Doc/library/asyncio-eventloop.rst

.. currentmodule:: asyncio


==========
Event Loop
==========


.. rubric:: Preface

The event loop is a central component of every asyncio application.
Event loops run asynchronous tasks and callbacks, perform network
IO operations, and run subprocesses.

Application developers will typically use high-level asyncio functions
to interact with the event loop. In general, high-level asyncio applications
should not need to work directly with event loops and, instead, should use
the :func:`asyncio.run` function to initialize, manage the event loop, and
run asynchronous code.

Alternatively, developers of low-level code, such as libraries and
framework, may need access to the event loop.

.. rubric:: Accessing Event Loop

The following low-level functions can be used to get, set, or create
an event loop:

.. function:: get_running_loop()

   Return the running event loop in the current OS thread.

   If there is no running event loop a :exc:`RuntimeError` is raised.
   This function can only be called from a coroutine or a callback.

   .. versionadded:: 3.7

.. function:: get_event_loop()

   Get the current event loop.  If there is no current event loop set
   in the current OS thread and :func:`set_event_loop` has not yet
   been called, asyncio will create a new event loop and set it as the
   current one.

   Because this function has rather complex behavior (especially
   when custom event loop policies are in use), using the
   :func:`get_running_loop` function is preferred to :func:`get_event_loop`
   in coroutines and callbacks.

   Consider also using the :func:`asyncio.run` function instead of using
   lower level functions to manually create and close an event loop.

.. function:: set_event_loop(loop)

   Set *loop* as a current event loop for the current OS thread.

.. function:: new_event_loop()

   Create a new event loop object.

Note that the behaviour of :func:`get_event_loop`, :func:`set_event_loop`,
and :func:`new_event_loop` functions can be altered by
:ref:`setting a custom event loop policy <asyncio-policies>`.


.. rubric:: Contents

This documentation page contains the following sections:

* The `Event Loop Methods`_ section is the reference documentation of
  event loop APIs;

* The `Callback Handles`_ section documents the :class:`Handle` and
  :class:`TimerHandle`, instances which are returned from functions, such
  as :meth:`loop.call_soon` and :meth:`loop.call_later`;

* The `Server Objects`_ sections document types returned from
  event loop methods like :meth:`loop.create_server`;

* The `Event Loops Implementations`_ section documents the
  :class:`SelectorEventLoop` and :class:`ProactorEventLoop` classes;

* The `Examples`_ section showcases how to work with some event
  loop APIs.


.. _asyncio-event-loop:

Event Loop Methods
==================

Event loops have **low-level** APIs for the following:

.. contents::
   :depth: 1
   :local:


Running and stopping the loop
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. method:: loop.run_until_complete(future)

   Run until the *future* (an instance of :class:`Future`) is
   completed.

   If the argument is a :ref:`coroutine object <coroutine>` it
   is implicitly wrapped into an :class:`asyncio.Task`.

   Return the Future's result or raise its exception.

.. method:: loop.run_forever()

   Run the event loop until :meth:`stop` is called.

   If :meth:`stop` is called before :meth:`run_forever()` is called,
   the loop will poll the I/O selector once with a timeout of zero,
   run all callbacks scheduled in response to I/O events (and
   those that were already scheduled), and then exit.

   If :meth:`stop` is called while :meth:`run_forever` is running,
   the loop will run the current batch of callbacks and then exit.
   Note that callbacks scheduled by callbacks will not run in this
   case; instead, they will run the next time :meth:`run_forever` or
   :meth:`run_until_complete` is called.

.. method:: loop.stop()

   Stop the event loop.

.. method:: loop.is_running()

   Return ``True`` if the event loop is currently running.

.. method:: loop.is_closed()

   Return ``True`` if the event loop was closed.

.. method:: loop.close()

   Close the event loop.

   The loop must be running when this function is called.
   Any pending callbacks will be discarded.

   This method clears all queues and shuts down the executor, but does
   not wait for the executor to finish.

   This method is idempotent and irreversible.  No other methods
   should be called after the event loop is closed.

.. coroutinemethod:: loop.shutdown_asyncgens()

   Schedule all currently open :term:`asynchronous generator` objects to
   close with an :meth:`~agen.aclose()` call.  After calling this method,
   the event loop will issue a warning if a new asynchronous generator
   is iterated. This should be used to reliably finalize all scheduled
   asynchronous generators, e.g.::


    try:
        loop.run_forever()
    finally:
        loop.run_until_complete(loop.shutdown_asyncgens())
        loop.close()

   .. versionadded:: 3.6


.. _asyncio-pass-keywords:

Scheduling callbacks
^^^^^^^^^^^^^^^^^^^^

.. method:: loop.call_soon(callback, *args, context=None)

   Schedule a *callback* to be called with *args* arguments at
   the next iteration of the event loop.

   Callbacks are called in the order in which they are registered.
   Each callback will be called exactly once.

   An optional keyword-only *context* argument allows specifying a
   custom :class:`contextvars.Context` for the *callback* to run in.
   The current context is used when no *context* is provided.

   An instance of :class:`asyncio.Handle` is returned, which can be
   used later to cancel the callback.

   This method is not thread-safe.

.. method:: loop.call_soon_threadsafe(callback, *args, context=None)

   A thread-safe variant of :meth:`call_soon`.  Must be used to
   schedule callbacks *from another thread*.

   See the :ref:`concurrency and multithreading <asyncio-multithreading>`
   section of the documentation.

.. versionchanged:: 3.7
   The *context* keyword-only parameter was added. See :pep:`567`
   for more details.

.. note::

   Most :mod:`asyncio` scheduling functions don't allow passing
   keyword arguments.  To do that, use :func:`functools.partial`,
   e.g.::

      # will schedule "print("Hello", flush=True)"
      loop.call_soon(
          functools.partial(print, "Hello", flush=True))

   Using partial objects is usually more convenient than using lambdas,
   as asyncio can better render partial objects in debug and error
   messages.


