# `selectors` — High-level I/O multiplexing

Added in version 3.4.

Source code: Lib/selectors.py

## Introduction

This module allows high-level and efficient I/O multiplexing, built upon the `select` module primitives. Users are encouraged to use this module instead, unless they want precise control over the OS-level primitives used.

It defines a `BaseSelector` abstract base class, along with several concrete implementations (`KqueueSelector`, `EpollSelector`…), that can be used to wait for I/O readiness notification on multiple file objects. In the following, “file object” refers to any object with a `fileno()` method, or a raw file descriptor. See file object.

`DefaultSelector` is an alias to the most efficient implementation available on the current platform: this should be the default choice for most users.

Note

The type of file objects supported depends on the platform: on Windows, sockets are supported, but not pipes, whereas on Unix, both are supported (some other types may be supported as well, such as fifos or special file devices).

See also

`select`

  Low-level I/O multiplexing module.

Availability: not WASI.

This module does not work or is not available on WebAssembly. See WebAssembly platforms for more information.

## Classes

Classes hierarchy:

    BaseSelector
    +-- SelectSelector
    +-- PollSelector
    +-- EpollSelector
    +-- DevpollSelector
    +-- KqueueSelector

In the following, events is a bitwise mask indicating which I/O events should be waited for on a given file object. It can be a combination of the module’s constants below:

> Constant | Meaning
> --- | ---

> selectors.EVENT_READ

> Available for read

> selectors.EVENT_WRITE

> Available for write

class selectors.SelectorKey

  A `SelectorKey` is a `namedtuple` used to associate a file object to its underlying file descriptor, selected event mask and attached data. It is returned by several `BaseSelector` methods.

  fileobj

    File object registered.

  fd

    Underlying file descriptor.

  events

    Events that must be waited for on this file object.

  data

    Optional opaque data associated to this file object: for example, this could be used to store a per-client session ID.

class selectors.BaseSelector

  A `BaseSelector` is used to wait for I/O event readiness on multiple file objects. It supports file stream registration, unregistration, and a method to wait for I/O events on those streams, with an optional timeout. It’s an abstract base class, so cannot be instantiated. Use `DefaultSelector` instead, or one of `SelectSelector`, `KqueueSelector` etc. if you want to specifically use an implementation, and your platform supports it. `BaseSelector` and its concrete implementations support the context manager protocol.

  abstractmethod register(fileobj, events, data=None)

    Register a file object for selection, monitoring it for I/O events.

    fileobj is the file object to monitor. It may either be an integer file descriptor or an object with a `fileno()` method. events is a bitwise mask of events to monitor. data is an opaque object.

    This returns a new `SelectorKey` instance, or raises a `ValueError` in case of invalid event mask or file descriptor, or `KeyError` if the file object is already registered.

  abstractmethod unregister(fileobj)

    Unregister a file object from selection, removing it from monitoring. A file object shall be unregistered prior to being closed.

    fileobj must be a file object previously registered.

    This returns the associated `SelectorKey` instance, or raises a `KeyError` if fileobj is not registered. It will raise `ValueError` if fileobj is invalid (e.g. it has no `fileno()` method or its `fileno()` method has an invalid return value).

  modify(fileobj, events, data=None)

    Change a registered file object’s monitored events or attached data.

    This is equivalent to `BaseSelector.unregister(fileobj)` followed by `BaseSelector.register(fileobj, events, data)`, except that it can be implemented more efficiently.

    This returns a new `SelectorKey` instance, or raises a `ValueError` in case of invalid event mask or file descriptor, or `KeyError` if the file object is not registered.

  abstractmethod select(timeout=None)

    Wait until some registered file objects become ready, or the timeout expires.

    If `timeout > 0`, this specifies the maximum wait time, in seconds. If `timeout <= 0`, the call won’t block, and will report the currently ready file objects. If timeout is `None`, the call will block until a monitored file object becomes ready.

    This returns a list of `(key, events)` tuples, one for each ready file object.

    key is the `SelectorKey` instance corresponding to a ready file object. events is a bitmask of events ready on this file object.

    Note

    This method can return before any file object becomes ready or the timeout has elapsed if the current process receives a signal: in this case, an empty list will be returned.

    Changed in version 3.5: The selector is now retried with a recomputed timeout when interrupted by a signal if the signal handler did not raise an exception (see PEP 475 for the rationale), instead of returning an empty list of events before the timeout.

  close()

    Close the selector.

    This must be called to make sure that any underlying resource is freed. The selector shall not be used once it has been closed.

  get_key(fileobj)

    Return the key associated with a registered file object.

    This returns the `SelectorKey` instance associated to this file object, or raises `KeyError` if the file object is not registered.

  abstractmethod get_map()

    Return a mapping of file objects to selector keys.

    This returns a `Mapping` instance mapping registered file objects to their associated `SelectorKey` instance.

class selectors.DefaultSelector

  The default selector class, using the most efficient implementation available on the current platform. This should be the default choice for most users.

class selectors.SelectSelector

  `select.select()`-based selector.

class selectors.PollSelector

  `select.poll()`-based selector.

class selectors.EpollSelector

  `select.epoll()`-based selector.

  fileno()

    This returns the file descriptor used by the underlying `select.epoll()` object.

class selectors.DevpollSelector

  `select.devpoll()`-based selector.

  fileno()

    This returns the file descriptor used by the underlying `select.devpoll()` object.

  Added in version 3.5.

class selectors.KqueueSelector

  `select.kqueue()`-based selector.

  fileno()

    This returns the file descriptor used by the underlying `select.kqueue()` object.

## Examples

Here is a simple echo server implementation:

    import selectors
    import socket

    sel = selectors.DefaultSelector()

    def accept(sock, mask):
        conn, addr = sock.accept()  # Should be ready
        print('accepted', conn, 'from', addr)
        conn.setblocking(False)
        sel.register(conn, selectors.EVENT_READ, read)

    def read(conn, mask):
        data = conn.recv(1000)  # Should be ready
        if data:
            print('echoing', repr(data), 'to', conn)
            conn.send(data)  # Hope it won't block
        else:
            print('closing', conn)
            sel.unregister(conn)
            conn.close()

    sock = socket.socket()
    sock.bind(('localhost', 1234))
    sock.listen(100)
    sock.setblocking(False)
    sel.register(sock, selectors.EVENT_READ, accept)

    while True:
        events = sel.select()
        for key, mask in events:
            callback = key.data
            callback(key.fileobj, mask)