:func:`~os.open` function.  They can be combined using the bitwise OR operator
``|``.  Some of them are not available on all platforms.  For descriptions of
their availability and use, consult the :manpage:`open(2)` manual page on Unix
or `the MSDN <https://msdn.microsoft.com/en-us/library/z0kc8e3z.aspx>`_ on Windows.


.. data:: O_RDONLY
          O_WRONLY
          O_RDWR
          O_APPEND
          O_CREAT
          O_EXCL
          O_TRUNC

   The above constants are available on Unix and Windows.


.. data:: O_DSYNC
          O_RSYNC
          O_SYNC
          O_NDELAY
          O_NONBLOCK
          O_NOCTTY
          O_CLOEXEC

   The above constants are only available on Unix.

   .. versionchanged:: 3.3
      Add :data:`O_CLOEXEC` constant.

.. data:: O_BINARY
          O_NOINHERIT
          O_SHORT_LIVED
          O_TEMPORARY
          O_RANDOM
          O_SEQUENTIAL
          O_TEXT

   The above constants are only available on Windows.


.. data:: O_ASYNC
          O_DIRECT
          O_DIRECTORY
          O_NOFOLLOW
          O_NOATIME
          O_PATH
          O_TMPFILE
          O_SHLOCK
          O_EXLOCK

   The above constants are extensions and not present if they are not defined by
   the C library.

   .. versionchanged:: 3.4
      Add :data:`O_PATH` on systems that support it.
      Add :data:`O_TMPFILE`, only available on Linux Kernel 3.11
        or newer.


.. function:: openpty()

   .. index:: module: pty

   Open a new pseudo-terminal pair. Return a pair of file descriptors
   ``(master, slave)`` for the pty and the tty, respectively. The new file
   descriptors are :ref:`non-inheritable <fd_inheritance>`. For a (slightly) more
   portable approach, use the :mod:`pty` module.

   Availability: some flavors of Unix.

   .. versionchanged:: 3.4
      The new file descriptors are now non-inheritable.


.. function:: pipe()

   Create a pipe.  Return a pair of file descriptors ``(r, w)`` usable for
   reading and writing, respectively. The new file descriptor is
   :ref:`non-inheritable <fd_inheritance>`.

   Availability: Unix, Windows.

   .. versionchanged:: 3.4
      The new file descriptors are now non-inheritable.


.. function:: pipe2(flags)

   Create a pipe with *flags* set atomically.
   *flags* can be constructed by ORing together one or more of these values:
   :data:`O_NONBLOCK`, :data:`O_CLOEXEC`.
   Return a pair of file descriptors ``(r, w)`` usable for reading and writing,
   respectively.

   Availability: some flavors of Unix.

   .. versionadded:: 3.3


.. function:: posix_fallocate(fd, offset, len)

   Ensures that enough disk space is allocated for the file specified by *fd*
   starting from *offset* and continuing for *len* bytes.

   Availability: Unix.

   .. versionadded:: 3.3


.. function:: posix_fadvise(fd, offset, len, advice)

   Announces an intention to access data in a specific pattern thus allowing
   the kernel to make optimizations.
   The advice applies to the region of the file specified by *fd* starting at
   *offset* and continuing for *len* bytes.
   *advice* is one of :data:`POSIX_FADV_NORMAL`, :data:`POSIX_FADV_SEQUENTIAL`,
   :data:`POSIX_FADV_RANDOM`, :data:`POSIX_FADV_NOREUSE`,
   :data:`POSIX_FADV_WILLNEED` or :data:`POSIX_FADV_DONTNEED`.

   Availability: Unix.

   .. versionadded:: 3.3


.. data:: POSIX_FADV_NORMAL
          POSIX_FADV_SEQUENTIAL
          POSIX_FADV_RANDOM
          POSIX_FADV_NOREUSE
          POSIX_FADV_WILLNEED
          POSIX_FADV_DONTNEED

   Flags that can be used in *advice* in :func:`posix_fadvise` that specify
   the access pattern that is likely to be used.

   Availability: Unix.

   .. versionadded:: 3.3


.. function:: pread(fd, buffersize, offset)

   Read from a file descriptor, *fd*, at a position of *offset*. It will read up
   to *buffersize* number of bytes. The file offset remains unchanged.

   Availability: Unix.

   .. versionadded:: 3.3


.. function:: pwrite(fd, str, offset)

   Write *bytestring* to a file descriptor, *fd*, from *offset*,
   leaving the file offset unchanged.

   Availability: Unix.

   .. versionadded:: 3.3


.. function:: pwritev(fd, buffers, offset, flags=0)

   Combines the functionality of :func:`os.writev` and :func:`os.pwrite`. It
   writes the contents of *buffers* to file descriptor *fd* at offset *offset*.
   *buffers* must be a sequence of :term:`bytes-like objects <bytes-like object>`.
   Buffers are processed in array order. Entire contents of first buffer is written
   before proceeding to second, and so on. The operating system may set a limit
   (sysconf() value SC_IOV_MAX) on the number of buffers that can be used.
   :func:`~os.pwritev` writes the contents of each object to the file descriptor
   and returns the total number of bytes written.

   The *flags* argument contains a bitwise OR of zero or more of the following
   flags:

   - RWF_DSYNC
   - RWF_SYNC

   Using non-zero flags requires Linux 4.7 or newer.

   Availability: Linux (version 2.6.30), FreeBSD 6.0 and newer,
   OpenBSD (version 2.7 and newer).

   .. versionadded:: 3.7

.. data:: RWF_DSYNC (since Linux 4.7)
   Provide a per-write equivalent of the O_DSYNC open(2) flag. This flag
   is meaningful only for pwritev2(), and its effect applies only to the
   data range written by the system call.

   .. versionadded:: 3.7

.. data:: RWF_SYNC (since Linux 4.7)
   Provide a per-write equivalent of the O_SYNC open(2) flag. This flag is
   meaningful only for pwritev2(), and its effect applies only to the data
   range written by the system call.

   .. versionadded:: 3.7


.. function:: read(fd, n)

   Read at most *n* bytes from file descriptor *fd*. Return a bytestring containing the
   bytes read.  If the end of the file referred to by *fd* has been reached, an
   empty bytes object is returned.

   .. note::

      This function is intended for low-level I/O and must be applied to a file
      descriptor as returned by :func:`os.open` or :func:`pipe`.  To read a
      "file object" returned by the built-in function :func:`open` or by
      :func:`popen` or :func:`fdopen`, or :data:`sys.stdin`, use its
      :meth:`~file.read` or :meth:`~file.readline` methods.

   .. versionchanged:: 3.5
      If the system call is interrupted and the signal handler does not raise an
      exception, the function now retries the system call instead of raising an
      :exc:`InterruptedError` exception (see :pep:`475` for the rationale).


.. function:: sendfile(out, in, offset, count)
              sendfile(out, in, offset, count, [headers], [trailers], flags=0)

   Copy *count* bytes from file descriptor *in* to file descriptor *out*
   starting at *offset*.
   Return the number of bytes sent. When EOF is reached return 0.

   The first function notation is supported by all platforms that define
   :func:`sendfile`.

   On Linux, if *offset* is given as ``None``, the bytes are read from the
   current position of *in* and the position of *in* is updated.

   The second case may be used on Mac OS X and FreeBSD where *headers* and
   *trailers* are arbitrary sequences of buffers that are written before and
   after the data from *in* is written. It returns the same as the first case.

   On Mac OS X and FreeBSD, a value of 0 for *count* specifies to send until
   the end of *in* is reached.

   All platforms support sockets as *out* file descriptor, and some platforms
   allow other types (e.g. regular file, pipe) as well.

   Cross-platform applications should not use *headers*, *trailers* and *flags*
   arguments.

   Availability: Unix.

   .. note::

      For a higher-level wrapper of :func:`sendfile`, see
      :meth:`socket.socket.sendfile`.

   .. versionadded:: 3.3


.. function:: set_blocking(fd, blocking)

   Set the blocking mode of the specified file descriptor. Set the
   :data:`O_NONBLOCK` flag if blocking is ``False``, clear the flag otherwise.

   See also :func:`get_blocking` and :meth:`socket.socket.setblocking`.

   Availability: Unix.

   .. versionadded:: 3.5


.. data:: SF_NODISKIO
          SF_MNOWAIT
          SF_SYNC

   Parameters to the :func:`sendfile` function, if the implementation supports
   them.

   Availability: Unix.

   .. versionadded:: 3.3


.. function:: readv(fd, buffers)

   Read from a file descriptor *fd* into a number of mutable :term:`bytes-like
   objects <bytes-like object>` *buffers*. :func:`~os.readv` will transfer data
   into each buffer until it is full and then move on to the next buffer in the
   sequence to hold the rest of the data. :func:`~os.readv` returns the total
   number of bytes read (which may be less than the total capacity of all the
   objects).

   Availability: Unix.

   .. versionadded:: 3.3


.. function:: preadv(fd, buffers, offset, flags=0)

   Combines the functionality of :func:`os.readv` and :func:`os.pread`. It
   reads from a file descriptor *fd* into a number of mutable :term:`bytes-like
   objects <bytes-like object>` *buffers*. As :func:`os.readv`, it will transfer
   data into each buffer until it is full and then move on to the next buffer in
   the sequence to hold the rest of the data. Its fourth argument, *offset*,
   specifies the file offset at which the input operation is to be performed.
   :func:`~os.preadv` return the total number of bytes read (which can be less than
   the total capacity of all the objects).

   The flags argument contains a bitwise OR of zero or more of the following
   flags:

   - RWF_HIPRI
   - RWF_NOWAIT

   Using non-zero flags requires Linux 4.6 or newer.

   Availability: Linux (version 2.6.30), FreeBSD 6.0 and newer,
   OpenBSD (version 2.7 and newer).

   .. versionadded:: 3.7


.. data:: RWF_HIPRI (since Linux 4.6)
   High priority read/write. Allows block-based filesystems to use polling
   of the device, which provides lower latency, but may use additional
   resources. (Currently, this feature is usable only on a file descriptor
   opened using the O_DIRECT flag.)

   .. versionadded:: 3.7


.. data:: RWF_NOWAIT (since Linux 4.14)
   Do not wait for data which is not immediately available. If this flag
   is  specified, the preadv2() system call will return instantly
   if it would have to read data from the backing storage or wait for a lock.
   If some data was successfully read, it will return the number of bytes
   read. If no bytes were read, it will return -1 and set errno to EAGAIN.
   Currently, this flag is meaningful only for preadv2().

   .. versionadded:: 3.7


.. function:: tcgetpgrp(fd)

   Return the process group associated with the terminal given by *fd* (an open
   file descriptor as returned by :func:`os.open`).

   Availability: Unix.


.. function:: tcsetpgrp(fd, pg)

   Set the process group associated with the terminal given by *fd* (an open file
   descriptor as returned by :func:`os.open`) to *pg*.

   Availability: Unix.


.. function:: ttyname(fd)

   Return a string which specifies the terminal device associated with
   file descriptor *fd*.  If *fd* is not associated with a terminal device, an
   exception is raised.

   Availability: Unix.


.. function:: write(fd, str)

   Write the bytestring in *str* to file descriptor *fd*. Return the number of
   bytes actually written.

   .. note::

      This function is intended for low-level I/O and must be applied to a file
      descriptor as returned by :func:`os.open` or :func:`pipe`.  To write a "file
      object" returned by the built-in function :func:`open` or by :func:`popen` or
      :func:`fdopen`, or :data:`sys.stdout` or :data:`sys.stderr`, use its
      :meth:`~file.write` method.

   .. versionchanged:: 3.5
      If the system call is interrupted and the signal handler does not raise an
      exception, the function now retries the system call instead of raising an
      :exc:`InterruptedError` exception (see :pep:`475` for the rationale).


.. function:: writev(fd, buffers)

   Write the contents of *buffers* to file descriptor *fd*. *buffers* must be a
   sequence of :term:`bytes-like objects <bytes-like object>`. Buffers are
   processed in array order. Entire contents of first buffer is written before
   proceeding to second, and so on. The operating system may set a limit
   (sysconf() value SC_IOV_MAX) on the number of buffers that can be used.

   :func:`~os.writev` writes the contents of each object to the file descriptor
   and returns the total number of bytes written.

   Availability: Unix.

   .. versionadded:: 3.3


.. _terminal-size:

Querying the size of a terminal
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. versionadded:: 3.3

.. function:: get_terminal_size(fd=STDOUT_FILENO)

   Return the size of the terminal window as ``(columns, lines)``,
   tuple of type :class:`terminal_size`.

   The optional argument ``fd`` (default ``STDOUT_FILENO``, or standard
   output) specifies which file descriptor should be queried.

   If the file descriptor is not connected to a terminal, an :exc:`OSError`
   is raised.

   :func:`shutil.get_terminal_size` is the high-level function which
   should normally be used, ``os.get_terminal_size`` is the low-level
   implementation.

   Availability: Unix, Windows.

.. class:: terminal_size

   A subclass of tuple, holding ``(columns, lines)`` of the terminal window size.

   .. attribute:: columns

      Width of the terminal window in characters.

