: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.
.. include:: ../includes/wasm-notavail.rst
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 |
+=======================+===============================================+
| .. data:: EVENT_READ | Available for read |
+-----------------------+-----------------------------------------------+
| .. data:: 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 descriptor, 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 ``BaseSelector.unregister(fileobj)`` followed
by ``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)