.. _asyncio-delayed-calls:

Scheduling delayed callbacks
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Event loop provides mechanisms to schedule callback functions
to be called at some point in the future.  Event loop uses monotonic
clocks to track time.


.. method:: loop.call_later(delay, callback, *args, context=None)

   Schedule *callback* to be called after the given *delay*
   number of seconds (can be either an int or a float).

   An instance of :class:`asyncio.TimerHandle` is returned which can
   be used to cancel the callback.

   *callback* will be called exactly once.  If two callbacks are
   scheduled for exactly the same time, it is undefined which one will
   be called first.

   The optional positional *args* will be passed to the callback when
   it is called. If you want the callback to be called with keyword
   arguments use :func:`functools.partial`.

   An optional keyword-only *context* argument allows specifying a
   custom :class:`contextvars.Context` for the *callback* to run in.
   The current context is used when no *context* is provided.

   .. versionchanged:: 3.7
      The *context* keyword-only parameter was added. See :pep:`567`
      for more details.

.. method:: loop.call_at(when, callback, *args, context=None)

   Schedule *callback* to be called at the given absolute timestamp
   *when* (an int or a float), using the same time reference as
   :meth:`loop.time`.

   This method's behavior is the same as :meth:`call_later`.

   An instance of :class:`asyncio.TimerHandle` is returned which can
   be used to cancel the callback.

   .. versionchanged:: 3.7
      The *context* keyword-only parameter was added. See :pep:`567`
      for more details.

.. method:: loop.time()

   Return the current time, as a :class:`float` value, according to
   the event loop's internal monotonic clock.

.. note::

   Timeouts (relative *delay* or absolute *when*) should not
   exceed one day.

.. seealso::

   The :func:`asyncio.sleep` function.


Creating Futures and Tasks
^^^^^^^^^^^^^^^^^^^^^^^^^^

.. method:: loop.create_future()

   Create an :class:`asyncio.Future` object attached to the event loop.

   This is the preferred way to create Futures in asyncio. This lets
   third-party event loops provide alternative implementations of
   the Future object (with better performance or instrumentation).

   .. versionadded:: 3.5.2

.. method:: loop.create_task(coro, \*, name=None)

   Schedule the execution of a :ref:`coroutine`.
   Return a :class:`Task` object.

   Third-party event loops can use their own subclass of :class:`Task`
   for interoperability. In this case, the result type is a subclass
   of :class:`Task`.

   If the *name* argument is provided and not ``None``, it is set as
   the name of the task using :meth:`Task.set_name`.

   .. versionchanged:: 3.8
      Added the ``name`` parameter.

.. method:: loop.set_task_factory(factory)

   Set a task factory that will be used by
   :meth:`loop.create_task`.

   If *factory* is ``None`` the default task factory will be set.

   If *factory* is a *callable*, it should have a signature matching
   ``(loop, coro)``, where *loop* will be a reference to the active
   event loop, *coro* will be a coroutine object.  The callable
   must return an :class:`asyncio.Future` compatible object.

.. method:: loop.get_task_factory()

   Return a task factory or ``None`` if the default one is in use.


Opening network connections
^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. coroutinemethod:: loop.create_connection(protocol_factory, \
                          host=None, port=None, \*, ssl=None, \
                          family=0, proto=0, flags=0, sock=None, \
                          local_addr=None, server_hostname=None, \
                          ssl_handshake_timeout=None)

   Open a streaming transport connection to a given
   address specified by *host* and *port*.

   The socket family can be either :py:data:`~socket.AF_INET` or
   :py:data:`~socket.AF_INET6` depending on *host* (or the *family*
   argument, if provided).

   The socket type will be :py:data:`~socket.SOCK_STREAM`.

   *protocol_factory* must be a callable returning an
   :ref:`asyncio protocol <asyncio-protocol>` implementation.

   This method will try to establish the connection in the background.
   When successful, it returns a ``(transport, protocol)`` pair.

   The chronological synopsis of the underlying operation is as follows:

   #. The connection is established and a :ref:`transport <asyncio-transport>`
      is created for it.

   #. *protocol_factory* is called without arguments and is expected to
      return a :ref:`protocol <asyncio-protocol>` instance.

   #. The protocol instance is coupled with the transport by calling its
      :meth:`~BaseProtocol.connection_made` method.

   #. A ``(transport, protocol)`` tuple is returned on success.

   The created transport is an implementation-dependent bidirectional
   stream.

   .. note::
      *protocol_factory* can be any kind of callable, not necessarily
      a class.  For example, if you want to use a pre-created
      protocol instance, you can pass ``lambda: my_protocol``.

   Other arguments:

   * *ssl*: if given and not false, an SSL/TLS transport is created
     (by default a plain TCP transport is created).  If *ssl* is
     a :class:`ssl.SSLContext` object, this context is used to create
     the transport; if *ssl* is :const:`True`, a context with some
     unspecified default settings is used.

     .. seealso:: :ref:`SSL/TLS security considerations <ssl-security>`

   * *server_hostname*, is only for use together with *ssl*,
     and sets or overrides the hostname that the target server's certificate
     will be matched against.  By default the value of the *host* argument
     is used.  If *host* is empty, there is no default and you must pass a
     value for *server_hostname*.  If *server_hostname* is an empty
     string, hostname matching is disabled (which is a serious security
     risk, allowing for man-in-the-middle-attacks).

   * *family*, *proto*, *flags* are the optional address family, protocol
     and flags to be passed through to getaddrinfo() for *host* resolution.
     If given, these should all be integers from the corresponding
     :mod:`socket` module constants.

   * *sock*, if given, should be an existing, already connected
     :class:`socket.socket` object to be used by the transport.
     If *sock* is given, none of *host*, *port*, *family*, *proto*, *flags*
     and *local_addr* should be specified.