   .. attribute:: lines

      Height of the terminal window in characters.


.. _fd_inheritance:

Inheritance of File Descriptors
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. versionadded:: 3.4

A file descriptor has an "inheritable" flag which indicates if the file descriptor
can be inherited by child processes.  Since Python 3.4, file descriptors
created by Python are non-inheritable by default.

On UNIX, non-inheritable file descriptors are closed in child processes at the
execution of a new program, other file descriptors are inherited.

On Windows, non-inheritable handles and file descriptors are closed in child
processes, except for standard streams (file descriptors 0, 1 and 2: stdin, stdout
and stderr), which are always inherited.  Using :func:`spawn\* <spawnl>` functions,
all inheritable handles and all inheritable file descriptors are inherited.
Using the :mod:`subprocess` module, all file descriptors except standard
streams are closed, and inheritable handles are only inherited if the
*close_fds* parameter is ``False``.

.. function:: get_inheritable(fd)

   Get the "inheritable" flag of the specified file descriptor (a boolean).

.. function:: set_inheritable(fd, inheritable)

   Set the "inheritable" flag of the specified file descriptor.

.. function:: get_handle_inheritable(handle)

   Get the "inheritable" flag of the specified handle (a boolean).

   Availability: Windows.

.. function:: set_handle_inheritable(handle, inheritable)

   Set the "inheritable" flag of the specified handle.

   Availability: Windows.


.. _os-file-dir:

Files and Directories
---------------------

On some Unix platforms, many of these functions support one or more of these
features:

.. _path_fd:

* **specifying a file descriptor:**
  For some functions, the *path* argument can be not only a string giving a path
  name, but also a file descriptor.  The function will then operate on the file
  referred to by the descriptor.  (For POSIX systems, Python will call the
  ``f...`` version of the function.)

  You can check whether or not *path* can be specified as a file descriptor on
  your platform using :data:`os.supports_fd`.  If it is unavailable, using it
  will raise a :exc:`NotImplementedError`.

  If the function also supports *dir_fd* or *follow_symlinks* arguments, it is
  an error to specify one of those when supplying *path* as a file descriptor.

.. _dir_fd:

* **paths relative to directory descriptors:** If *dir_fd* is not ``None``, it
  should be a file descriptor referring to a directory, and the path to operate
  on should be relative; path will then be relative to that directory.  If the
  path is absolute, *dir_fd* is ignored.  (For POSIX systems, Python will call
  the ``...at`` or ``f...at`` version of the function.)

  You can check whether or not *dir_fd* is supported on your platform using
  :data:`os.supports_dir_fd`.  If it is unavailable, using it will raise a
  :exc:`NotImplementedError`.

.. _follow_symlinks:

* **not following symlinks:** If *follow_symlinks* is
  ``False``, and the last element of the path to operate on is a symbolic link,
  the function will operate on the symbolic link itself instead of the file the
  link points to.  (For POSIX systems, Python will call the ``l...`` version of
  the function.)

  You can check whether or not *follow_symlinks* is supported on your platform
  using :data:`os.supports_follow_symlinks`.  If it is unavailable, using it
  will raise a :exc:`NotImplementedError`.



.. function:: access(path, mode, *, dir_fd=None, effective_ids=False, follow_symlinks=True)

   Use the real uid/gid to test for access to *path*.  Note that most operations
   will use the effective uid/gid, therefore this routine can be used in a
   suid/sgid environment to test if the invoking user has the specified access to
   *path*.  *mode* should be :const:`F_OK` to test the existence of *path*, or it
   can be the inclusive OR of one or more of :const:`R_OK`, :const:`W_OK`, and
   :const:`X_OK` to test permissions.  Return :const:`True` if access is allowed,
   :const:`False` if not. See the Unix man page :manpage:`access(2)` for more
   information.

   This function can support specifying :ref:`paths relative to directory
   descriptors <dir_fd>` and :ref:`not following symlinks <follow_symlinks>`.

   If *effective_ids* is ``True``, :func:`access` will perform its access
   checks using the effective uid/gid instead of the real uid/gid.
   *effective_ids* may not be supported on your platform; you can check whether
   or not it is available using :data:`os.supports_effective_ids`.  If it is
   unavailable, using it will raise a :exc:`NotImplementedError`.

   .. note::

      Using :func:`access` to check if a user is authorized to e.g. open a file
      before actually doing so using :func:`open` creates a security hole,
      because the user might exploit the short time interval between checking
      and opening the file to manipulate it. It's preferable to use :term:`EAFP`
      techniques. For example::

         if os.access("myfile", os.R_OK):
             with open("myfile") as fp:
                 return fp.read()
         return "some default data"

      is better written as::

         try:
             fp = open("myfile")
         except PermissionError:
             return "some default data"
         else:
             with fp:
                 return fp.read()

   .. note::

      I/O operations may fail even when :func:`access` indicates that they would
      succeed, particularly for operations on network filesystems which may have
      permissions semantics beyond the usual POSIX permission-bit model.

   .. versionchanged:: 3.3
      Added the *dir_fd*, *effective_ids*, and *follow_symlinks* parameters.

   .. versionchanged:: 3.6
      Accepts a :term:`path-like object`.


.. data:: F_OK
          R_OK
          W_OK
          X_OK

   Values to pass as the *mode* parameter of :func:`access` to test the
   existence, readability, writability and executability of *path*,
   respectively.


.. function:: chdir(path)

   .. index:: single: directory; changing

   Change the current working directory to *path*.

   This function can support :ref:`specifying a file descriptor <path_fd>`.  The
   descriptor must refer to an opened directory, not an open file.

   .. versionadded:: 3.3
      Added support for specifying *path* as a file descriptor
      on some platforms.

   .. versionchanged:: 3.6
      Accepts a :term:`path-like object`.


.. function:: chflags(path, flags, *, follow_symlinks=True)

   Set the flags of *path* to the numeric *flags*. *flags* may take a combination
   (bitwise OR) of the following values (as defined in the :mod:`stat` module):

   * :data:`stat.UF_NODUMP`
   * :data:`stat.UF_IMMUTABLE`
   * :data:`stat.UF_APPEND`
   * :data:`stat.UF_OPAQUE`
   * :data:`stat.UF_NOUNLINK`
   * :data:`stat.UF_COMPRESSED`
   * :data:`stat.UF_HIDDEN`
   * :data:`stat.SF_ARCHIVED`
   * :data:`stat.SF_IMMUTABLE`
   * :data:`stat.SF_APPEND`
   * :data:`stat.SF_NOUNLINK`
   * :data:`stat.SF_SNAPSHOT`

   This function can support :ref:`not following symlinks <follow_symlinks>`.

   Availability: Unix.

   .. versionadded:: 3.3
      The *follow_symlinks* argument.

   .. versionchanged:: 3.6
      Accepts a :term:`path-like object`.


.. function:: chmod(path, mode, *, dir_fd=None, follow_symlinks=True)

   Change the mode of *path* to the numeric *mode*. *mode* may take one of the
   following values (as defined in the :mod:`stat` module) or bitwise ORed
   combinations of them:

   * :data:`stat.S_ISUID`
   * :data:`stat.S_ISGID`
   * :data:`stat.S_ENFMT`
   * :data:`stat.S_ISVTX`
   * :data:`stat.S_IREAD`
   * :data:`stat.S_IWRITE`
   * :data:`stat.S_IEXEC`
   * :data:`stat.S_IRWXU`
   * :data:`stat.S_IRUSR`
   * :data:`stat.S_IWUSR`
   * :data:`stat.S_IXUSR`
   * :data:`stat.S_IRWXG`
   * :data:`stat.S_IRGRP`
   * :data:`stat.S_IWGRP`
   * :data:`stat.S_IXGRP`
   * :data:`stat.S_IRWXO`
   * :data:`stat.S_IROTH`
   * :data:`stat.S_IWOTH`
   * :data:`stat.S_IXOTH`

   This function can support :ref:`specifying a file descriptor <path_fd>`,
   :ref:`paths relative to directory descriptors <dir_fd>` and :ref:`not
   following symlinks <follow_symlinks>`.

   .. note::

      Although Windows supports :func:`chmod`, you can only set the file's
      read-only flag with it (via the ``stat.S_IWRITE`` and ``stat.S_IREAD``
      constants or a corresponding integer value).  All other bits are ignored.

   .. versionadded:: 3.3
      Added support for specifying *path* as an open file descriptor,
      and the *dir_fd* and *follow_symlinks* arguments.

   .. versionchanged:: 3.6
      Accepts a :term:`path-like object`.


.. function:: chown(path, uid, gid, *, dir_fd=None, follow_symlinks=True)

   Change the owner and group id of *path* to the numeric *uid* and *gid*.  To
   leave one of the ids unchanged, set it to -1.

   This function can support :ref:`specifying a file descriptor <path_fd>`,
   :ref:`paths relative to directory descriptors <dir_fd>` and :ref:`not
   following symlinks <follow_symlinks>`.

   See :func:`shutil.chown` for a higher-level function that accepts names in
   addition to numeric ids.

   Availability: Unix.

   .. versionadded:: 3.3
      Added support for specifying an open file descriptor for *path*,
      and the *dir_fd* and *follow_symlinks* arguments.

   .. versionchanged:: 3.6
      Supports a :term:`path-like object`.


.. function:: chroot(path)

   Change the root directory of the current process to *path*.

   Availability: Unix.

   .. versionchanged:: 3.6
      Accepts a :term:`path-like object`.


.. function:: fchdir(fd)

   Change the current working directory to the directory represented by the file
   descriptor *fd*.  The descriptor must refer to an opened directory, not an
   open file.  As of Python 3.3, this is equivalent to ``os.chdir(fd)``.

   Availability: Unix.


.. function:: getcwd()

   Return a string representing the current working directory.


.. function:: getcwdb()

   Return a bytestring representing the current working directory.


.. function:: lchflags(path, flags)

   Set the flags of *path* to the numeric *flags*, like :func:`chflags`, but do
   not follow symbolic links.  As of Python 3.3, this is equivalent to
   ``os.chflags(path, flags, follow_symlinks=False)``.

   Availability: Unix.

   .. versionchanged:: 3.6
      Accepts a :term:`path-like object`.


.. function:: lchmod(path, mode)

   Change the mode of *path* to the numeric *mode*. If path is a symlink, this
   affects the symlink rather than the target.  See the docs for :func:`chmod`
   for possible values of *mode*.  As of Python 3.3, this is equivalent to
   ``os.chmod(path, mode, follow_symlinks=False)``.

   Availability: Unix.

   .. versionchanged:: 3.6
      Accepts a :term:`path-like object`.

.. function:: lchown(path, uid, gid)

   Change the owner and group id of *path* to the numeric *uid* and *gid*.  This
   function will not follow symbolic links.  As of Python 3.3, this is equivalent
   to ``os.chown(path, uid, gid, follow_symlinks=False)``.

   Availability: Unix.

   .. versionchanged:: 3.6
      Accepts a :term:`path-like object`.


.. function:: link(src, dst, *, src_dir_fd=None, dst_dir_fd=None, follow_symlinks=True)

   Create a hard link pointing to *src* named *dst*.

   This function can support specifying *src_dir_fd* and/or *dst_dir_fd* to
   supply :ref:`paths relative to directory descriptors <dir_fd>`, and :ref:`not
   following symlinks <follow_symlinks>`.

   Availability: Unix, Windows.

   .. versionchanged:: 3.2
      Added Windows support.

   .. versionadded:: 3.3
      Added the *src_dir_fd*, *dst_dir_fd*, and *follow_symlinks* arguments.

   .. versionchanged:: 3.6
      Accepts a :term:`path-like object` for *src* and *dst*.


.. function:: listdir(path='.')

   Return a list containing the names of the entries in the directory given by
   *path*.  The list is in arbitrary order, and does not include the special
   entries ``'.'`` and ``'..'`` even if they are present in the directory.

   *path* may be a :term:`path-like object`.  If *path* is of type ``bytes``
   (directly or indirectly through the :class:`PathLike` interface),
   the filenames returned will also be of type ``bytes``;
   in all other circumstances, they will be of type ``str``.

   This function can also support :ref:`specifying a file descriptor
   <path_fd>`; the file descriptor must refer to a directory.

   .. note::
      To encode ``str`` filenames to ``bytes``, use :func:`~os.fsencode`.

   .. seealso::

      The :func:`scandir` function returns directory entries along with
      file attribute information, giving better performance for many
      common use cases.

   .. versionchanged:: 3.2
      The *path* parameter became optional.

   .. versionadded:: 3.3
      Added support for specifying an open file descriptor for *path*.

   .. versionchanged:: 3.6
      Accepts a :term:`path-like object`.


.. function:: lstat(path, \*, dir_fd=None)

   Perform the equivalent of an :c:func:`lstat` system call on the given path.
   Similar to :func:`~os.stat`, but does not follow symbolic links. Return a
   :class:`stat_result` object.

   On platforms that do not support symbolic links, this is an alias for
   :func:`~os.stat`.

   As of Python 3.3, this is equivalent to ``os.stat(path, dir_fd=dir_fd,
   follow_symlinks=False)``.

