.. currentmodule:: asyncio
.. _asyncio-event-loop:
==========
Event Loop
==========
**Source code:** :source:`Lib/asyncio/events.py`,
:source:`Lib/asyncio/base_events.py`
------------------------------------
.. rubric:: Preface
The event loop is the core of every asyncio application.
Event loops run asynchronous tasks and callbacks, perform network
IO operations, and run subprocesses.
Application developers should typically use the high-level asyncio functions,
such as :func:`asyncio.run`, and should rarely need to reference the loop
object or call its methods. This section is intended mostly for authors
of lower-level code, libraries, and frameworks, who need finer control over
the event loop behavior.
.. rubric:: Obtaining the 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.
Raise a :exc:`RuntimeError` if there is no running event loop.
This function can only be called from a coroutine or a callback.
.. versionadded:: 3.7
.. function:: get_event_loop()
Get the current event loop.
When called from a coroutine or a callback (e.g. scheduled with
call_soon or similar API), this function will always return the
running event loop.
If there is no running event loop set, the function will return
the result of the ``get_event_loop_policy().get_event_loop()`` call.
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.
As noted above, consider using the higher-level :func:`asyncio.run` function,
instead of using these lower level functions to manually create and close an
event loop.
.. note::
In Python versions 3.10.0--3.10.8 and 3.11.0 this function
(and other functions which use it implicitly) emitted a
:exc:`DeprecationWarning` if there was no running event loop, even if
the current loop was set on the policy.
In Python versions 3.10.9, 3.11.1 and 3.12 they emit a
:exc:`DeprecationWarning` if there is no running event loop and no
current loop is set.
In some future Python release this will become an error.
.. function:: set_event_loop(loop)
Set *loop* as the current event loop for the current OS thread.
.. function:: new_event_loop()
Create and return 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
the event loop APIs;
* The `Callback Handles`_ section documents the :class:`Handle` and
:class:`TimerHandle` instances which are returned from scheduling
methods such as :meth:`loop.call_soon` and :meth:`loop.call_later`;
* The `Server Objects`_ section documents types returned from
event loop methods like :meth:`loop.create_server`;
* The `Event Loop 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-methods:
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`) has
completed.
If the argument is a :ref:`coroutine object <coroutine>` it
is implicitly scheduled to run as a :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 new 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 not 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.
Note that there is no need to call this function when
:func:`asyncio.run` is used.
Example::
try:
loop.run_forever()
finally:
loop.run_until_complete(loop.shutdown_asyncgens())
loop.close()
.. versionadded:: 3.6
.. coroutinemethod:: loop.shutdown_default_executor()
Schedule the closure of the default executor and wait for it to join all of
the threads in the :class:`ThreadPoolExecutor`. After calling this method, a
:exc:`RuntimeError` will be raised if :meth:`loop.run_in_executor` is called
while using the default executor.
Note that there is no need to call this function when
:func:`asyncio.run` is used.
.. versionadded:: 3.9
Scheduling callbacks
^^^^^^^^^^^^^^^^^^^^
.. method:: loop.call_soon(callback, *args, context=None)
Schedule the *callback* :term:`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*.
Raises :exc:`RuntimeError` if called on a loop that's been closed.
This can happen on a secondary thread when the main application is
shutting down.
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.
.. _asyncio-pass-keywords:
.. note::
Most :mod:`asyncio` scheduling functions don't allow passing
keyword arguments. To do that, use :func:`functools.partial`::
# 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 render partial objects better 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, the order in which they
are called is undefined.
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.
.. versionchanged:: 3.8
In Python 3.7 and earlier with the default event loop implementation,
the *delay* could not exceed one day.
This has been fixed in Python 3.8.
.. 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.
.. versionchanged:: 3.8
In Python 3.7 and earlier with the default event loop implementation,
the difference between *when* and the current time could not exceed
one day. This has been fixed in Python 3.8.
.. method:: loop.time()
Return the current time, as a :class:`float` value, according to
the event loop's internal monotonic clock.
.. note::
.. versionchanged:: 3.8
In Python 3.7 and earlier timeouts (relative *delay* or absolute *when*)
should not exceed one day. This has been fixed in Python 3.8.
.. 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, context=None)
Schedule the execution of :ref:`coroutine <coroutine>` *coro*.