   * *local_addr*, if given, is a ``(local_host, local_port)`` tuple used
     to bind the socket to locally.  The *local_host* and *local_port*
     are looked up using ``getaddrinfo()``, similarly to *host* and *port*.

   * *ssl_handshake_timeout* is (for an SSL connection) the time in seconds
     to wait for the SSL handshake to complete before aborting the connection.
     ``60.0`` seconds if ``None`` (default).

   .. versionadded:: 3.7

      The *ssl_handshake_timeout* parameter.

   .. versionchanged:: 3.6

      The socket option :py:data:`~socket.TCP_NODELAY` is set by default
      for all TCP connections.

   .. versionchanged:: 3.5

      Added support for SSL/TLS for :class:`ProactorEventLoop`.

   .. seealso::

      The :func:`open_connection` function is a high-level alternative
      API.  It returns a pair of (:class:`StreamReader`, :class:`StreamWriter`)
      that can be used directly in async/await code.

.. coroutinemethod:: loop.create_datagram_endpoint(protocol_factory, \
                        local_addr=None, remote_addr=None, \*, \
                        family=0, proto=0, flags=0, \
                        reuse_address=None, reuse_port=None, \
                        allow_broadcast=None, sock=None)

   Create a datagram connection.

   The socket family can be either :py:data:`~socket.AF_INET`,
   :py:data:`~socket.AF_INET6`, or :py:data:`~socket.AF_UNIX`,
   depending on *host* (or the *family* argument, if provided).

   The socket type will be :py:data:`~socket.SOCK_DGRAM`.

   *protocol_factory* must be a callable returning a
   :ref:`protocol <asyncio-protocol>` implementation.

   A tuple of ``(transport, protocol)`` is returned on success.

   Other arguments:

   * *local_addr*, if given, is a ``(local_host, local_port)`` tuple used
     to bind the socket to locally.  The *local_host* and *local_port*
     are looked up using :meth:`getaddrinfo`.

   * *remote_addr*, if given, is a ``(remote_host, remote_port)`` tuple used
     to connect the socket to a remote address.  The *remote_host* and
     *remote_port* are looked up using :meth:`getaddrinfo`.

   * *family*, *proto*, *flags* are the optional address family, protocol
     and flags to be passed through to :meth:`getaddrinfo` for *host*
     resolution. If given, these should all be integers from the
     corresponding :mod:`socket` module constants.

   * *reuse_address* tells the kernel to reuse a local socket in
     ``TIME_WAIT`` state, without waiting for its natural timeout to
     expire. If not specified will automatically be set to ``True`` on
     UNIX.

   * *reuse_port* tells the kernel to allow this endpoint to be bound to the
     same port as other existing endpoints are bound to, so long as they all
     set this flag when being created. This option is not supported on Windows
     and some UNIX's. If the :py:data:`~socket.SO_REUSEPORT` constant is not
     defined then this capability is unsupported.

   * *allow_broadcast* tells the kernel to allow this endpoint to send
     messages to the broadcast address.

   * *sock* can optionally be specified in order to use a preexisting,
     already connected, :class:`socket.socket` object to be used by the
     transport. If specified, *local_addr* and *remote_addr* should be omitted
     (must be :const:`None`).

   On Windows with :class:`ProactorEventLoop`, this method is not supported.

   See :ref:`UDP echo client protocol <asyncio-udp-echo-client-protocol>` and
   :ref:`UDP echo server protocol <asyncio-udp-echo-server-protocol>` examples.

   .. versionchanged:: 3.4.4
      The *family*, *proto*, *flags*, *reuse_address*, *reuse_port,
      *allow_broadcast*, and *sock* parameters were added.

.. coroutinemethod:: loop.create_unix_connection(protocol_factory, \
                        path=None, \*, ssl=None, sock=None, \
                        server_hostname=None, ssl_handshake_timeout=None)

   Create UNIX connection.

   The socket family will be :py:data:`~socket.AF_UNIX`; socket
   type will be :py:data:`~socket.SOCK_STREAM`.

   A tuple of ``(transport, protocol)`` is returned on success.

   *path* is the name of a UNIX domain socket and is required,
   unless a *sock* parameter is specified.  Abstract UNIX sockets,
   :class:`str`, :class:`bytes`, and :class:`~pathlib.Path` paths are
   supported.

   See the documentation of the :meth:`loop.create_connection` method
   for information about arguments to this method.

   Availability: UNIX.

   .. versionadded:: 3.7

      The *ssl_handshake_timeout* parameter.

   .. versionchanged:: 3.7

      The *path* parameter can now be a :term:`path-like object`.


Creating network servers
^^^^^^^^^^^^^^^^^^^^^^^^

.. coroutinemethod:: loop.create_server(protocol_factory, \
                        host=None, port=None, \*, \
                        family=socket.AF_UNSPEC, \
                        flags=socket.AI_PASSIVE, \
                        sock=None, backlog=100, ssl=None, \
                        reuse_address=None, reuse_port=None, \
                        ssl_handshake_timeout=None, start_serving=True)

   Create a TCP server (socket type :data:`~socket.SOCK_STREAM`) listening
   on the *host* and *port* address.

   Returns a :class:`Server` object.

   Arguments:

   * The *host* parameter can be set to several types which determine behavior:
       - If *host* is a string, the TCP server is bound to *host* and *port*.
       - if *host* is a sequence of strings, the TCP server is bound to all
         hosts of the sequence.
       - If *host* is an empty string or ``None``, all interfaces are
         assumed and a list of multiple sockets will be returned (most likely
         one for IPv4 and another one for IPv6).

   * *family* can be set to either :data:`socket.AF_INET` or
     :data:`~socket.AF_INET6` to force the socket to use IPv4 or IPv6.
     If not set, the *family* will be determined from host name
     (defaults to :data:`~socket.AF_UNSPEC`).

   * *flags* is a bitmask for :meth:`getaddrinfo`.

   * *sock* can optionally be specified in order to use a preexisting
     socket object. If specified, *host* and *port* should be omitted (must be
     :const:`None`).

