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ReStructuredText
278 lines
8.8 KiB
ReStructuredText
:mod:`!selectors` --- High-level I/O multiplexing
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=================================================
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.. module:: selectors
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:synopsis: High-level I/O multiplexing.
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.. versionadded:: 3.4
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**Source code:** :source:`Lib/selectors.py`
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--------------
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Introduction
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------------
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This module allows high-level and efficient I/O multiplexing, built upon the
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:mod:`select` module primitives. Users are encouraged to use this module
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instead, unless they want precise control over the OS-level primitives used.
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It defines a :class:`BaseSelector` abstract base class, along with several
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concrete implementations (:class:`KqueueSelector`, :class:`EpollSelector`...),
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that can be used to wait for I/O readiness notification on multiple file
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objects. In the following, "file object" refers to any object with a
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:meth:`~io.IOBase.fileno` method, or a raw file descriptor. See :term:`file object`.
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:class:`DefaultSelector` is an alias to the most efficient implementation
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available on the current platform: this should be the default choice for most
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users.
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.. note::
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The type of file objects supported depends on the platform: on Windows,
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sockets are supported, but not pipes, whereas on Unix, both are supported
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(some other types may be supported as well, such as fifos or special file
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devices).
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.. seealso::
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:mod:`select`
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Low-level I/O multiplexing module.
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.. include:: ../includes/wasm-notavail.rst
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Classes
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-------
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Classes hierarchy::
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BaseSelector
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+-- SelectSelector
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+-- PollSelector
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+-- EpollSelector
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+-- DevpollSelector
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+-- KqueueSelector
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In the following, *events* is a bitwise mask indicating which I/O events should
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be waited for on a given file object. It can be a combination of the modules
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constants below:
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+-----------------------+-----------------------------------------------+
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| Constant | Meaning |
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+=======================+===============================================+
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| .. data:: EVENT_READ | Available for read |
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+-----------------------+-----------------------------------------------+
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| .. data:: EVENT_WRITE | Available for write |
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+-----------------------+-----------------------------------------------+
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.. class:: SelectorKey
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A :class:`SelectorKey` is a :class:`~collections.namedtuple` used to
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associate a file object to its underlying file descriptor, selected event
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mask and attached data. It is returned by several :class:`BaseSelector`
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methods.
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.. attribute:: fileobj
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File object registered.
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.. attribute:: fd
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Underlying file descriptor.
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.. attribute:: events
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Events that must be waited for on this file object.
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.. attribute:: data
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Optional opaque data associated to this file object: for example, this
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could be used to store a per-client session ID.
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.. class:: BaseSelector
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A :class:`BaseSelector` is used to wait for I/O event readiness on multiple
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file objects. It supports file stream registration, unregistration, and a
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method to wait for I/O events on those streams, with an optional timeout.
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It's an abstract base class, so cannot be instantiated. Use
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:class:`DefaultSelector` instead, or one of :class:`SelectSelector`,
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:class:`KqueueSelector` etc. if you want to specifically use an
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implementation, and your platform supports it.
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:class:`BaseSelector` and its concrete implementations support the
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:term:`context manager` protocol.
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.. abstractmethod:: register(fileobj, events, data=None)
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Register a file object for selection, monitoring it for I/O events.
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*fileobj* is the file object to monitor. It may either be an integer
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file descriptor or an object with a ``fileno()`` method.
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*events* is a bitwise mask of events to monitor.
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*data* is an opaque object.
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This returns a new :class:`SelectorKey` instance, or raises a
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:exc:`ValueError` in case of invalid event mask or file descriptor, or
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:exc:`KeyError` if the file object is already registered.
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.. abstractmethod:: unregister(fileobj)
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Unregister a file object from selection, removing it from monitoring. A
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file object shall be unregistered prior to being closed.
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*fileobj* must be a file object previously registered.