   This function can also support :ref:`paths relative to directory descriptors
   <dir_fd>`.

   .. seealso::

      The :func:`.stat` function.

   .. versionchanged:: 3.2
      Added support for Windows 6.0 (Vista) symbolic links.

   .. versionchanged:: 3.3
      Added the *dir_fd* parameter.

   .. versionchanged:: 3.6
      Accepts a :term:`path-like object` for *src* and *dst*.


.. function:: mkdir(path, mode=0o777, *, dir_fd=None)

   Create a directory named *path* with numeric mode *mode*.

   If the directory already exists, :exc:`FileExistsError` is raised.

   .. _mkdir_modebits:

   On some systems, *mode* is ignored.  Where it is used, the current umask
   value is first masked out.  If bits other than the last 9 (i.e. the last 3
   digits of the octal representation of the *mode*) are set, their meaning is
   platform-dependent.  On some platforms, they are ignored and you should call
   :func:`chmod` explicitly to set them.

   This function can also support :ref:`paths relative to directory descriptors
   <dir_fd>`.

   It is also possible to create temporary directories; see the
   :mod:`tempfile` module's :func:`tempfile.mkdtemp` function.

   .. versionadded:: 3.3
      The *dir_fd* argument.

   .. versionchanged:: 3.6
      Accepts a :term:`path-like object`.


.. function:: makedirs(name, mode=0o777, exist_ok=False)

   .. index::
      single: directory; creating
      single: UNC paths; and os.makedirs()

   Recursive directory creation function.  Like :func:`mkdir`, but makes all
   intermediate-level directories needed to contain the leaf directory.

   The *mode* parameter is passed to :func:`mkdir` for creating the leaf
   directory; see :ref:`the mkdir() description <mkdir_modebits>` for how it
   is interpreted.  To set the file permission bits of any newly-created parent
   directories you can set the umask before invoking :func:`makedirs`.  The
   file permission bits of existing parent directories are not changed.

   If *exist_ok* is ``False`` (the default), an :exc:`OSError` is raised if the
   target directory already exists.

   .. note::

      :func:`makedirs` will become confused if the path elements to create
      include :data:`pardir` (eg. ".." on UNIX systems).

   This function handles UNC paths correctly.

   .. versionadded:: 3.2
      The *exist_ok* parameter.

   .. versionchanged:: 3.4.1

      Before Python 3.4.1, if *exist_ok* was ``True`` and the directory existed,
      :func:`makedirs` would still raise an error if *mode* did not match the
      mode of the existing directory. Since this behavior was impossible to
      implement safely, it was removed in Python 3.4.1. See :issue:`21082`.

   .. versionchanged:: 3.6
      Accepts a :term:`path-like object`.

   .. versionchanged:: 3.7
      The *mode* argument no longer affects the file permission bits of
      newly-created intermediate-level directories.


.. function:: mkfifo(path, mode=0o666, *, dir_fd=None)

   Create a FIFO (a named pipe) named *path* with numeric mode *mode*.
   The current umask value is first masked out from the mode.

   This function can also support :ref:`paths relative to directory descriptors
   <dir_fd>`.

   FIFOs are pipes that can be accessed like regular files.  FIFOs exist until they
   are deleted (for example with :func:`os.unlink`). Generally, FIFOs are used as
   rendezvous between "client" and "server" type processes: the server opens the
   FIFO for reading, and the client opens it for writing.  Note that :func:`mkfifo`
   doesn't open the FIFO --- it just creates the rendezvous point.

   Availability: Unix.

   .. versionadded:: 3.3
      The *dir_fd* argument.

   .. versionchanged:: 3.6
      Accepts a :term:`path-like object`.


.. function:: mknod(path, mode=0o600, device=0, *, dir_fd=None)

   Create a filesystem node (file, device special file or named pipe) named
   *path*. *mode* specifies both the permissions to use and the type of node
   to be created, being combined (bitwise OR) with one of ``stat.S_IFREG``,
   ``stat.S_IFCHR``, ``stat.S_IFBLK``, and ``stat.S_IFIFO`` (those constants are
   available in :mod:`stat`).  For ``stat.S_IFCHR`` and ``stat.S_IFBLK``,
   *device* defines the newly created device special file (probably using
   :func:`os.makedev`), otherwise it is ignored.

   This function can also support :ref:`paths relative to directory descriptors
   <dir_fd>`.

   Availability: Unix.

   .. versionadded:: 3.3
      The *dir_fd* argument.

   .. versionchanged:: 3.6
      Accepts a :term:`path-like object`.


.. function:: major(device)

   Extract the device major number from a raw device number (usually the
   :attr:`st_dev` or :attr:`st_rdev` field from :c:type:`stat`).


.. function:: minor(device)

   Extract the device minor number from a raw device number (usually the
   :attr:`st_dev` or :attr:`st_rdev` field from :c:type:`stat`).


.. function:: makedev(major, minor)

   Compose a raw device number from the major and minor device numbers.


.. function:: pathconf(path, name)

   Return system configuration information relevant to a named file. *name*
   specifies the configuration value to retrieve; it may be a string which is the
   name of a defined system value; these names are specified in a number of
   standards (POSIX.1, Unix 95, Unix 98, and others).  Some platforms define
   additional names as well.  The names known to the host operating system are
   given in the ``pathconf_names`` dictionary.  For configuration variables not
   included in that mapping, passing an integer for *name* is also accepted.

   If *name* is a string and is not known, :exc:`ValueError` is raised.  If a
   specific value for *name* is not supported by the host system, even if it is
   included in ``pathconf_names``, an :exc:`OSError` is raised with
   :const:`errno.EINVAL` for the error number.

   This function can support :ref:`specifying a file descriptor
   <path_fd>`.

   Availability: Unix.

   .. versionchanged:: 3.6
      Accepts a :term:`path-like object`.


.. data:: pathconf_names

   Dictionary mapping names accepted by :func:`pathconf` and :func:`fpathconf` to
   the integer values defined for those names by the host operating system.  This
   can be used to determine the set of names known to the system.

   Availability: Unix.


.. function:: readlink(path, *, dir_fd=None)

   Return a string representing the path to which the symbolic link points.  The
   result may be either an absolute or relative pathname; if it is relative, it
   may be converted to an absolute pathname using
   ``os.path.join(os.path.dirname(path), result)``.

   If the *path* is a string object (directly or indirectly through a
   :class:`PathLike` interface), the result will also be a string object,
   and the call may raise a UnicodeDecodeError. If the *path* is a bytes
   object (direct or indirectly), the result will be a bytes object.

   This function can also support :ref:`paths relative to directory descriptors
   <dir_fd>`.

   Availability: Unix, Windows

   .. versionchanged:: 3.2
      Added support for Windows 6.0 (Vista) symbolic links.

   .. versionadded:: 3.3
      The *dir_fd* argument.

   .. versionchanged:: 3.6
      Accepts a :term:`path-like object`.


.. function:: remove(path, *, dir_fd=None)

   Remove (delete) the file *path*.  If *path* is a directory, :exc:`OSError` is
   raised.  Use :func:`rmdir` to remove directories.

   This function can support :ref:`paths relative to directory descriptors
   <dir_fd>`.

   On Windows, attempting to remove a file that is in use causes an exception to
   be raised; on Unix, the directory entry is removed but the storage allocated
   to the file is not made available until the original file is no longer in use.

   This function is semantically identical to :func:`unlink`.

   .. versionadded:: 3.3
      The *dir_fd* argument.

   .. versionchanged:: 3.6
      Accepts a :term:`path-like object`.


.. function:: removedirs(name)

   .. index:: single: directory; deleting

   Remove directories recursively.  Works like :func:`rmdir` except that, if the
   leaf directory is successfully removed, :func:`removedirs`  tries to
   successively remove every parent directory mentioned in  *path* until an error
   is raised (which is ignored, because it generally means that a parent directory
   is not empty). For example, ``os.removedirs('foo/bar/baz')`` will first remove
   the directory ``'foo/bar/baz'``, and then remove ``'foo/bar'`` and ``'foo'`` if
   they are empty. Raises :exc:`OSError` if the leaf directory could not be
   successfully removed.

   .. versionchanged:: 3.6
      Accepts a :term:`path-like object`.


.. function:: rename(src, dst, *, src_dir_fd=None, dst_dir_fd=None)

   Rename the file or directory *src* to *dst*.  If *dst* is a directory,
   :exc:`OSError` will be raised.  On Unix, if *dst* exists and is a file, it will
   be replaced silently if the user has permission.  The operation may fail on some
   Unix flavors if *src* and *dst* are on different filesystems.  If successful,
   the renaming will be an atomic operation (this is a POSIX requirement).  On
   Windows, if *dst* already exists, :exc:`OSError` will be raised even if it is a
   file.

   This function can support specifying *src_dir_fd* and/or *dst_dir_fd* to
   supply :ref:`paths relative to directory descriptors <dir_fd>`.

   If you want cross-platform overwriting of the destination, use :func:`replace`.

   .. versionadded:: 3.3
      The *src_dir_fd* and *dst_dir_fd* arguments.

   .. versionchanged:: 3.6
      Accepts a :term:`path-like object` for *src* and *dst*.


.. function:: renames(old, new)

   Recursive directory or file renaming function. Works like :func:`rename`, except
   creation of any intermediate directories needed to make the new pathname good is
   attempted first. After the rename, directories corresponding to rightmost path
   segments of the old name will be pruned away using :func:`removedirs`.

   .. note::

      This function can fail with the new directory structure made if you lack
      permissions needed to remove the leaf directory or file.

   .. versionchanged:: 3.6
      Accepts a :term:`path-like object` for *old* and *new*.


.. function:: replace(src, dst, *, src_dir_fd=None, dst_dir_fd=None)

   Rename the file or directory *src* to *dst*.  If *dst* is a directory,
   :exc:`OSError` will be raised.  If *dst* exists and is a file, it will
   be replaced silently if the user has permission.  The operation may fail
   if *src* and *dst* are on different filesystems.  If successful,
   the renaming will be an atomic operation (this is a POSIX requirement).

   This function can support specifying *src_dir_fd* and/or *dst_dir_fd* to
   supply :ref:`paths relative to directory descriptors <dir_fd>`.

   .. versionadded:: 3.3

   .. versionchanged:: 3.6
      Accepts a :term:`path-like object` for *src* and *dst*.


.. function:: rmdir(path, *, dir_fd=None)

   Remove (delete) the directory *path*.  Only works when the directory is
   empty, otherwise, :exc:`OSError` is raised.  In order to remove whole
   directory trees, :func:`shutil.rmtree` can be used.

   This function can support :ref:`paths relative to directory descriptors
   <dir_fd>`.

   .. versionadded:: 3.3
      The *dir_fd* parameter.

   .. versionchanged:: 3.6
      Accepts a :term:`path-like object`.


.. function:: scandir(path='.')

   Return an iterator of :class:`os.DirEntry` objects corresponding to the
   entries in the directory given by *path*. The entries are yielded in
   arbitrary order, and the special entries ``'.'`` and ``'..'`` are not
   included.

   Using :func:`scandir` instead of :func:`listdir` can significantly
   increase the performance of code that also needs file type or file
   attribute information, because :class:`os.DirEntry` objects expose this
   information if the operating system provides it when scanning a directory.
   All :class:`os.DirEntry` methods may perform a system call, but
   :func:`~os.DirEntry.is_dir` and :func:`~os.DirEntry.is_file` usually only
   require a system call for symbolic links; :func:`os.DirEntry.stat`
   always requires a system call on Unix but only requires one for
   symbolic links on Windows.

   *path* may be a :term:`path-like object`.  If *path* is of type ``bytes``
   (directly or indirectly through the :class:`PathLike` interface),
   the type of the :attr:`~os.DirEntry.name` and :attr:`~os.DirEntry.path`
   attributes of each :class:`os.DirEntry` will be ``bytes``; in all other
   circumstances, they will be of type ``str``.

   This function can also support :ref:`specifying a file descriptor
   <path_fd>`; the file descriptor must refer to a directory.

   The :func:`scandir` iterator supports the :term:`context manager` protocol
   and has the following method:

   .. method:: scandir.close()

      Close the iterator and free acquired resources.

      This is called automatically when the iterator is exhausted or garbage
      collected, or when an error happens during iterating.  However it
      is advisable to call it explicitly or use the :keyword:`with`
      statement.

      .. versionadded:: 3.6

   The following example shows a simple use of :func:`scandir` to display all
   the files (excluding directories) in the given *path* that don't start with
   ``'.'``. The ``entry.is_file()`` call will generally not make an additional
   system call::

      with os.scandir(path) as it:
          for entry in it:
              if not entry.name.startswith('.') and entry.is_file():
                  print(entry.name)

   .. note::

      On Unix-based systems, :func:`scandir` uses the system's
      `opendir() <http://pubs.opengroup.org/onlinepubs/009695399/functions/opendir.html>`_
      and
      `readdir() <http://pubs.opengroup.org/onlinepubs/009695399/functions/readdir_r.html>`_
      functions. On Windows, it uses the Win32
      `FindFirstFileW <https://msdn.microsoft.com/en-us/library/windows/desktop/aa364418(v=vs.85).aspx>`_
      and
      `FindNextFileW <https://msdn.microsoft.com/en-us/library/windows/desktop/aa364428(v=vs.85).aspx>`_
      functions.