   * *backlog* is the maximum number of queued connections passed to
     :meth:`~socket.socket.listen` (defaults to 100).

   * *ssl* can be set to an :class:`~ssl.SSLContext` to enable SSL over the
     accepted connections.

   * *reuse_address* tells the kernel to reuse a local socket in
     ``TIME_WAIT`` state, without waiting for its natural timeout to
     expire. If not specified will automatically be set to ``True`` on
     UNIX.

   * *reuse_port* tells the kernel to allow this endpoint to be bound to the
     same port as other existing endpoints are bound to, so long as they all
     set this flag when being created. This option is not supported on
     Windows.

   * *ssl_handshake_timeout* is (for an SSL server) the time in seconds to wait
     for the SSL handshake to complete before aborting the connection.
     ``60.0`` seconds if ``None`` (default).

   * *start_serving* set to ``True`` (the default) causes the created server
     to start accepting connections immediately.  When set to ``False``,
     the user should await on :meth:`Server.start_serving` or
     :meth:`Server.serve_forever` to make the server to start accepting
     connections.

   .. versionadded:: 3.7

      Added *ssl_handshake_timeout* and *start_serving* parameters.

   .. versionchanged:: 3.6

      The socket option :py:data:`~socket.TCP_NODELAY` is set by default
      for all TCP connections.

   .. versionchanged:: 3.5

      Added support for SSL/TLS on Windows with
      :class:`ProactorEventLoop`.

   .. versionchanged:: 3.5.1

      The *host* parameter can be a sequence of strings.

   .. seealso::

      The :func:`start_server` function is a higher-level alternative API
      that returns a pair of :class:`StreamReader` and :class:`StreamWriter`
      that can be used in an async/await code.


.. coroutinemethod:: loop.create_unix_server(protocol_factory, path=None, \
                          \*, sock=None, backlog=100, ssl=None, \
                          ssl_handshake_timeout=None, start_serving=True)

   Similar to :meth:`loop.create_server` but works with the
   :py:data:`~socket.AF_UNIX` socket family.

   *path* is the name of a UNIX domain socket, and is required,
   unless a *sock* argument is provided.  Abstract UNIX sockets,
   :class:`str`, :class:`bytes`, and :class:`~pathlib.Path` paths
   are supported.

   Availability: UNIX.

   .. versionadded:: 3.7

      The *ssl_handshake_timeout* and *start_serving* parameters.

   .. versionchanged:: 3.7

      The *path* parameter can now be a :class:`~pathlib.Path` object.

.. coroutinemethod:: loop.connect_accepted_socket(protocol_factory, \
                        sock, \*, ssl=None, ssl_handshake_timeout=None)

   Wrap an already accepted connection into a transport/protocol pair.

   This method can be used by servers that accept connections outside
   of asyncio but that use asyncio to handle them.

   Parameters:

   * *sock* is a preexisting socket object returned from
     :meth:`socket.accept <socket.socket.accept>`.

   * *ssl* can be set to an :class:`~ssl.SSLContext` to enable SSL over
     the accepted connections.

   * *ssl_handshake_timeout* is (for an SSL connection) the time in seconds to
     wait for the SSL handshake to complete before aborting the connection.
     ``60.0`` seconds if ``None`` (default).

   Returns a ``(transport, protocol)`` pair.

   .. versionadded:: 3.7

      The *ssl_handshake_timeout* parameter.

   .. versionadded:: 3.5.3


Transferring files
^^^^^^^^^^^^^^^^^^

.. coroutinemethod:: loop.sendfile(transport, file, \
                                   offset=0, count=None, *, fallback=True)

   Send a *file* over a *transport*.  Return the total number of bytes
   sent.

   The method uses high-performance :meth:`os.sendfile` if available.

   *file* must be a regular file object opened in binary mode.

   *offset* tells from where to start reading the file. If specified,
   *count* is the total number of bytes to transmit as opposed to
   sending the file until EOF is reached. File position is updated on
   return or also in case of error in which case :meth:`file.tell()
   <io.IOBase.tell>` can be used to figure out the number of bytes
   which were sent.

   *fallback* set to ``True`` makes asyncio to manually read and send
   the file when the platform does not support the sendfile system call
   (e.g. Windows or SSL socket on Unix).

   Raise :exc:`SendfileNotAvailableError` if the system does not support
   *sendfile* syscall and *fallback* is ``False``.

   .. versionadded:: 3.7


TLS Upgrade
^^^^^^^^^^^

.. coroutinemethod:: loop.start_tls(transport, protocol, \
                        sslcontext, \*, server_side=False, \
                        server_hostname=None, ssl_handshake_timeout=None)

   Upgrade an existing transport-based connection to TLS.

   Return a new transport instance, that the *protocol* must start using
   immediately after the *await*.  The *transport* instance passed to
   the *start_tls* method should never be used again.

   Parameters:

   * *transport* and *protocol* instances that methods like
     :meth:`~loop.create_server` and
     :meth:`~loop.create_connection` return.

   * *sslcontext*: a configured instance of :class:`~ssl.SSLContext`.

   * *server_side* pass ``True`` when a server-side connection is being
     upgraded (like the one created by :meth:`~loop.create_server`).

   * *server_hostname*: sets or overrides the host name that the target
     server's certificate will be matched against.

   * *ssl_handshake_timeout* is (for an SSL connection) the time in seconds to
     wait for the SSL handshake to complete before aborting the connection.
     ``60.0`` seconds if ``None`` (default).

   .. versionadded:: 3.7


Watching file descriptors
^^^^^^^^^^^^^^^^^^^^^^^^^

.. method:: loop.add_reader(fd, callback, \*args)

   Start watching the file descriptor *fd* for read availability and
   call the *callback* with specified arguments.

.. method:: loop.remove_reader(fd)

   Stop watching the file descriptor *fd* for read availability.