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This returns the associated :class:`SelectorKey` instance, or raises a
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:exc:`KeyError` if *fileobj* is not registered. It will raise
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:exc:`ValueError` if *fileobj* is invalid (e.g. it has no ``fileno()``
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method or its ``fileno()`` method has an invalid return value).
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.. method:: modify(fileobj, events, data=None)
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Change a registered file object's monitored events or attached data.
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This is equivalent to ``BaseSelector.unregister(fileobj)`` followed
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by ``BaseSelector.register(fileobj, events, data)``, except that it
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can be implemented more efficiently.
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This returns a new :class:`SelectorKey` instance, or raises a
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:exc:`ValueError` in case of invalid event mask or file descriptor, or
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:exc:`KeyError` if the file object is not registered.
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.. abstractmethod:: select(timeout=None)
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Wait until some registered file objects become ready, or the timeout
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expires.
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If ``timeout > 0``, this specifies the maximum wait time, in seconds.
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If ``timeout <= 0``, the call won't block, and will report the currently
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ready file objects.
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If *timeout* is ``None``, the call will block until a monitored file object
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becomes ready.
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This returns a list of ``(key, events)`` tuples, one for each ready file
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object.
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*key* is the :class:`SelectorKey` instance corresponding to a ready file
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object.
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*events* is a bitmask of events ready on this file object.
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.. note::
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This method can return before any file object becomes ready or the
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timeout has elapsed if the current process receives a signal: in this
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case, an empty list will be returned.
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.. versionchanged:: 3.5
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The selector is now retried with a recomputed timeout when interrupted
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by a signal if the signal handler did not raise an exception (see
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:pep:`475` for the rationale), instead of returning an empty list
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of events before the timeout.
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.. method:: close()
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Close the selector.
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This must be called to make sure that any underlying resource is freed.
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The selector shall not be used once it has been closed.
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.. method:: get_key(fileobj)
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Return the key associated with a registered file object.
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This returns the :class:`SelectorKey` instance associated to this file
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object, or raises :exc:`KeyError` if the file object is not registered.
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.. abstractmethod:: get_map()
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Return a mapping of file objects to selector keys.
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This returns a :class:`~collections.abc.Mapping` instance mapping
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registered file objects to their associated :class:`SelectorKey`
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instance.
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.. class:: DefaultSelector()
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The default selector class, using the most efficient implementation
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available on the current platform. This should be the default choice for
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most users.
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.. class:: SelectSelector()
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:func:`select.select`-based selector.
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.. class:: PollSelector()
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:func:`select.poll`-based selector.
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.. class:: EpollSelector()
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:func:`select.epoll`-based selector.
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.. method:: fileno()
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This returns the file descriptor used by the underlying
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:func:`select.epoll` object.
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.. class:: DevpollSelector()
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:func:`select.devpoll`-based selector.
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.. method:: fileno()
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This returns the file descriptor used by the underlying
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:func:`select.devpoll` object.
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.. versionadded:: 3.5
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.. class:: KqueueSelector()
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:func:`select.kqueue`-based selector.
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.. method:: fileno()
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This returns the file descriptor used by the underlying
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:func:`select.kqueue` object.
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Examples
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--------
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Here is a simple echo server implementation::
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import selectors
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import socket
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sel = selectors.DefaultSelector()
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def accept(sock, mask):
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conn, addr = sock.accept() # Should be ready
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print('accepted', conn, 'from', addr)
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conn.setblocking(False)
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sel.register(conn, selectors.EVENT_READ, read)
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def read(conn, mask):
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data = conn.recv(1000) # Should be ready
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if data:
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print('echoing', repr(data), 'to', conn)
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conn.send(data) # Hope it won't block
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else:
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print('closing', conn)
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sel.unregister(conn)
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conn.close()
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sock = socket.socket()
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sock.bind(('localhost', 1234))
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sock.listen(100)
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sock.setblocking(False)
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sel.register(sock, selectors.EVENT_READ, accept)
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while True:
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events = sel.select()
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for key, mask in events:
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callback = key.data
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callback(key.fileobj, mask)
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