   .. versionadded:: 3.5

   .. versionadded:: 3.6
      Added support for the :term:`context manager` protocol and the
      :func:`~scandir.close()` method.  If a :func:`scandir` iterator is neither
      exhausted nor explicitly closed a :exc:`ResourceWarning` will be emitted
      in its destructor.

      The function accepts a :term:`path-like object`.

   .. versionchanged:: 3.7
      Added support for :ref:`file descriptors <path_fd>` on Unix.


.. class:: DirEntry

   Object yielded by :func:`scandir` to expose the file path and other file
   attributes of a directory entry.

   :func:`scandir` will provide as much of this information as possible without
   making additional system calls. When a ``stat()`` or ``lstat()`` system call
   is made, the ``os.DirEntry`` object will cache the result.

   ``os.DirEntry`` instances are not intended to be stored in long-lived data
   structures; if you know the file metadata has changed or if a long time has
   elapsed since calling :func:`scandir`, call ``os.stat(entry.path)`` to fetch
   up-to-date information.

   Because the ``os.DirEntry`` methods can make operating system calls, they may
   also raise :exc:`OSError`. If you need very fine-grained
   control over errors, you can catch :exc:`OSError` when calling one of the
   ``os.DirEntry`` methods and handle as appropriate.

   To be directly usable as a :term:`path-like object`, ``os.DirEntry``
   implements the :class:`PathLike` interface.

   Attributes and methods on a ``os.DirEntry`` instance are as follows:

   .. attribute:: name

      The entry's base filename, relative to the :func:`scandir` *path*
      argument.

      The :attr:`name` attribute will be ``bytes`` if the :func:`scandir`
      *path* argument is of type ``bytes`` and ``str`` otherwise.  Use
      :func:`~os.fsdecode` to decode byte filenames.

   .. attribute:: path

      The entry's full path name: equivalent to ``os.path.join(scandir_path,
      entry.name)`` where *scandir_path* is the :func:`scandir` *path*
      argument.  The path is only absolute if the :func:`scandir` *path*
      argument was absolute.  If the :func:`scandir` *path*
      argument was a :ref:`file descriptor <path_fd>`, the :attr:`path`
      attribute is the same as the :attr:`name` attribute.

      The :attr:`path` attribute will be ``bytes`` if the :func:`scandir`
      *path* argument is of type ``bytes`` and ``str`` otherwise.  Use
      :func:`~os.fsdecode` to decode byte filenames.

   .. method:: inode()

      Return the inode number of the entry.

      The result is cached on the ``os.DirEntry`` object. Use
      ``os.stat(entry.path, follow_symlinks=False).st_ino`` to fetch up-to-date
      information.

      On the first, uncached call, a system call is required on Windows but
      not on Unix.

   .. method:: is_dir(\*, follow_symlinks=True)

      Return ``True`` if this entry is a directory or a symbolic link pointing
      to a directory; return ``False`` if the entry is or points to any other
      kind of file, or if it doesn't exist anymore.

      If *follow_symlinks* is ``False``, return ``True`` only if this entry
      is a directory (without following symlinks); return ``False`` if the
      entry is any other kind of file or if it doesn't exist anymore.

      The result is cached on the ``os.DirEntry`` object, with a separate cache
      for *follow_symlinks* ``True`` and ``False``. Call :func:`os.stat` along
      with :func:`stat.S_ISDIR` to fetch up-to-date information.

      On the first, uncached call, no system call is required in most cases.
      Specifically, for non-symlinks, neither Windows or Unix require a system
      call, except on certain Unix file systems, such as network file systems,
      that return ``dirent.d_type == DT_UNKNOWN``. If the entry is a symlink,
      a system call will be required to follow the symlink unless
      *follow_symlinks* is ``False``.

      This method can raise :exc:`OSError`, such as :exc:`PermissionError`,
      but :exc:`FileNotFoundError` is caught and not raised.

   .. method:: is_file(\*, follow_symlinks=True)

      Return ``True`` if this entry is a file or a symbolic link pointing to a
      file; return ``False`` if the entry is or points to a directory or other
      non-file entry, or if it doesn't exist anymore.

      If *follow_symlinks* is ``False``, return ``True`` only if this entry
      is a file (without following symlinks); return ``False`` if the entry is
      a directory or other non-file entry, or if it doesn't exist anymore.

      The result is cached on the ``os.DirEntry`` object. Caching, system calls
      made, and exceptions raised are as per :func:`~os.DirEntry.is_dir`.

   .. method:: is_symlink()

      Return ``True`` if this entry is a symbolic link (even if broken);
      return ``False`` if the entry points to a directory or any kind of file,
      or if it doesn't exist anymore.

      The result is cached on the ``os.DirEntry`` object. Call
      :func:`os.path.islink` to fetch up-to-date information.

      On the first, uncached call, no system call is required in most cases.
      Specifically, neither Windows or Unix require a system call, except on
      certain Unix file systems, such as network file systems, that return
      ``dirent.d_type == DT_UNKNOWN``.

      This method can raise :exc:`OSError`, such as :exc:`PermissionError`,
      but :exc:`FileNotFoundError` is caught and not raised.

   .. method:: stat(\*, follow_symlinks=True)

      Return a :class:`stat_result` object for this entry. This method
      follows symbolic links by default; to stat a symbolic link add the
      ``follow_symlinks=False`` argument.

      On Unix, this method always requires a system call. On Windows, it
      only requires a system call if *follow_symlinks* is ``True`` and the
      entry is a symbolic link.

      On Windows, the ``st_ino``, ``st_dev`` and ``st_nlink`` attributes of the
      :class:`stat_result` are always set to zero. Call :func:`os.stat` to
      get these attributes.

      The result is cached on the ``os.DirEntry`` object, with a separate cache
      for *follow_symlinks* ``True`` and ``False``. Call :func:`os.stat` to
      fetch up-to-date information.

   Note that there is a nice correspondence between several attributes
   and methods of ``os.DirEntry`` and of :class:`pathlib.Path`.  In
   particular, the ``name`` attribute has the same
   meaning, as do the ``is_dir()``, ``is_file()``, ``is_symlink()``
   and ``stat()`` methods.

   .. versionadded:: 3.5

   .. versionchanged:: 3.6
      Added support for the :class:`~os.PathLike` interface.  Added support
      for :class:`bytes` paths on Windows.


.. function:: stat(path, \*, dir_fd=None, follow_symlinks=True)

   Get the status of a file or a file descriptor. Perform the equivalent of a
   :c:func:`stat` system call on the given path. *path* may be specified as
   either a string or bytes -- directly or indirectly through the :class:`PathLike`
   interface -- or as an open file descriptor. Return a :class:`stat_result`
   object.

   This function normally follows symlinks; to stat a symlink add the argument
   ``follow_symlinks=False``, or use :func:`lstat`.

   This function can support :ref:`specifying a file descriptor <path_fd>` and
   :ref:`not following symlinks <follow_symlinks>`.

   .. index:: module: stat

   Example::

      >>> import os
      >>> statinfo = os.stat('somefile.txt')
      >>> statinfo
      os.stat_result(st_mode=33188, st_ino=7876932, st_dev=234881026,
      st_nlink=1, st_uid=501, st_gid=501, st_size=264, st_atime=1297230295,
      st_mtime=1297230027, st_ctime=1297230027)
      >>> statinfo.st_size
      264

   .. seealso::

      :func:`fstat` and :func:`lstat` functions.

   .. versionadded:: 3.3
      Added the *dir_fd* and *follow_symlinks* arguments, specifying a file
      descriptor instead of a path.

   .. versionchanged:: 3.6
      Accepts a :term:`path-like object`.


.. class:: stat_result

   Object whose attributes correspond roughly to the members of the
   :c:type:`stat` structure. It is used for the result of :func:`os.stat`,
   :func:`os.fstat` and :func:`os.lstat`.

   Attributes:

   .. attribute:: st_mode

      File mode: file type and file mode bits (permissions).

   .. attribute:: st_ino

      Inode number.

   .. attribute:: st_dev

      Identifier of the device on which this file resides.

   .. attribute:: st_nlink

      Number of hard links.

   .. attribute:: st_uid

      User identifier of the file owner.

   .. attribute:: st_gid

      Group identifier of the file owner.

   .. attribute:: st_size

      Size of the file in bytes, if it is a regular file or a symbolic link.
      The size of a symbolic link is the length of the pathname it contains,
      without a terminating null byte.

   Timestamps:

   .. attribute:: st_atime

      Time of most recent access expressed in seconds.

   .. attribute:: st_mtime

      Time of most recent content modification expressed in seconds.

   .. attribute:: st_ctime

      Platform dependent:

      * the time of most recent metadata change on Unix,
      * the time of creation on Windows, expressed in seconds.

   .. attribute:: st_atime_ns

      Time of most recent access expressed in nanoseconds as an integer.

   .. attribute:: st_mtime_ns

      Time of most recent content modification expressed in nanoseconds as an
      integer.

   .. attribute:: st_ctime_ns

      Platform dependent:

      * the time of most recent metadata change on Unix,
      * the time of creation on Windows, expressed in nanoseconds as an
        integer.

   .. note::

      The exact meaning and resolution of the :attr:`st_atime`,
      :attr:`st_mtime`, and :attr:`st_ctime` attributes depend on the operating
      system and the file system. For example, on Windows systems using the FAT
      or FAT32 file systems, :attr:`st_mtime` has 2-second resolution, and
      :attr:`st_atime` has only 1-day resolution.  See your operating system
      documentation for details.

      Similarly, although :attr:`st_atime_ns`, :attr:`st_mtime_ns`,
      and :attr:`st_ctime_ns` are always expressed in nanoseconds, many
      systems do not provide nanosecond precision.  On systems that do
      provide nanosecond precision, the floating-point object used to
      store :attr:`st_atime`, :attr:`st_mtime`, and :attr:`st_ctime`
      cannot preserve all of it, and as such will be slightly inexact.
      If you need the exact timestamps you should always use
      :attr:`st_atime_ns`, :attr:`st_mtime_ns`, and :attr:`st_ctime_ns`.

   On some Unix systems (such as Linux), the following attributes may also be
   available:

   .. attribute:: st_blocks

      Number of 512-byte blocks allocated for file.
      This may be smaller than :attr:`st_size`/512 when the file has holes.

   .. attribute:: st_blksize

      "Preferred" blocksize for efficient file system I/O. Writing to a file in
      smaller chunks may cause an inefficient read-modify-rewrite.

   .. attribute:: st_rdev

      Type of device if an inode device.

   .. attribute:: st_flags

      User defined flags for file.

   On other Unix systems (such as FreeBSD), the following attributes may be
   available (but may be only filled out if root tries to use them):

   .. attribute:: st_gen

      File generation number.

   .. attribute:: st_birthtime

      Time of file creation.

   On Solaris and derivatives, the following attributes may also be
   available:

   .. attribute:: st_fstype

      String that uniquely identifies the type of the filesystem that
      contains the file.

   On Mac OS systems, the following attributes may also be available:

   .. attribute:: st_rsize

      Real size of the file.

   .. attribute:: st_creator

      Creator of the file.

   .. attribute:: st_type

      File type.

   On Windows systems, the following attribute is also available:

   .. attribute:: st_file_attributes

      Windows file attributes: ``dwFileAttributes`` member of the
      ``BY_HANDLE_FILE_INFORMATION`` structure returned by
      :c:func:`GetFileInformationByHandle`. See the ``FILE_ATTRIBUTE_*``
      constants in the :mod:`stat` module.

   The standard module :mod:`stat` defines functions and constants that are
   useful for extracting information from a :c:type:`stat` structure. (On
   Windows, some items are filled with dummy values.)

   For backward compatibility, a :class:`stat_result` instance is also
   accessible as a tuple of at least 10 integers giving the most important (and
   portable) members of the :c:type:`stat` structure, in the order
   :attr:`st_mode`, :attr:`st_ino`, :attr:`st_dev`, :attr:`st_nlink`,
   :attr:`st_uid`, :attr:`st_gid`, :attr:`st_size`, :attr:`st_atime`,
   :attr:`st_mtime`, :attr:`st_ctime`. More items may be added at the end by
   some implementations. For compatibility with older Python versions,
   accessing :class:`stat_result` as a tuple always returns integers.

   .. versionadded:: 3.3
      Added the :attr:`st_atime_ns`, :attr:`st_mtime_ns`, and
      :attr:`st_ctime_ns` members.

   .. versionadded:: 3.5
      Added the :attr:`st_file_attributes` member on Windows.

   .. versionadded:: 3.7
      Added the :attr:`st_fstype` member to Solaris/derivatives.

.. function:: statvfs(path)

   Perform a :c:func:`statvfs` system call on the given path.  The return value is
   an object whose attributes describe the filesystem on the given path, and
   correspond to the members of the :c:type:`statvfs` structure, namely:
   :attr:`f_bsize`, :attr:`f_frsize`, :attr:`f_blocks`, :attr:`f_bfree`,
   :attr:`f_bavail`, :attr:`f_files`, :attr:`f_ffree`, :attr:`f_favail`,
   :attr:`f_flag`, :attr:`f_namemax`, :attr:`f_fsid`.