.. method:: loop.add_writer(fd, callback, \*args)

   Start watching the file descriptor *fd* for write availability and then
   call the *callback* with specified arguments.

   Use :func:`functools.partial` :ref:`to pass keywords
   <asyncio-pass-keywords>` to *func*.

.. method:: loop.remove_writer(fd)

   Stop watching the file descriptor *fd* for write availability.

See also :ref:`Platform Support <asyncio-platform-support>` section
for some limitations of these methods.


Working with socket objects directly
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

In general, protocol implementations that use transport-based APIs
such as :meth:`loop.create_connection` and :meth:`loop.create_server`
are faster than implementations that work with sockets directly.
However, there are some use cases when performance is not critical, and
working with :class:`~socket.socket` objects directly is more
convenient.

.. coroutinemethod:: loop.sock_recv(sock, nbytes)

   Receive data.  Asynchronous version of
   :meth:`socket.recv() <socket.socket.recv>`.

   The received data is returned as a bytes object.  The maximum amount
   of data to be received is specified by the *nbytes* argument.

   The socket *sock* must be non-blocking.

   .. versionchanged:: 3.7
      Even though this method was always documented as a coroutine
      method, releases before Python 3.7 returned a :class:`Future`.
      Since Python 3.7 this is an ``async def`` method.

.. coroutinemethod:: loop.sock_recv_into(sock, buf)

   Receive data into a buffer.  Modeled after the blocking
   :meth:`socket.recv_into() <socket.socket.recv_into>` method.

   Return the number of bytes written to the buffer.

   The socket *sock* must be non-blocking.

   .. versionadded:: 3.7

.. coroutinemethod:: loop.sock_sendall(sock, data)

   Send data to the socket. Asynchronous version of
   :meth:`socket.sendall() <socket.socket.sendall>`.

   This method continues to send data from *data* to the socket until either
   all data in *data* has been sent or an error occurs.  ``None`` is returned
   on success.  On error, an exception is raised. Additionally, there is no way
   to determine how much data, if any, was successfully processed by the
   receiving end of the connection.

   The socket *sock* must be non-blocking.

   .. versionchanged:: 3.7
      Even though the method was always documented as a coroutine
      method, before Python 3.7 it returned an :class:`Future`.
      Since Python 3.7, this is an ``async def`` method.

.. coroutinemethod:: loop.sock_connect(sock, address)

   Connect to a remote socket at *address*.

   Asynchronous version of :meth:`socket.connect() <socket.socket.connect>`.

   The socket *sock* must be non-blocking.

   .. versionchanged:: 3.5.2
      ``address`` no longer needs to be resolved.  ``sock_connect``
      will try to check if the *address* is already resolved by calling
      :func:`socket.inet_pton`.  If not,
      :meth:`loop.getaddrinfo` will be used to resolve the
      *address*.

   .. seealso::

      :meth:`loop.create_connection`
      and  :func:`asyncio.open_connection() <open_connection>`.


.. coroutinemethod:: loop.sock_accept(sock)

   Accept a connection.  Modeled after the blocking
   :meth:`socket.accept() <socket.socket.accept>` method.

   The socket must be bound to an address and listening
   for connections. The return value is a pair ``(conn, address)`` where *conn*
   is a *new* socket object usable to send and receive data on the connection,
   and *address* is the address bound to the socket on the other end of the
   connection.

   The socket *sock* must be non-blocking.

   .. versionchanged:: 3.7
      Even though the method was always documented as a coroutine
      method, before Python 3.7 it returned a :class:`Future`.
      Since Python 3.7, this is an ``async def`` method.

   .. seealso::

      :meth:`loop.create_server` and :func:`start_server`.

.. coroutinemethod:: loop.sock_sendfile(sock, file, offset=0, count=None, \
                                        \*, fallback=True)

   Send a file using high-performance :mod:`os.sendfile` if possible.
   Return the total number of bytes which were sent.

   Asynchronous version of :meth:`socket.sendfile() <socket.socket.sendfile>`.

   *sock* must be non-blocking :class:`~socket.socket` of
   :const:`socket.SOCK_STREAM` type.

   *file* must be a regular file object opened in binary mode.

   *offset* tells from where to start reading the file. If specified,
   *count* is the total number of bytes to transmit as opposed to
   sending the file until EOF is reached. File position is updated on
   return or also in case of error in which case :meth:`file.tell()
   <io.IOBase.tell>` can be used to figure out the number of bytes
   which were sent.

   *fallback*, when set to ``True``, makes asyncio manually read and send
   the file when the platform does not support the sendfile syscall
   (e.g. Windows or SSL socket on Unix).

   Raise :exc:`SendfileNotAvailableError` if the system does not support
   *sendfile* syscall and *fallback* is ``False``.

   The socket *sock* must be non-blocking.

   .. versionadded:: 3.7


DNS
^^^

.. coroutinemethod:: loop.getaddrinfo(host, port, \*, family=0, \
                        type=0, proto=0, flags=0)

   Asynchronous version of :meth:`socket.getaddrinfo`.

.. coroutinemethod:: loop.getnameinfo(sockaddr, flags=0)

   Asynchronous version of :meth:`socket.getnameinfo`.

.. versionchanged:: 3.7
   Both *getaddrinfo* and *getnameinfo* methods were always documented
   to return a coroutine, but prior to Python 3.7 they were, in fact,
   returning :class:`asyncio.Future` objects.  Starting with Python 3.7
   both methods are coroutines.


Working with pipes
^^^^^^^^^^^^^^^^^^

.. coroutinemethod:: loop.connect_read_pipe(protocol_factory, pipe)

   Register a read-pipe in the event loop.

   *protocol_factory* must be a callable returning an
   :ref:`asyncio protocol <asyncio-protocol>` implementation.

   *pipe* is a :term:`file-like object <file object>`.

   Return pair ``(transport, protocol)``, where *transport* supports
   the :class:`ReadTransport` interface.

