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# 18.4. `selectors` – High-level I/O multiplexing

New in version 3.4.

Source code: Lib/selectors.py

## 18.4.1. 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.

## 18.4.2. 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 modules constants below:

> Constant | Meaning
> --- | ---
> `EVENT_READ` | Available for read
> `EVENT_WRITE` | Available for write

class `selectors.``SelectorKey`

  A `SelectorKey` is a `namedtuple` used to associate a file object to its underlying file decriptor, 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.

  New in version 3.5.

class `selectors.``KqueueSelector`

  `select.kqueue()`-based selector.

  `fileno`()

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

## 18.4.3. 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)

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