   Two module-level constants are defined for the :attr:`f_flag` attribute's
   bit-flags: if :const:`ST_RDONLY` is set, the filesystem is mounted
   read-only, and if :const:`ST_NOSUID` is set, the semantics of
   setuid/setgid bits are disabled or not supported.

   Additional module-level constants are defined for GNU/glibc based systems.
   These are :const:`ST_NODEV` (disallow access to device special files),
   :const:`ST_NOEXEC` (disallow program execution), :const:`ST_SYNCHRONOUS`
   (writes are synced at once), :const:`ST_MANDLOCK` (allow mandatory locks on an FS),
   :const:`ST_WRITE` (write on file/directory/symlink), :const:`ST_APPEND`
   (append-only file), :const:`ST_IMMUTABLE` (immutable file), :const:`ST_NOATIME`
   (do not update access times), :const:`ST_NODIRATIME` (do not update directory access
   times), :const:`ST_RELATIME` (update atime relative to mtime/ctime).

   This function can support :ref:`specifying a file descriptor <path_fd>`.

   Availability: Unix.

   .. versionchanged:: 3.2
      The :const:`ST_RDONLY` and :const:`ST_NOSUID` constants were added.

   .. versionadded:: 3.3
      Added support for specifying an open file descriptor for *path*.

   .. versionchanged:: 3.4
      The :const:`ST_NODEV`, :const:`ST_NOEXEC`, :const:`ST_SYNCHRONOUS`,
      :const:`ST_MANDLOCK`, :const:`ST_WRITE`, :const:`ST_APPEND`,
      :const:`ST_IMMUTABLE`, :const:`ST_NOATIME`, :const:`ST_NODIRATIME`,
      and :const:`ST_RELATIME` constants were added.

   .. versionchanged:: 3.6
      Accepts a :term:`path-like object`.

   .. versionadded:: 3.7
      Added :attr:`f_fsid`.


.. data:: supports_dir_fd

   A :class:`~collections.abc.Set` object indicating which functions in the
   :mod:`os` module permit use of their *dir_fd* parameter.  Different platforms
   provide different functionality, and an option that might work on one might
   be unsupported on another.  For consistency's sakes, functions that support
   *dir_fd* always allow specifying the parameter, but will raise an exception
   if the functionality is not actually available.

   To check whether a particular function permits use of its *dir_fd*
   parameter, use the ``in`` operator on ``supports_dir_fd``.  As an example,
   this expression determines whether the *dir_fd* parameter of :func:`os.stat`
   is locally available::

       os.stat in os.supports_dir_fd

   Currently *dir_fd* parameters only work on Unix platforms; none of them work
   on Windows.

   .. versionadded:: 3.3


.. data:: supports_effective_ids

   A :class:`~collections.abc.Set` object indicating which functions in the
   :mod:`os` module permit use of the *effective_ids* parameter for
   :func:`os.access`.  If the local platform supports it, the collection will
   contain :func:`os.access`, otherwise it will be empty.

   To check whether you can use the *effective_ids* parameter for
   :func:`os.access`, use the ``in`` operator on ``supports_effective_ids``,
   like so::

       os.access in os.supports_effective_ids

   Currently *effective_ids* only works on Unix platforms; it does not work on
   Windows.

   .. versionadded:: 3.3


.. data:: supports_fd

   A :class:`~collections.abc.Set` object indicating which functions in the
   :mod:`os` module permit specifying their *path* parameter as an open file
   descriptor.  Different platforms provide different functionality, and an
   option that might work on one might be unsupported on another.  For
   consistency's sakes, functions that support *fd* always allow specifying
   the parameter, but will raise an exception if the functionality is not
   actually available.

   To check whether a particular function permits specifying an open file
   descriptor for its *path* parameter, use the ``in`` operator on
   ``supports_fd``. As an example, this expression determines whether
   :func:`os.chdir` accepts open file descriptors when called on your local
   platform::

       os.chdir in os.supports_fd

   .. versionadded:: 3.3


.. data:: supports_follow_symlinks

   A :class:`~collections.abc.Set` object indicating which functions in the
   :mod:`os` module permit use of their *follow_symlinks* parameter.  Different
   platforms provide different functionality, and an option that might work on
   one might be unsupported on another.  For consistency's sakes, functions that
   support *follow_symlinks* always allow specifying the parameter, but will
   raise an exception if the functionality is not actually available.

   To check whether a particular function permits use of its *follow_symlinks*
   parameter, use the ``in`` operator on ``supports_follow_symlinks``.  As an
   example, this expression determines whether the *follow_symlinks* parameter
   of :func:`os.stat` is locally available::

       os.stat in os.supports_follow_symlinks

   .. versionadded:: 3.3


.. function:: symlink(src, dst, target_is_directory=False, *, dir_fd=None)

   Create a symbolic link pointing to *src* named *dst*.

   On Windows, a symlink represents either a file or a directory, and does not
   morph to the target dynamically.  If the target is present, the type of the
   symlink will be created to match. Otherwise, the symlink will be created
   as a directory if *target_is_directory* is ``True`` or a file symlink (the
   default) otherwise.  On non-Window platforms, *target_is_directory* is ignored.

   Symbolic link support was introduced in Windows 6.0 (Vista).  :func:`symlink`
   will raise a :exc:`NotImplementedError` on Windows versions earlier than 6.0.

   This function can support :ref:`paths relative to directory descriptors
   <dir_fd>`.

   .. note::

      On Windows, the *SeCreateSymbolicLinkPrivilege* is required in order to
      successfully create symlinks. This privilege is not typically granted to
      regular users but is available to accounts which can escalate privileges
      to the administrator level. Either obtaining the privilege or running your
      application as an administrator are ways to successfully create symlinks.


      :exc:`OSError` is raised when the function is called by an unprivileged
      user.

   Availability: Unix, Windows.

   .. versionchanged:: 3.2
      Added support for Windows 6.0 (Vista) symbolic links.

   .. versionadded:: 3.3
      Added the *dir_fd* argument, and now allow *target_is_directory*
      on non-Windows platforms.

   .. versionchanged:: 3.6
      Accepts a :term:`path-like object` for *src* and *dst*.


.. function:: sync()

   Force write of everything to disk.

   Availability: Unix.

   .. versionadded:: 3.3


.. function:: truncate(path, length)

   Truncate the file corresponding to *path*, so that it is at most
   *length* bytes in size.

   This function can support :ref:`specifying a file descriptor <path_fd>`.

   Availability: Unix, Windows.

   .. versionadded:: 3.3

   .. versionchanged:: 3.5
      Added support for Windows

   .. versionchanged:: 3.6
      Accepts a :term:`path-like object`.


.. function:: unlink(path, *, dir_fd=None)

   Remove (delete) the file *path*.  This function is semantically
   identical to :func:`remove`; the ``unlink`` name is its
   traditional Unix name.  Please see the documentation for
   :func:`remove` for further information.

   .. versionadded:: 3.3
      The *dir_fd* parameter.

   .. versionchanged:: 3.6
      Accepts a :term:`path-like object`.


.. function:: utime(path, times=None, *[, ns], dir_fd=None, follow_symlinks=True)

   Set the access and modified times of the file specified by *path*.

   :func:`utime` takes two optional parameters, *times* and *ns*.
   These specify the times set on *path* and are used as follows:

   - If *ns* is specified,
     it must be a 2-tuple of the form ``(atime_ns, mtime_ns)``
     where each member is an int expressing nanoseconds.
   - If *times* is not ``None``,
     it must be a 2-tuple of the form ``(atime, mtime)``
     where each member is an int or float expressing seconds.
   - If *times* is ``None`` and *ns* is unspecified,
     this is equivalent to specifying ``ns=(atime_ns, mtime_ns)``
     where both times are the current time.

   It is an error to specify tuples for both *times* and *ns*.

   Whether a directory can be given for *path*
   depends on whether the operating system implements directories as files
   (for example, Windows does not).  Note that the exact times you set here may
   not be returned by a subsequent :func:`~os.stat` call, depending on the
   resolution with which your operating system records access and modification
   times; see :func:`~os.stat`.  The best way to preserve exact times is to
   use the *st_atime_ns* and *st_mtime_ns* fields from the :func:`os.stat`
   result object with the *ns* parameter to `utime`.

   This function can support :ref:`specifying a file descriptor <path_fd>`,
   :ref:`paths relative to directory descriptors <dir_fd>` and :ref:`not
   following symlinks <follow_symlinks>`.

   .. versionadded:: 3.3
      Added support for specifying an open file descriptor for *path*,
      and the *dir_fd*, *follow_symlinks*, and *ns* parameters.

   .. versionchanged:: 3.6
      Accepts a :term:`path-like object`.


.. function:: walk(top, topdown=True, onerror=None, followlinks=False)

   .. index::
      single: directory; walking
      single: directory; traversal

   Generate the file names in a directory tree by walking the tree
   either top-down or bottom-up. For each directory in the tree rooted at directory
   *top* (including *top* itself), it yields a 3-tuple ``(dirpath, dirnames,
   filenames)``.

   *dirpath* is a string, the path to the directory.  *dirnames* is a list of the
   names of the subdirectories in *dirpath* (excluding ``'.'`` and ``'..'``).
   *filenames* is a list of the names of the non-directory files in *dirpath*.
   Note that the names in the lists contain no path components.  To get a full path
   (which begins with *top*) to a file or directory in *dirpath*, do
   ``os.path.join(dirpath, name)``.

   If optional argument *topdown* is ``True`` or not specified, the triple for a
   directory is generated before the triples for any of its subdirectories
   (directories are generated top-down).  If *topdown* is ``False``, the triple
   for a directory is generated after the triples for all of its subdirectories
   (directories are generated bottom-up). No matter the value of *topdown*, the
   list of subdirectories is retrieved before the tuples for the directory and
   its subdirectories are generated.

   When *topdown* is ``True``, the caller can modify the *dirnames* list in-place
   (perhaps using :keyword:`del` or slice assignment), and :func:`walk` will only
   recurse into the subdirectories whose names remain in *dirnames*; this can be
   used to prune the search, impose a specific order of visiting, or even to inform
   :func:`walk` about directories the caller creates or renames before it resumes
   :func:`walk` again.  Modifying *dirnames* when *topdown* is ``False`` has
   no effect on the behavior of the walk, because in bottom-up mode the directories
   in *dirnames* are generated before *dirpath* itself is generated.

   By default, errors from the :func:`listdir` call are ignored.  If optional
   argument *onerror* is specified, it should be a function; it will be called with
   one argument, an :exc:`OSError` instance.  It can report the error to continue
   with the walk, or raise the exception to abort the walk.  Note that the filename
   is available as the ``filename`` attribute of the exception object.

   By default, :func:`walk` will not walk down into symbolic links that resolve to
   directories. Set *followlinks* to ``True`` to visit directories pointed to by
   symlinks, on systems that support them.

   .. note::

      Be aware that setting *followlinks* to ``True`` can lead to infinite
      recursion if a link points to a parent directory of itself. :func:`walk`
      does not keep track of the directories it visited already.

   .. note::

      If you pass a relative pathname, don't change the current working directory
      between resumptions of :func:`walk`.  :func:`walk` never changes the current
      directory, and assumes that its caller doesn't either.

   This example displays the number of bytes taken by non-directory files in each
   directory under the starting directory, except that it doesn't look under any
   CVS subdirectory::

      import os
      from os.path import join, getsize
      for root, dirs, files in os.walk('python/Lib/email'):
          print(root, "consumes", end=" ")
          print(sum(getsize(join(root, name)) for name in files), end=" ")
          print("bytes in", len(files), "non-directory files")
          if 'CVS' in dirs:
              dirs.remove('CVS')  # don't visit CVS directories

   In the next example (simple implementation of :func:`shutil.rmtree`),
   walking the tree bottom-up is essential, :func:`rmdir` doesn't allow
   deleting a directory before the directory is empty::

      # Delete everything reachable from the directory named in "top",
      # assuming there are no symbolic links.
      # CAUTION:  This is dangerous!  For example, if top == '/', it
      # could delete all your disk files.
      import os
      for root, dirs, files in os.walk(top, topdown=False):
          for name in files:
              os.remove(os.path.join(root, name))
          for name in dirs:
              os.rmdir(os.path.join(root, name))

   .. versionchanged:: 3.5
      This function now calls :func:`os.scandir` instead of :func:`os.listdir`,
      making it faster by reducing the number of calls to :func:`os.stat`.

   .. versionchanged:: 3.6
      Accepts a :term:`path-like object`.


.. function:: fwalk(top='.', topdown=True, onerror=None, *, follow_symlinks=False, dir_fd=None)

   .. index::
      single: directory; walking
      single: directory; traversal

   This behaves exactly like :func:`walk`, except that it yields a 4-tuple
   ``(dirpath, dirnames, filenames, dirfd)``, and it supports ``dir_fd``.