   With :class:`SelectorEventLoop` event loop, the *pipe* is set to
   non-blocking mode.

.. coroutinemethod:: loop.connect_write_pipe(protocol_factory, pipe)

   Register a write-pipe in the event loop.

   *protocol_factory* must be a callable returning an
   :ref:`asyncio protocol <asyncio-protocol>` implementation.

   *pipe* is :term:`file-like object <file object>`.

   Return pair ``(transport, protocol)``, where *transport* supports
   :class:`WriteTransport` interface.

   With :class:`SelectorEventLoop` event loop, the *pipe* is set to
   non-blocking mode.

.. note::

   :class:`SelectorEventLoop` does not support the above methods on
   Windows.  Use :class:`ProactorEventLoop` instead for Windows.

.. seealso::

   The :meth:`loop.subprocess_exec` and
   :meth:`loop.subprocess_shell` methods.


UNIX signals
^^^^^^^^^^^^

.. method:: loop.add_signal_handler(signum, callback, \*args)

   Add a handler for a signal.

   Raise :exc:`ValueError` if the signal number is invalid or uncatchable.
   Raise :exc:`RuntimeError` if there is a problem setting up the handler.

   Use :func:`functools.partial` :ref:`to pass keywords
   <asyncio-pass-keywords>` to *func*.

.. method:: loop.remove_signal_handler(sig)

   Remove a handler for a signal.

   Return ``True`` if a signal handler was removed, ``False`` if not.

Availability: UNIX.

.. seealso::

   The :mod:`signal` module.


Executing code in thread or process pools
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. method:: loop.run_in_executor(executor, func, \*args)

   Arrange for a *func* to be called in the specified executor.

   The *executor* argument should be an :class:`concurrent.futures.Executor`
   instance. The default executor is used if *executor* is ``None``.

   Use :func:`functools.partial` :ref:`to pass keywords
   <asyncio-pass-keywords>` to *func*.

   This method returns a :class:`asyncio.Future` object.

   .. versionchanged:: 3.5.3
      :meth:`loop.run_in_executor` no longer configures the
      ``max_workers`` of the thread pool executor it creates, instead
      leaving it up to the thread pool executor
      (:class:`~concurrent.futures.ThreadPoolExecutor`) to set the
      default.

.. method:: loop.set_default_executor(executor)

   Set *executor* as the default executor used by :meth:`run_in_executor`.
   *executor* should be an instance of
   :class:`~concurrent.futures.ThreadPoolExecutor`.

   .. deprecated:: 3.8
      Using an executor that is not an instance of
      :class:`~concurrent.futures.ThreadPoolExecutor` is deprecated and
      will trigger an error in Python 3.9.

   *executor* must be an instance of
   :class:`concurrent.futures.ThreadPoolExecutor`.


Error Handling API
^^^^^^^^^^^^^^^^^^

Allows customizing how exceptions are handled in the event loop.

.. method:: loop.set_exception_handler(handler)

   Set *handler* as the new event loop exception handler.

   If *handler* is ``None``, the default exception handler will
   be set.

   If *handler* is a callable object, it should have a
   matching signature to ``(loop, context)``, where ``loop``
   will be a reference to the active event loop, ``context``
   will be a ``dict`` object (see :meth:`call_exception_handler`
   documentation for details about context).

.. method:: loop.get_exception_handler()

   Return the exception handler, or ``None`` if the default one
   is in use.

   .. versionadded:: 3.5.2

.. method:: loop.default_exception_handler(context)

   Default exception handler.

   This is called when an exception occurs and no exception
   handler is set. This can be called by a custom exception
   handler that wants to defer to the default handler behavior.

   *context* parameter has the same meaning as in
   :meth:`call_exception_handler`.

.. method:: loop.call_exception_handler(context)

   Call the current event loop exception handler.

   *context* is a ``dict`` object containing the following keys
   (new keys may be introduced later):

   * 'message': Error message;
   * 'exception' (optional): Exception object;
   * 'future' (optional): :class:`asyncio.Future` instance;
   * 'handle' (optional): :class:`asyncio.Handle` instance;
   * 'protocol' (optional): :ref:`Protocol <asyncio-protocol>` instance;
   * 'transport' (optional): :ref:`Transport <asyncio-transport>` instance;
   * 'socket' (optional): :class:`socket.socket` instance.

   .. note::

       This method should not be overloaded in subclassed
       event loops.  For any custom exception handling, use
       :meth:`set_exception_handler()` method.

Enabling debug mode
^^^^^^^^^^^^^^^^^^^

.. method:: loop.get_debug()

   Get the debug mode (:class:`bool`) of the event loop.

   The default value is ``True`` if the environment variable
   :envvar:`PYTHONASYNCIODEBUG` is set to a non-empty string, ``False``
   otherwise.

.. method:: loop.set_debug(enabled: bool)

   Set the debug mode of the event loop.

.. seealso::

   The :ref:`debug mode of asyncio <asyncio-debug-mode>`.


Running Subprocesses
^^^^^^^^^^^^^^^^^^^^

Methods described in this subsections are low-level.  In an
async/await code consider using high-level convenient
:func:`asyncio.create_subprocess_shell` and
:func:`asyncio.create_subprocess_exec` functions instead.

.. note::

   The default event loop that asyncio is pre-configured
   to use on **Windows** does not support subprocesses.
   See :ref:`Subprocess Support on Windows <asyncio-windows-subprocess>`
   for details.

.. coroutinemethod:: loop.subprocess_exec(protocol_factory, \*args, \
                      stdin=subprocess.PIPE, stdout=subprocess.PIPE, \
                      stderr=subprocess.PIPE, \*\*kwargs)

   Create a subprocess from one or more string arguments specified by
   *args*.

   *args* must be a list of strings represented by:

   * :class:`str`;
   * or :class:`bytes`, encoded to the
     :ref:`filesystem encoding <filesystem-encoding>`.

