cpython/Doc/library/selectors.rst

:mod:`selectors` -- High-level I/O multiplexing
===============================================

.. module:: selectors
   :synopsis: High-level I/O multiplexing.

.. versionadded:: 3.4

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

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

Introduction
------------

This module allows high-level and efficient I/O multiplexing, built upon the
:mod:`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 :class:`BaseSelector` abstract base class, along with several
concrete implementations (:class:`KqueueSelector`, :class:`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
:meth:`fileno()` method, or a raw file descriptor. See :term:`file object`.

:class:`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).

.. seealso::

   :mod:`select`
      Low-level I/O multiplexing module.


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                                       |
   +=======================+===============================================+
   | :const:`EVENT_READ`   | Available for read                            |
   +-----------------------+-----------------------------------------------+
   | :const:`EVENT_WRITE`  | Available for write                           |
   +-----------------------+-----------------------------------------------+


.. class:: SelectorKey

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

   .. attribute:: fileobj

      File object registered.

   .. attribute:: fd

      Underlying file descriptor.

   .. attribute:: events

      Events that must be waited for on this file object.

   .. attribute:: data

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


.. class:: BaseSelector

   A :class:`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
   :class:`DefaultSelector` instead, or one of :class:`SelectSelector`,
   :class:`KqueueSelector` etc. if you want to specifically use an
   implementation, and your platform supports it.
   :class:`BaseSelector` and its concrete implementations support the
   :term:`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 :class:`SelectorKey` instance, or raises a
      :exc:`ValueError` in case of invalid event mask or file descriptor, or
      :exc:`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 :class:`SelectorKey` instance, or raises a
      :exc:`KeyError` if *fileobj* is not registered.  It will raise
      :exc:`ValueError` if *fileobj* is invalid (e.g. it has no ``fileno()``
      method or its ``fileno()`` method has an invalid return value).

   .. method:: modify(fileobj, events, data=None)

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

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

      This returns a new :class:`SelectorKey` instance, or raises a
      :exc:`ValueError` in case of invalid event mask or file descriptor, or
      :exc:`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 :class:`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.

      .. versionchanged:: 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.

   .. method:: 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.

   .. method:: get_key(fileobj)

      Return the key associated with a registered file object.

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

   .. abstractmethod:: get_map()

      Return a mapping of file objects to selector keys.

      This returns a :class:`~collections.abc.Mapping` instance mapping
      registered file objects to their associated :class:`SelectorKey`
      instance.


.. class:: 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:: SelectSelector()

   :func:`select.select`-based selector.


.. class:: PollSelector()

   :func:`select.poll`-based selector.


.. class:: EpollSelector()

   :func:`select.epoll`-based selector.

   .. method:: fileno()

      This returns the file descriptor used by the underlying
      :func:`select.epoll` object.

.. class:: DevpollSelector()

   :func:`select.devpoll`-based selector.

   .. method:: fileno()

      This returns the file descriptor used by the underlying
      :func:`select.devpoll` object.

   .. versionadded:: 3.5

.. class:: KqueueSelector()

   :func:`select.kqueue`-based selector.

   .. method:: fileno()

      This returns the file descriptor used by the underlying
      :func:`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)