   *dirpath*, *dirnames* and *filenames* are identical to :func:`walk` output,
   and *dirfd* is a file descriptor referring to the directory *dirpath*.

   This function always supports :ref:`paths relative to directory descriptors
   <dir_fd>` and :ref:`not following symlinks <follow_symlinks>`.  Note however
   that, unlike other functions, the :func:`fwalk` default value for
   *follow_symlinks* is ``False``.

   .. note::

      Since :func:`fwalk` yields file descriptors, those are only valid until
      the next iteration step, so you should duplicate them (e.g. with
      :func:`dup`) if you want to keep them longer.

   This example displays the number of bytes taken by non-directory files in each
   directory under the starting directory, except that it doesn't look under any
   CVS subdirectory::

      import os
      for root, dirs, files, rootfd in os.fwalk('python/Lib/email'):
          print(root, "consumes", end="")
          print(sum([os.stat(name, dir_fd=rootfd).st_size for name in files]),
                end="")
          print("bytes in", len(files), "non-directory files")
          if 'CVS' in dirs:
              dirs.remove('CVS')  # don't visit CVS directories

   In the next example, walking the tree bottom-up is essential:
   :func:`rmdir` doesn't allow deleting a directory before the directory is
   empty::

      # Delete everything reachable from the directory named in "top",
      # assuming there are no symbolic links.
      # CAUTION:  This is dangerous!  For example, if top == '/', it
      # could delete all your disk files.
      import os
      for root, dirs, files, rootfd in os.fwalk(top, topdown=False):
          for name in files:
              os.unlink(name, dir_fd=rootfd)
          for name in dirs:
              os.rmdir(name, dir_fd=rootfd)

   Availability: Unix.

   .. versionadded:: 3.3

   .. versionchanged:: 3.6
      Accepts a :term:`path-like object`.

   .. versionchanged:: 3.7
      Added support for :class:`bytes` paths.