   The first string specifies the program to execute,
   and the remaining strings specify the arguments.  Together, string
   arguments form the ``argv`` of the program.

   This is similar to the standard library :class:`subprocess.Popen`
   class called with ``shell=False`` and the list of strings passed as
   the first argument; however, where :class:`~subprocess.Popen` takes
   a single argument which is list of strings, *subprocess_exec*
   takes multiple string arguments.

   The *protocol_factory* must instantiate a subclass of the
   :class:`asyncio.SubprocessProtocol` class.

   Other parameters:

   * *stdin*: either a file-like object representing a pipe to be
     connected to the subprocess's standard input stream using
     :meth:`~loop.connect_write_pipe`, or the
     :const:`subprocess.PIPE`  constant (default). By default a new
     pipe will be created and connected.

   * *stdout*: either a file-like object representing the pipe to be
     connected to the subprocess's standard output stream using
     :meth:`~loop.connect_read_pipe`, or the
     :const:`subprocess.PIPE` constant (default). By default a new pipe
     will be created and connected.

   * *stderr*: either a file-like object representing the pipe to be
     connected to the subprocess's standard error stream using
     :meth:`~loop.connect_read_pipe`, or one of
     :const:`subprocess.PIPE` (default) or :const:`subprocess.STDOUT`
     constants.

     By default a new pipe will be created and connected. When
     :const:`subprocess.STDOUT` is specified, the subprocess' standard
     error stream will be connected to the same pipe as the standard
     output stream.

   * All other keyword arguments are passed to :class:`subprocess.Popen`
     without interpretation, except for *bufsize*, *universal_newlines*
     and *shell*, which should not be specified at all.

   See the constructor of the :class:`subprocess.Popen` class
   for documentation on other arguments.

   Returns a pair of ``(transport, protocol)``, where *transport*
   conforms to the :class:`asyncio.SubprocessTransport` base class.

.. coroutinemethod:: loop.subprocess_shell(protocol_factory, cmd, \*, \
                        stdin=subprocess.PIPE, stdout=subprocess.PIPE, \
                        stderr=subprocess.PIPE, \*\*kwargs)

   Create a subprocess from *cmd*, which can be a :class:`str` or a
   :class:`bytes` string encoded to the
   :ref:`filesystem encoding <filesystem-encoding>`,
   using the platform's "shell" syntax.

   This is similar to the standard library :class:`subprocess.Popen`
   class called with ``shell=True``.

   The *protocol_factory* must instantiate a subclass of the
   :class:`SubprocessProtocol` class.

   See :meth:`~loop.subprocess_exec` for more details about
   the remaining arguments.

   Returns a pair of ``(transport, protocol)``, where *transport*
   conforms to the :class:`SubprocessTransport` base class.

.. note::
   It is the application's responsibility to ensure that all whitespace
   and metacharacters are quoted appropriately to avoid `shell injection
   <https://en.wikipedia.org/wiki/Shell_injection#Shell_injection>`_
   vulnerabilities. The :func:`shlex.quote` function can be used to
   properly escape whitespace and shell metacharacters in strings that
   are going to be used to construct shell commands.


Callback Handles
================

.. class:: Handle

   A callback wrapper object returned by :meth:`loop.call_soon`,
   :meth:`loop.call_soon_threadsafe`.

   .. method:: cancel()

      Cancel the call.  If the callback is already canceled or executed,
      this method has no effect.

   .. method:: cancelled()

      Return ``True`` if the call was cancelled.

      .. versionadded:: 3.7

.. class:: TimerHandle

   A callback wrapper object returned by :meth:`loop.call_later`,
   and :meth:`loop.call_at`.

   The class is inherited from :class:`Handle`.

   .. method:: when()

      Return a scheduled callback time as :class:`float` seconds.

      The time is an absolute timestamp, using the same time
      reference as :meth:`loop.time`.

      .. versionadded:: 3.7


Server Objects
==============

Server objects are created by :meth:`loop.create_server`,
:meth:`loop.create_unix_server`, :func:`start_server`,
and :func:`start_unix_server` functions.

Do not instantiate the class directly.

.. class:: Server

   *Server* objects are asynchronous context managers.  When used in an
   ``async with`` statement, it's guaranteed that the Server object is
   closed and not accepting new connections when the ``async with``
   statement is completed::

      srv = await loop.create_server(...)

      async with srv:
          # some code

      # At this point, srv is closed and no longer accepts new connections.


   .. versionchanged:: 3.7
      Server object is an asynchronous context manager since Python 3.7.

   .. method:: close()

      Stop serving: close listening sockets and set the :attr:`sockets`
      attribute to ``None``.

      The sockets that represent existing incoming client connections
      are left open.

      The server is closed asynchronously, use the :meth:`wait_closed`
      coroutine to wait until the server is closed.

   .. method:: get_loop()

      Gives the event loop associated with the server object.

      .. versionadded:: 3.7

   .. coroutinemethod:: start_serving()

      Start accepting connections.

      This method is idempotent, so it can be called when
      the server is already being serving.

      The new *start_serving* keyword-only parameter to
      :meth:`loop.create_server` and
      :meth:`asyncio.start_server` allows to create a Server object
      that is not accepting connections right away.  In which case
      this method, or :meth:`Server.serve_forever` can be used
      to make the Server object to start accepting connections.

      .. versionadded:: 3.7

   .. coroutinemethod:: serve_forever()

      Start accepting connections until the coroutine is cancelled.
      Cancellation of ``serve_forever`` task causes the server
      to be closed.

      This method can be called if the server is already accepting
      connections.  Only one ``serve_forever`` task can exist per
      one *Server* object.

      Example::

          async def client_connected(reader, writer):
              # Communicate with the client with
              # reader/writer streams.  For example:
              await reader.readline()

          async def main(host, port):
              srv = await asyncio.start_server(
                  client_connected, host, port)
              await srv.serve_forever()

          asyncio.run(main('127.0.0.1', 0))

      .. versionadded:: 3.7

   .. method:: is_serving()

      Return ``True`` if the server is accepting new connections.