Linux extended attributes
~~~~~~~~~~~~~~~~~~~~~~~~~

.. versionadded:: 3.3

These functions are all available on Linux only.

.. function:: getxattr(path, attribute, *, follow_symlinks=True)

   Return the value of the extended filesystem attribute *attribute* for
   *path*. *attribute* can be bytes or str (directly or indirectly through the
   :class:`PathLike` interface). If it is str, it is encoded with the filesystem
   encoding.

   This function can support :ref:`specifying a file descriptor <path_fd>` and
   :ref:`not following symlinks <follow_symlinks>`.

   .. versionchanged:: 3.6
      Accepts a :term:`path-like object` for *path* and *attribute*.


.. function:: listxattr(path=None, *, follow_symlinks=True)

   Return a list of the extended filesystem attributes on *path*.  The
   attributes in the list are represented as strings decoded with the filesystem
   encoding.  If *path* is ``None``, :func:`listxattr` will examine the current
   directory.

   This function can support :ref:`specifying a file descriptor <path_fd>` and
   :ref:`not following symlinks <follow_symlinks>`.

   .. versionchanged:: 3.6
      Accepts a :term:`path-like object`.


.. function:: removexattr(path, attribute, *, follow_symlinks=True)

   Removes the extended filesystem attribute *attribute* from *path*.
   *attribute* should be bytes or str (directly or indirectly through the
   :class:`PathLike` interface). If it is a string, it is encoded
   with the filesystem encoding.

   This function can support :ref:`specifying a file descriptor <path_fd>` and
   :ref:`not following symlinks <follow_symlinks>`.

   .. versionchanged:: 3.6
      Accepts a :term:`path-like object` for *path* and *attribute*.


.. function:: setxattr(path, attribute, value, flags=0, *, follow_symlinks=True)

   Set the extended filesystem attribute *attribute* on *path* to *value*.
   *attribute* must be a bytes or str with no embedded NULs (directly or
   indirectly through the :class:`PathLike` interface). If it is a str,
   it is encoded with the filesystem encoding.  *flags* may be
   :data:`XATTR_REPLACE` or :data:`XATTR_CREATE`. If :data:`XATTR_REPLACE` is
   given and the attribute does not exist, ``EEXISTS`` will be raised.
   If :data:`XATTR_CREATE` is given and the attribute already exists, the
   attribute will not be created and ``ENODATA`` will be raised.

   This function can support :ref:`specifying a file descriptor <path_fd>` and
   :ref:`not following symlinks <follow_symlinks>`.

   .. note::

      A bug in Linux kernel versions less than 2.6.39 caused the flags argument
      to be ignored on some filesystems.

   .. versionchanged:: 3.6
      Accepts a :term:`path-like object` for *path* and *attribute*.


.. data:: XATTR_SIZE_MAX

   The maximum size the value of an extended attribute can be. Currently, this
   is 64 KiB on Linux.


.. data:: XATTR_CREATE

   This is a possible value for the flags argument in :func:`setxattr`. It
   indicates the operation must create an attribute.


.. data:: XATTR_REPLACE

   This is a possible value for the flags argument in :func:`setxattr`. It
   indicates the operation must replace an existing attribute.


.. _os-process:

Process Management
------------------

These functions may be used to create and manage processes.

The various :func:`exec\* <execl>` functions take a list of arguments for the new
program loaded into the process.  In each case, the first of these arguments is
passed to the new program as its own name rather than as an argument a user may
have typed on a command line.  For the C programmer, this is the ``argv[0]``
passed to a program's :c:func:`main`.  For example, ``os.execv('/bin/echo',
['foo', 'bar'])`` will only print ``bar`` on standard output; ``foo`` will seem
to be ignored.


.. function:: abort()

   Generate a :const:`SIGABRT` signal to the current process.  On Unix, the default
   behavior is to produce a core dump; on Windows, the process immediately returns
   an exit code of ``3``.  Be aware that calling this function will not call the
   Python signal handler registered for :const:`SIGABRT` with
   :func:`signal.signal`.


.. function:: execl(path, arg0, arg1, ...)
              execle(path, arg0, arg1, ..., env)
              execlp(file, arg0, arg1, ...)
              execlpe(file, arg0, arg1, ..., env)
              execv(path, args)
              execve(path, args, env)
              execvp(file, args)
              execvpe(file, args, env)

   These functions all execute a new program, replacing the current process; they
   do not return.  On Unix, the new executable is loaded into the current process,
   and will have the same process id as the caller.  Errors will be reported as
   :exc:`OSError` exceptions.

   The current process is replaced immediately. Open file objects and
   descriptors are not flushed, so if there may be data buffered
   on these open files, you should flush them using
   :func:`sys.stdout.flush` or :func:`os.fsync` before calling an
   :func:`exec\* <execl>` function.

   The "l" and "v" variants of the :func:`exec\* <execl>` functions differ in how
   command-line arguments are passed.  The "l" variants are perhaps the easiest
   to work with if the number of parameters is fixed when the code is written; the
   individual parameters simply become additional parameters to the :func:`execl\*`
   functions.  The "v" variants are good when the number of parameters is
   variable, with the arguments being passed in a list or tuple as the *args*
   parameter.  In either case, the arguments to the child process should start with
   the name of the command being run, but this is not enforced.

   The variants which include a "p" near the end (:func:`execlp`,
   :func:`execlpe`, :func:`execvp`, and :func:`execvpe`) will use the
   :envvar:`PATH` environment variable to locate the program *file*.  When the
   environment is being replaced (using one of the :func:`exec\*e <execl>` variants,
   discussed in the next paragraph), the new environment is used as the source of
   the :envvar:`PATH` variable. The other variants, :func:`execl`, :func:`execle`,
   :func:`execv`, and :func:`execve`, will not use the :envvar:`PATH` variable to
   locate the executable; *path* must contain an appropriate absolute or relative
   path.

   For :func:`execle`, :func:`execlpe`, :func:`execve`, and :func:`execvpe` (note
   that these all end in "e"), the *env* parameter must be a mapping which is
   used to define the environment variables for the new process (these are used
   instead of the current process' environment); the functions :func:`execl`,
   :func:`execlp`, :func:`execv`, and :func:`execvp` all cause the new process to
   inherit the environment of the current process.

   For :func:`execve` on some platforms, *path* may also be specified as an open
   file descriptor.  This functionality may not be supported on your platform;
   you can check whether or not it is available using :data:`os.supports_fd`.
   If it is unavailable, using it will raise a :exc:`NotImplementedError`.

   Availability: Unix, Windows.

   .. versionadded:: 3.3
      Added support for specifying an open file descriptor for *path*
      for :func:`execve`.

   .. versionchanged:: 3.6
      Accepts a :term:`path-like object`.

.. function:: _exit(n)

   Exit the process with status *n*, without calling cleanup handlers, flushing
   stdio buffers, etc.

   .. note::

      The standard way to exit is ``sys.exit(n)``.  :func:`_exit` should
      normally only be used in the child process after a :func:`fork`.

The following exit codes are defined and can be used with :func:`_exit`,
although they are not required.  These are typically used for system programs
written in Python, such as a mail server's external command delivery program.

.. note::

   Some of these may not be available on all Unix platforms, since there is some
   variation.  These constants are defined where they are defined by the underlying
   platform.


.. data:: EX_OK

   Exit code that means no error occurred.

   Availability: Unix.


.. data:: EX_USAGE

   Exit code that means the command was used incorrectly, such as when the wrong
   number of arguments are given.

   Availability: Unix.


.. data:: EX_DATAERR

   Exit code that means the input data was incorrect.

   Availability: Unix.


.. data:: EX_NOINPUT

   Exit code that means an input file did not exist or was not readable.

   Availability: Unix.


.. data:: EX_NOUSER

   Exit code that means a specified user did not exist.

   Availability: Unix.


.. data:: EX_NOHOST

   Exit code that means a specified host did not exist.

   Availability: Unix.


.. data:: EX_UNAVAILABLE

   Exit code that means that a required service is unavailable.

   Availability: Unix.


.. data:: EX_SOFTWARE

   Exit code that means an internal software error was detected.

   Availability: Unix.


.. data:: EX_OSERR

   Exit code that means an operating system error was detected, such as the
   inability to fork or create a pipe.

   Availability: Unix.


.. data:: EX_OSFILE

   Exit code that means some system file did not exist, could not be opened, or had
   some other kind of error.

   Availability: Unix.


.. data:: EX_CANTCREAT

   Exit code that means a user specified output file could not be created.

   Availability: Unix.


.. data:: EX_IOERR

   Exit code that means that an error occurred while doing I/O on some file.

   Availability: Unix.


.. data:: EX_TEMPFAIL

   Exit code that means a temporary failure occurred.  This indicates something
   that may not really be an error, such as a network connection that couldn't be
   made during a retryable operation.

   Availability: Unix.


.. data:: EX_PROTOCOL

   Exit code that means that a protocol exchange was illegal, invalid, or not
   understood.

   Availability: Unix.


.. data:: EX_NOPERM

   Exit code that means that there were insufficient permissions to perform the
   operation (but not intended for file system problems).

   Availability: Unix.


.. data:: EX_CONFIG

   Exit code that means that some kind of configuration error occurred.

   Availability: Unix.


.. data:: EX_NOTFOUND

   Exit code that means something like "an entry was not found".

   Availability: Unix.


.. function:: fork()

   Fork a child process.  Return ``0`` in the child and the child's process id in the
   parent.  If an error occurs :exc:`OSError` is raised.

   Note that some platforms including FreeBSD <= 6.3 and Cygwin have
   known issues when using fork() from a thread.

   .. warning::

      See :mod:`ssl` for applications that use the SSL module with fork().

   Availability: Unix.


.. function:: forkpty()

   Fork a child process, using a new pseudo-terminal as the child's controlling
   terminal. Return a pair of ``(pid, fd)``, where *pid* is ``0`` in the child, the
   new child's process id in the parent, and *fd* is the file descriptor of the
   master end of the pseudo-terminal.  For a more portable approach, use the
   :mod:`pty` module.  If an error occurs :exc:`OSError` is raised.

   Availability: some flavors of Unix.


.. function:: kill(pid, sig)

   .. index::
      single: process; killing
      single: process; signalling

   Send signal *sig* to the process *pid*.  Constants for the specific signals
   available on the host platform are defined in the :mod:`signal` module.

   Windows: The :data:`signal.CTRL_C_EVENT` and
   :data:`signal.CTRL_BREAK_EVENT` signals are special signals which can
   only be sent to console processes which share a common console window,
   e.g., some subprocesses. Any other value for *sig* will cause the process
   to be unconditionally killed by the TerminateProcess API, and the exit code
   will be set to *sig*. The Windows version of :func:`kill` additionally takes
   process handles to be killed.

   See also :func:`signal.pthread_kill`.

   .. versionadded:: 3.2
      Windows support.


.. function:: killpg(pgid, sig)

   .. index::
      single: process; killing
      single: process; signalling

   Send the signal *sig* to the process group *pgid*.

   Availability: Unix.


.. function:: nice(increment)

   Add *increment* to the process's "niceness".  Return the new niceness.

   Availability: Unix.


.. function:: plock(op)

   Lock program segments into memory.  The value of *op* (defined in
   ``<sys/lock.h>``) determines which segments are locked.

   Availability: Unix.


.. function:: popen(cmd, mode='r', buffering=-1)

   Open a pipe to or from command *cmd*.
   The return value is an open file object
   connected to the pipe, which can be read or written depending on whether *mode*
   is ``'r'`` (default) or ``'w'``. The *buffering* argument has the same meaning as
   the corresponding argument to the built-in :func:`open` function. The
   returned file object reads or writes text strings rather than bytes.

   The ``close`` method returns :const:`None` if the subprocess exited
   successfully, or the subprocess's return code if there was an
   error. On POSIX systems, if the return code is positive it
   represents the return value of the process left-shifted by one
   byte.  If the return code is negative, the process was terminated
   by the signal given by the negated value of the return code.  (For
   example, the return value might be ``- signal.SIGKILL`` if the
   subprocess was killed.)  On Windows systems, the return value
   contains the signed integer return code from the child process.

   This is implemented using :class:`subprocess.Popen`; see that class's
   documentation for more powerful ways to manage and communicate with
   subprocesses.


.. function:: register_at_fork(*, before=None, after_in_parent=None, \
                               after_in_child=None)

   Register callables to be executed when a new child process is forked
   using :func:`os.fork` or similar process cloning APIs.
   The parameters are optional and keyword-only.
   Each specifies a different call point.

   * *before* is a function called before forking a child process.
   * *after_in_parent* is a function called from the parent process
     after forking a child process.
   * *after_in_child* is a function called from the child process.

   These calls are only made if control is expected to return to the
   Python interpreter.  A typical :mod:`subprocess` launch will not
   trigger them as the child is not going to re-enter the interpreter.

   Functions registered for execution before forking are called in
   reverse registration order.  Functions registered for execution
   after forking (either in the parent or in the child) are called
   in registration order.

   Note that :c:func:`fork` calls made by third-party C code may not
   call those functions, unless it explicitly calls :c:func:`PyOS_BeforeFork`,
   :c:func:`PyOS_AfterFork_Parent` and :c:func:`PyOS_AfterFork_Child`.

   There is no way to unregister a function.

   Availability: Unix.

   .. versionadded:: 3.7


.. function:: spawnl(mode, path, ...)
              spawnle(mode, path, ..., env)
              spawnlp(mode, file, ...)
              spawnlpe(mode, file, ..., env)
              spawnv(mode, path, args)
              spawnve(mode, path, args, env)
              spawnvp(mode, file, args)
              spawnvpe(mode, file, args, env)

   Execute the program *path* in a new process.

   (Note that the :mod:`subprocess` module provides more powerful facilities for
   spawning new processes and retrieving their results; using that module is
   preferable to using these functions.  Check especially the
   :ref:`subprocess-replacements` section.)

   If *mode* is :const:`P_NOWAIT`, this function returns the process id of the new
   process; if *mode* is :const:`P_WAIT`, returns the process's exit code if it
   exits normally, or ``-signal``, where *signal* is the signal that killed the
   process.  On Windows, the process id will actually be the process handle, so can
   be used with the :func:`waitpid` function.

   The "l" and "v" variants of the :func:`spawn\* <spawnl>` functions differ in how
   command-line arguments are passed.  The "l" variants are perhaps the easiest
   to work with if the number of parameters is fixed when the code is written; the
   individual parameters simply become additional parameters to the
   :func:`spawnl\*` functions.  The "v" variants are good when the number of
   parameters is variable, with the arguments being passed in a list or tuple as
   the *args* parameter.  In either case, the arguments to the child process must
   start with the name of the command being run.

   The variants which include a second "p" near the end (:func:`spawnlp`,
   :func:`spawnlpe`, :func:`spawnvp`, and :func:`spawnvpe`) will use the
   :envvar:`PATH` environment variable to locate the program *file*.  When the
   environment is being replaced (using one of the :func:`spawn\*e <spawnl>` variants,
   discussed in the next paragraph), the new environment is used as the source of
   the :envvar:`PATH` variable.  The other variants, :func:`spawnl`,
   :func:`spawnle`, :func:`spawnv`, and :func:`spawnve`, will not use the
   :envvar:`PATH` variable to locate the executable; *path* must contain an
   appropriate absolute or relative path.

   For :func:`spawnle`, :func:`spawnlpe`, :func:`spawnve`, and :func:`spawnvpe`
   (note that these all end in "e"), the *env* parameter must be a mapping
   which is used to define the environment variables for the new process (they are
   used instead of the current process' environment); the functions
   :func:`spawnl`, :func:`spawnlp`, :func:`spawnv`, and :func:`spawnvp` all cause
   the new process to inherit the environment of the current process.  Note that
   keys and values in the *env* dictionary must be strings; invalid keys or
   values will cause the function to fail, with a return value of ``127``.

   As an example, the following calls to :func:`spawnlp` and :func:`spawnvpe` are
   equivalent::

      import os
      os.spawnlp(os.P_WAIT, 'cp', 'cp', 'index.html', '/dev/null')

      L = ['cp', 'index.html', '/dev/null']
      os.spawnvpe(os.P_WAIT, 'cp', L, os.environ)

   Availability: Unix, Windows.  :func:`spawnlp`, :func:`spawnlpe`, :func:`spawnvp`
   and :func:`spawnvpe` are not available on Windows.  :func:`spawnle` and
   :func:`spawnve` are not thread-safe on Windows; we advise you to use the
   :mod:`subprocess` module instead.

   .. versionchanged:: 3.6
      Accepts a :term:`path-like object`.


.. data:: P_NOWAIT
          P_NOWAITO

   Possible values for the *mode* parameter to the :func:`spawn\* <spawnl>` family of
   functions.  If either of these values is given, the :func:`spawn\*` functions
   will return as soon as the new process has been created, with the process id as
   the return value.

   Availability: Unix, Windows.


.. data:: P_WAIT

   Possible value for the *mode* parameter to the :func:`spawn\* <spawnl>` family of
   functions.  If this is given as *mode*, the :func:`spawn\*` functions will not
   return until the new process has run to completion and will return the exit code
   of the process the run is successful, or ``-signal`` if a signal kills the
   process.

   Availability: Unix, Windows.


.. data:: P_DETACH
          P_OVERLAY

   Possible values for the *mode* parameter to the :func:`spawn\* <spawnl>` family of
   functions.  These are less portable than those listed above. :const:`P_DETACH`
   is similar to :const:`P_NOWAIT`, but the new process is detached from the
   console of the calling process. If :const:`P_OVERLAY` is used, the current
   process will be replaced; the :func:`spawn\* <spawnl>` function will not return.

   Availability: Windows.


.. function:: startfile(path[, operation])

   Start a file with its associated application.

   When *operation* is not specified or ``'open'``, this acts like double-clicking
   the file in Windows Explorer, or giving the file name as an argument to the
   :program:`start` command from the interactive command shell: the file is opened
   with whatever application (if any) its extension is associated.

   When another *operation* is given, it must be a "command verb" that specifies
   what should be done with the file. Common verbs documented by Microsoft are
   ``'print'`` and  ``'edit'`` (to be used on files) as well as ``'explore'`` and
   ``'find'`` (to be used on directories).

   :func:`startfile` returns as soon as the associated application is launched.
   There is no option to wait for the application to close, and no way to retrieve
   the application's exit status.  The *path* parameter is relative to the current
   directory.  If you want to use an absolute path, make sure the first character
   is not a slash (``'/'``); the underlying Win32 :c:func:`ShellExecute` function
   doesn't work if it is.  Use the :func:`os.path.normpath` function to ensure that
   the path is properly encoded for Win32.

   To reduce interpreter startup overhead, the Win32 :c:func:`ShellExecute`
   function is not resolved until this function is first called.  If the function
   cannot be resolved, :exc:`NotImplementedError` will be raised.

   Availability: Windows.


.. function:: system(command)

   Execute the command (a string) in a subshell.  This is implemented by calling
   the Standard C function :c:func:`system`, and has the same limitations.
   Changes to :data:`sys.stdin`, etc. are not reflected in the environment of
   the executed command. If *command* generates any output, it will be sent to
   the interpreter standard output stream.

   On Unix, the return value is the exit status of the process encoded in the
   format specified for :func:`wait`.  Note that POSIX does not specify the
   meaning of the return value of the C :c:func:`system` function, so the return
   value of the Python function is system-dependent.

   On Windows, the return value is that returned by the system shell after
   running *command*.  The shell is given by the Windows environment variable
   :envvar:`COMSPEC`: it is usually :program:`cmd.exe`, which returns the exit
   status of the command run; on systems using a non-native shell, consult your
   shell documentation.

   The :mod:`subprocess` module provides more powerful facilities for spawning
   new processes and retrieving their results; using that module is preferable
   to using this function.  See the :ref:`subprocess-replacements` section in
   the :mod:`subprocess` documentation for some helpful recipes.

   Availability: Unix, Windows.


.. function:: times()

   Returns the current global process times.
   The return value is an object with five attributes:

   * :attr:`user` - user time
   * :attr:`system` - system time
   * :attr:`children_user` - user time of all child processes
   * :attr:`children_system` - system time of all child processes
   * :attr:`elapsed` - elapsed real time since a fixed point in the past

   For backwards compatibility, this object also behaves like a five-tuple
   containing :attr:`user`, :attr:`system`, :attr:`children_user`,
   :attr:`children_system`, and :attr:`elapsed` in that order.

   See the Unix manual page
   :manpage:`times(2)` or the corresponding Windows Platform API documentation.
   On Windows, only :attr:`user` and :attr:`system` are known; the other
   attributes are zero.

   Availability: Unix, Windows.

   .. versionchanged:: 3.3
      Return type changed from a tuple to a tuple-like object
      with named attributes.


.. function:: wait()

   Wait for completion of a child process, and return a tuple containing its pid
   and exit status indication: a 16-bit number, whose low byte is the signal number
   that killed the process, and whose high byte is the exit status (if the signal
   number is zero); the high bit of the low byte is set if a core file was
   produced.

   Availability: Unix.

.. function:: waitid(idtype, id, options)

   Wait for the completion of one or more child processes.
   *idtype* can be :data:`P_PID`, :data:`P_PGID` or :data:`P_ALL`.
   *id* specifies the pid to wait on.
   *options* is constructed from the ORing of one or more of :data:`WEXITED`,
   :data:`WSTOPPED` or :data:`WCONTINUED` and additionally may be ORed with
   :data:`WNOHANG` or :data:`WNOWAIT`. The return value is an object
   representing the data contained in the :c:type:`siginfo_t` structure, namely:
   :attr:`si_pid`, :attr:`si_uid`, :attr:`si_signo`, :attr:`si_status`,
   :attr:`si_code` or ``None`` if :data:`WNOHANG` is specified and there are no
   children in a waitable state.

   Availability: Unix.

   .. versionadded:: 3.3

.. data:: P_PID
          P_PGID
          P_ALL

   These are the possible values for *idtype* in :func:`waitid`. They affect
   how *id* is interpreted.

   Availability: Unix.

   .. versionadded:: 3.3

.. data:: WEXITED
          WSTOPPED
          WNOWAIT

   Flags that can be used in *options* in :func:`waitid` that specify what
   child signal to wait for.

   Availability: Unix.

   .. versionadded:: 3.3


.. data:: CLD_EXITED
          CLD_DUMPED
          CLD_TRAPPED
          CLD_CONTINUED

   These are the possible values for :attr:`si_code` in the result returned by
   :func:`waitid`.

   Availability: Unix.

   .. versionadded:: 3.3


.. function:: waitpid(pid, options)

   The details of this function differ on Unix and Windows.

   On Unix: Wait for completion of a child process given by process id *pid*, and
   return a tuple containing its process id and exit status indication (encoded as
   for :func:`wait`).  The semantics of the call are affected by the value of the
   integer *options*, which should be ``0`` for normal operation.

   If *pid* is greater than ``0``, :func:`waitpid` requests status information for
   that specific process.  If *pid* is ``0``, the request is for the status of any
   child in the process group of the current process.  If *pid* is ``-1``, the
   request pertains to any child of the current process.  If *pid* is less than
   ``-1``, status is requested for any process in the process group ``-pid`` (the
   absolute value of *pid*).

   An :exc:`OSError` is raised with the value of errno when the syscall
   returns -1.

   On Windows: Wait for completion of a process given by process handle *pid*, and
   return a tuple containing *pid*, and its exit status shifted left by 8 bits
   (shifting makes cross-platform use of the function easier). A *pid* less than or
   equal to ``0`` has no special meaning on Windows, and raises an exception. The
   value of integer *options* has no effect. *pid* can refer to any process whose
   id is known, not necessarily a child process. The :func:`spawn\* <spawnl>`
   functions called with :const:`P_NOWAIT` return suitable process handles.

   .. versionchanged:: 3.5
      If the system call is interrupted and the signal handler does not raise an
      exception, the function now retries the system call instead of raising an
      :exc:`InterruptedError` exception (see :pep:`475` for the rationale).


.. function:: wait3(options)

   Similar to :func:`waitpid`, except no process id argument is given and a
   3-element tuple containing the child's process id, exit status indication, and
   resource usage information is returned.  Refer to :mod:`resource`.\
   :func:`~resource.getrusage` for details on resource usage information.  The
   option argument is the same as that provided to :func:`waitpid` and
   :func:`wait4`.

   Availability: Unix.


.. function:: wait4(pid, options)

   Similar to :func:`waitpid`, except a 3-element tuple, containing the child's
   process id, exit status indication, and resource usage information is returned.
   Refer to :mod:`resource`.\ :func:`~resource.getrusage` for details on
   resource usage information.  The arguments to :func:`wait4` are the same
   as those provided to :func:`waitpid`.

   Availability: Unix.


.. data:: WNOHANG

   The option for :func:`waitpid` to return immediately if no child process status
   is available immediately. The function returns ``(0, 0)`` in this case.

   Availability: Unix.


.. data:: WCONTINUED

   This option causes child processes to be reported if they have been continued
   from a job control stop since their status was last reported.

   Availability: some Unix systems.


.. data:: WUNTRACED

   This option causes child processes to be reported if they have been stopped but
   their current state has not been reported since they were stopped.

   Availability: Unix.


The following functions take a process status code as returned by
:func:`system`, :func:`wait`, or :func:`waitpid` as a parameter.  They may be
used to determine the disposition of a process.

.. function:: WCOREDUMP(status)

   Return ``True`` if a core dump was generated for the process, otherwise
   return ``False``.

   Availability: Unix.


.. function:: WIFCONTINUED(status)

   Return ``True`` if the process has been continued from a job control stop,
   otherwise return ``False``.

   Availability: Unix.


.. function:: WIFSTOPPED(status)

   Return ``True`` if the process has been stopped, otherwise return
   ``False``.

   Availability: Unix.


.. function:: WIFSIGNALED(status)

   Return ``True`` if the process exited due to a signal, otherwise return
   ``False``.

   Availability: Unix.


.. function:: WIFEXITED(status)

   Return ``True`` if the process exited using the :manpage:`exit(2)` system call,
   otherwise return ``False``.

   Availability: Unix.


.. function:: WEXITSTATUS(status)

   If ``WIFEXITED(status)`` is true, return the integer parameter to the
   :manpage:`exit(2)` system call.  Otherwise, the return value is meaningless.

   Availability: Unix.


.. function:: WSTOPSIG(status)

   Return the signal which caused the process to stop.

   Availability: Unix.


.. function:: WTERMSIG(status)

   Return the signal which caused the process to exit.

   Availability: Unix.


Interface to the scheduler
--------------------------

These functions control how a process is allocated CPU time by the operating
system. They are only available on some Unix platforms. For more detailed
information, consult your Unix manpages.

.. versionadded:: 3.3

The following scheduling policies are exposed if they are supported by the
operating system.

.. data:: SCHED_OTHER

   The default scheduling policy.

.. data:: SCHED_BATCH

   Scheduling policy for CPU-intensive processes that tries to preserve
   interactivity on the rest of the computer.

.. data:: SCHED_IDLE

   Scheduling policy for extremely low priority background tasks.

.. data:: SCHED_SPORADIC

   Scheduling policy for sporadic server programs.

.. data:: SCHED_FIFO

   A First In First Out scheduling policy.

.. data:: SCHED_RR

   A round-robin scheduling policy.

.. data:: SCHED_RESET_ON_FORK

   This flag can be OR'ed with any other scheduling policy. When a process with
   this flag set forks, its child's scheduling policy and priority are reset to
   the default.


.. class:: sched_param(sched_priority)

   This class represents tunable scheduling parameters used in
   :func:`sched_setparam`, :func:`sched_setscheduler`, and
   :func:`sched_getparam`. It is immutable.

   At the moment, there is only one possible parameter:

   .. attribute:: sched_priority

      The scheduling priority for a scheduling policy.


.. function:: sched_get_priority_min(policy)

   Get the minimum priority value for *policy*. *policy* is one of the
   scheduling policy constants above.


.. function:: sched_get_priority_max(policy)

   Get the maximum priority value for *policy*. *policy* is one of the
   scheduling policy constants above.


.. function:: sched_setscheduler(pid, policy, param)

   Set the scheduling policy for the process with PID *pid*. A *pid* of 0 means
   the calling process. *policy* is one of the scheduling policy constants
   above. *param* is a :class:`sched_param` instance.


.. function:: sched_getscheduler(pid)

   Return the scheduling policy for the process with PID *pid*. A *pid* of 0
   means the calling process. The result is one of the scheduling policy
   constants above.


.. function:: sched_setparam(pid, param)

   Set a scheduling parameters for the process with PID *pid*. A *pid* of 0 means
   the calling process. *param* is a :class:`sched_param` instance.


.. function:: sched_getparam(pid)

   Return the scheduling parameters as a :class:`sched_param` instance for the
   process with PID *pid*. A *pid* of 0 means the calling process.


.. function:: sched_rr_get_interval(pid)

   Return the round-robin quantum in seconds for the process with PID *pid*. A
   *pid* of 0 means the calling process.


.. function:: sched_yield()

   Voluntarily relinquish the CPU.


.. function:: sched_setaffinity(pid, mask)

   Restrict the process with PID *pid* (or the current process if zero) to a
   set of CPUs.  *mask* is an iterable of integers representing the set of
   CPUs to which the process should be restricted.


.. function:: sched_getaffinity(pid)

   Return the set of CPUs the process with PID *pid* (or the current process
   if zero) is restricted to.


.. _os-path:

Miscellaneous System Information
--------------------------------


.. function:: confstr(name)

   Return string-valued system configuration values. *name* specifies the
   configuration value to retrieve; it may be a string which is the name of a
   defined system value; these names are specified in a number of standards (POSIX,
   Unix 95, Unix 98, and others).  Some platforms define additional names as well.
   The names known to the host operating system are given as the keys of the
   ``confstr_names`` dictionary.  For configuration variables not included in that
   mapping, passing an integer for *name* is also accepted.

   If the configuration value specified by *name* isn't defined, ``None`` is
   returned.

   If *name* is a string and is not known, :exc:`ValueError` is raised.  If a
   specific value for *name* is not supported by the host system, even if it is
   included in ``confstr_names``, an :exc:`OSError` is raised with
   :const:`errno.EINVAL` for the error number.

   Availability: Unix.


.. data:: confstr_names

   Dictionary mapping names accepted by :func:`confstr` to the integer values
   defined for those names by the host operating system. This can be used to
   determine the set of names known to the system.

   Availability: Unix.


.. function:: cpu_count()

   Return the number of CPUs in the system. Returns ``None`` if undetermined.

   This number is not equivalent to the number of CPUs the current process can
   use.  The number of usable CPUs can be obtained with
   ``len(os.sched_getaffinity(0))``


   .. versionadded:: 3.4


.. function:: getloadavg()

   Return the number of processes in the system run queue averaged over the last
   1, 5, and 15 minutes or raises :exc:`OSError` if the load average was
   unobtainable.

   Availability: Unix.


.. function:: sysconf(name)

   Return integer-valued system configuration values. If the configuration value
   specified by *name* isn't defined, ``-1`` is returned.  The comments regarding
   the *name* parameter for :func:`confstr` apply here as well; the dictionary that
   provides information on the known names is given by ``sysconf_names``.

   Availability: Unix.


.. data:: sysconf_names

   Dictionary mapping names accepted by :func:`sysconf` to the integer values
   defined for those names by the host operating system. This can be used to
   determine the set of names known to the system.

   Availability: Unix.

The following data values are used to support path manipulation operations.  These
are defined for all platforms.

Higher-level operations on pathnames are defined in the :mod:`os.path` module.


.. data:: curdir

   The constant string used by the operating system to refer to the current
   directory. This is ``'.'`` for Windows and POSIX. Also available via
   :mod:`os.path`.


.. data:: pardir

   The constant string used by the operating system to refer to the parent
   directory. This is ``'..'`` for Windows and POSIX. Also available via
   :mod:`os.path`.


.. data:: sep

   The character used by the operating system to separate pathname components.
   This is ``'/'`` for POSIX and ``'\\'`` for Windows.  Note that knowing this
   is not sufficient to be able to parse or concatenate pathnames --- use
   :func:`os.path.split` and :func:`os.path.join` --- but it is occasionally
   useful. Also available via :mod:`os.path`.


.. data:: altsep

   An alternative character used by the operating system to separate pathname
   components, or ``None`` if only one separator character exists.  This is set to
   ``'/'`` on Windows systems where ``sep`` is a backslash. Also available via
   :mod:`os.path`.


.. data:: extsep

   The character which separates the base filename from the extension; for example,
   the ``'.'`` in :file:`os.py`. Also available via :mod:`os.path`.


.. data:: pathsep

   The character conventionally used by the operating system to separate search
   path components (as in :envvar:`PATH`), such as ``':'`` for POSIX or ``';'`` for
   Windows. Also available via :mod:`os.path`.


.. data:: defpath

   The default search path used by :func:`exec\*p\* <execl>` and
   :func:`spawn\*p\* <spawnl>` if the environment doesn't have a ``'PATH'``
   key. Also available via :mod:`os.path`.


.. data:: linesep

   The string used to separate (or, rather, terminate) lines on the current
   platform.  This may be a single character, such as ``'\n'`` for POSIX, or
   multiple characters, for example, ``'\r\n'`` for Windows. Do not use
   *os.linesep* as a line terminator when writing files opened in text mode (the
   default); use a single ``'\n'`` instead, on all platforms.


.. data:: devnull

   The file path of the null device. For example: ``'/dev/null'`` for
   POSIX, ``'nul'`` for Windows.  Also available via :mod:`os.path`.

.. data:: RTLD_LAZY
          RTLD_NOW
          RTLD_GLOBAL
          RTLD_LOCAL
          RTLD_NODELETE
          RTLD_NOLOAD
          RTLD_DEEPBIND

   Flags for use with the :func:`~sys.setdlopenflags` and
   :func:`~sys.getdlopenflags` functions.  See the Unix manual page
   :manpage:`dlopen(3)` for what the different flags mean.

   .. versionadded:: 3.3


Random numbers
--------------


.. function:: getrandom(size, flags=0)

   Get up to *size* random bytes. The function can return less bytes than
   requested.

   These bytes can be used to seed user-space random number generators or for
   cryptographic purposes.

   ``getrandom()`` relies on entropy gathered from device drivers and other
   sources of environmental noise. Unnecessarily reading large quantities of
   data will have a negative impact on  other users  of the ``/dev/random`` and
   ``/dev/urandom`` devices.

   The flags argument is a bit mask that can contain zero or more of the
   following values ORed together: :py:data:`os.GRND_RANDOM` and
   :py:data:`GRND_NONBLOCK`.

   See also the `Linux getrandom() manual page
   <http://man7.org/linux/man-pages/man2/getrandom.2.html>`_.

   Availability: Linux 3.17 and newer.

   .. versionadded:: 3.6

.. function:: urandom(size)

   Return a string of *size* random bytes suitable for cryptographic use.

   This function returns random bytes from an OS-specific randomness source.  The
   returned data should be unpredictable enough for cryptographic applications,
   though its exact quality depends on the OS implementation.

   On Linux, if the ``getrandom()`` syscall is available, it is used in
   blocking mode: block until the system urandom entropy pool is initialized
   (128 bits of entropy are collected by the kernel). See the :pep:`524` for
   the rationale. On Linux, the :func:`getrandom` function can be used to get
   random bytes in non-blocking mode (using the :data:`GRND_NONBLOCK` flag) or
   to poll until the system urandom entropy pool is initialized.

   On a Unix-like system, random bytes are read from the ``/dev/urandom``
   device. If the ``/dev/urandom`` device is not available or not readable, the
   :exc:`NotImplementedError` exception is raised.

   On Windows, it will use ``CryptGenRandom()``.

   .. seealso::
      The :mod:`secrets` module provides higher level functions. For an
      easy-to-use interface to the random number generator provided by your
      platform, please see :class:`random.SystemRandom`.

   .. versionchanged:: 3.6.0
      On Linux, ``getrandom()`` is now used in blocking mode to increase the
      security.

   .. versionchanged:: 3.5.2
      On Linux, if the ``getrandom()`` syscall blocks (the urandom entropy pool
      is not initialized yet), fall back on reading ``/dev/urandom``.

   .. versionchanged:: 3.5
      On Linux 3.17 and newer, the ``getrandom()`` syscall is now used
      when available.  On OpenBSD 5.6 and newer, the C ``getentropy()``
      function is now used. These functions avoid the usage of an internal file
      descriptor.

.. data:: GRND_NONBLOCK

   By  default, when reading from ``/dev/random``, :func:`getrandom` blocks if
   no random bytes are available, and when reading from ``/dev/urandom``, it blocks
   if the entropy pool has not yet been initialized.

   If the :py:data:`GRND_NONBLOCK` flag is set, then :func:`getrandom` does not
   block in these cases, but instead immediately raises :exc:`BlockingIOError`.

   .. versionadded:: 3.6

.. data:: GRND_RANDOM

   If  this  bit  is  set,  then  random bytes are drawn from the
   ``/dev/random`` pool instead of the ``/dev/urandom`` pool.

   .. versionadded:: 3.6