      .. versionadded:: 3.7

   .. coroutinemethod:: wait_closed()

      Wait until the :meth:`close` method completes.

   .. attribute:: sockets

      List of :class:`socket.socket` objects the server is listening to,
      or ``None`` if the server is closed.

      .. versionchanged:: 3.7
         Prior to Python 3.7 ``Server.sockets`` used to return the
         internal list of server's sockets directly.  In 3.7 a copy
         of that list is returned.


.. _asyncio-event-loops:

Event Loops Implementations
===========================

asyncio ships with two different event loop implementations:
:class:`SelectorEventLoop` and :class:`ProactorEventLoop`.

By default asyncio is configured to use :class:`SelectorEventLoop`
on all platforms.


.. class:: SelectorEventLoop

   An event loop based on the :mod:`selectors` module.

   Uses the most efficient *selector* available for the given
   platform.  It is also possible to manually configure what
   exact selector implementation should be used::

      import asyncio
      import selectors

      selector = selectors.SelectSelector()
      loop = asyncio.SelectorEventLoop(selector)
      asyncio.set_event_loop(loop)


   Availability: UNIX, Windows.


.. class:: ProactorEventLoop

   An event loop for Windows that uses "I/O Completion Ports" (IOCP).

   Availability: Windows.

   An example how to use :class:`ProactorEventLoop` on Windows::

        import asyncio
        import sys

        if sys.platform == 'win32':
            loop = asyncio.ProactorEventLoop()
            asyncio.set_event_loop(loop)

   .. seealso::

      `MSDN documentation on I/O Completion Ports
      <https://docs.microsoft.com/en-ca/windows/desktop/FileIO/i-o-completion-ports>`_.


.. class:: AbstractEventLoop

   Abstract base class for asyncio-compliant event loops.

   The :ref:`Event Loop Methods <asyncio-event-loop>` section lists all
   methods that an alternative implementation of ``AbstractEventLoop``
   should have defined.


Examples
========

Note that all examples in this section **purposefully** show how
to use low-level event loop APIs such as :meth:`loop.run_forever`
and :meth:`loop.call_soon`.  Modern asyncio applications rarely
need to be written this way; consider using high-level functions
like :func:`asyncio.run`.


.. _asyncio-hello-world-callback:

Hello World with call_soon()
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

An example using the :meth:`loop.call_soon` method to schedule a
callback. The callback displays ``"Hello World"`` and then stops the
event loop::

    import asyncio

    def hello_world(loop):
        """A callback to print 'Hello World' and stop the event loop"""
        print('Hello World')
        loop.stop()

    loop = asyncio.get_event_loop()

    # Schedule a call to hello_world()
    loop.call_soon(hello_world, loop)

    # Blocking call interrupted by loop.stop()
    try:
        loop.run_forever()
    finally:
        loop.close()

.. seealso::

   A similar :ref:`Hello World <asyncio-hello-world-coroutine>`
   example created with a coroutine and the :func:`run` function.


.. _asyncio-date-callback:

Display the current date with call_later()
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

An example of callback displaying the current date every second. The
callback uses the :meth:`loop.call_later` method to reschedule itself
during 5 seconds, and then stops the event loop::

    import asyncio
    import datetime

    def display_date(end_time, loop):
        print(datetime.datetime.now())
        if (loop.time() + 1.0) < end_time:
            loop.call_later(1, display_date, end_time, loop)
        else:
            loop.stop()

    loop = asyncio.get_event_loop()

    # Schedule the first call to display_date()
    end_time = loop.time() + 5.0
    loop.call_soon(display_date, end_time, loop)

    # Blocking call interrupted by loop.stop()
    try:
        loop.run_forever()
    finally:
        loop.close()

.. seealso::

   A similar :ref:`current date <asyncio-date-coroutine>` example
   created with a coroutine and the :func:`run` function.


.. _asyncio-watch-read-event:

Watch a file descriptor for read events
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Wait until a file descriptor received some data using the
:meth:`loop.add_reader` method and then close the event loop::

    import asyncio
    from socket import socketpair

    # Create a pair of connected file descriptors
    rsock, wsock = socketpair()

    loop = asyncio.get_event_loop()

    def reader():
        data = rsock.recv(100)
        print("Received:", data.decode())

        # We are done: unregister the file descriptor
        loop.remove_reader(rsock)

        # Stop the event loop
        loop.stop()

    # Register the file descriptor for read event
    loop.add_reader(rsock, reader)

    # Simulate the reception of data from the network
    loop.call_soon(wsock.send, 'abc'.encode())

    try:
        # Run the event loop
        loop.run_forever()
    finally:
        # We are done. Close sockets and the event loop.
        rsock.close()
        wsock.close()
        loop.close()

.. seealso::

   * A similar :ref:`example <asyncio-register-socket>`
     using transports, protocols, and the
     :meth:`loop.create_connection` method.

   * Another similar :ref:`example <asyncio-register-socket-streams>`
     using the high-level :func:`asyncio.open_connection` function
     and streams.


Set signal handlers for SIGINT and SIGTERM
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

(This ``signals`` example only works on UNIX.)

Register handlers for signals :py:data:`SIGINT` and :py:data:`SIGTERM`
using the :meth:`loop.add_signal_handler` method::

    import asyncio
    import functools
    import os
    import signal

    def ask_exit(signame):
        print("got signal %s: exit" % signame)
        loop.stop()

    async def main():
        loop = asyncio.get_running_loop()

        for signame in {'SIGINT', 'SIGTERM'}:
            loop.add_signal_handler(
                getattr(signal, signame),
                functools.partial(ask_exit, signame))

        await asyncio.sleep(3600)

    print("Event loop running for 1 hour, press Ctrl+C to interrupt.")
    print(f"pid {os.getpid()}: send SIGINT or SIGTERM to exit.")

    asyncio.run(main())