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`.
An optional keyword-only *context* argument allows specifying a
custom :class:`contextvars.Context` for the *coro* to run in.
The current context copy is created when no *context* is provided.
.. versionchanged:: 3.8
Added the *name* parameter.
.. versionchanged:: 3.11
Added the *context* 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.
Otherwise, *factory* must be a *callable* with the signature matching
``(loop, coro, context=None)``, where *loop* is a reference to the active
event loop, and *coro* is a coroutine object. The callable
must return a :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, \
ssl_shutdown_timeout=None, \
happy_eyeballs_delay=None, interleave=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.
Other arguments:
* *ssl*: if given and not false, a 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 default context returned
from :func:`ssl.create_default_context` is used.
.. seealso:: :ref:`SSL/TLS security considerations <ssl-security>`
* *server_hostname* sets or overrides the hostname that the target
server's certificate will be matched against. Should only be passed
if *ssl* is not ``None``. 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 potential 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.
* *happy_eyeballs_delay*, if given, enables Happy Eyeballs for this
connection. It should
be a floating-point number representing the amount of time in seconds
to wait for a connection attempt to complete, before starting the next
attempt in parallel. This is the "Connection Attempt Delay" as defined
in :rfc:`8305`. A sensible default value recommended by the RFC is ``0.25``
(250 milliseconds).
* *interleave* controls address reordering when a host name resolves to
multiple IP addresses.
If ``0`` or unspecified, no reordering is done, and addresses are
tried in the order returned by :meth:`getaddrinfo`. If a positive integer
is specified, the addresses are interleaved by address family, and the
given integer is interpreted as "First Address Family Count" as defined
in :rfc:`8305`. The default is ``0`` if *happy_eyeballs_delay* is not
specified, and ``1`` if it is.
* *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*,
*happy_eyeballs_delay*, *interleave*
and *local_addr* should be specified.
.. note::
The *sock* argument transfers ownership of the socket to the
transport created. To close the socket, call the transport's
:meth:`~asyncio.BaseTransport.close` method.
* *local_addr*, if given, is a ``(local_host, local_port)`` tuple used
to bind the socket locally. The *local_host* and *local_port*
are looked up using ``getaddrinfo()``, similarly to *host* and *port*.
* *ssl_handshake_timeout* is (for a TLS connection) the time in seconds
to wait for the TLS handshake to complete before aborting the connection.
``60.0`` seconds if ``None`` (default).
* *ssl_shutdown_timeout* is the time in seconds to wait for the SSL shutdown
to complete before aborting the connection. ``30.0`` seconds if ``None``
(default).
.. versionchanged:: 3.5
Added support for SSL/TLS in :class:`ProactorEventLoop`.
.. versionchanged:: 3.6
The socket option :py:data:`~socket.TCP_NODELAY` is set by default
for all TCP connections.
.. versionchanged:: 3.7
Added the *ssl_handshake_timeout* parameter.
.. versionchanged:: 3.8
Added the *happy_eyeballs_delay* and *interleave* parameters.
Happy Eyeballs Algorithm: Success with Dual-Stack Hosts.
When a server's IPv4 path and protocol are working, but the server's
IPv6 path and protocol are not working, a dual-stack client
application experiences significant connection delay compared to an
IPv4-only client. This is undesirable because it causes the dual-
stack client to have a worse user experience. This document
specifies requirements for algorithms that reduce this user-visible
delay and provides an algorithm.
For more information: https://tools.ietf.org/html/rfc6555
.. versionchanged:: 3.11
Added the *ssl_shutdown_timeout* parameter.
.. 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_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 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_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 Unixes. 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`).
.. note::
The *sock* argument transfers ownership of the socket to the
transport created. To close the socket, call the transport's
:meth:`~asyncio.BaseTransport.close` method.
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.
.. versionchanged:: 3.8.1
The *reuse_address* parameter is no longer supported, as using
:py:data:`~sockets.SO_REUSEADDR` poses a significant security concern for
UDP. Explicitly passing ``reuse_address=True`` will raise an exception.
When multiple processes with differing UIDs assign sockets to an
identical UDP socket address with ``SO_REUSEADDR``, incoming packets can
become randomly distributed among the sockets.
For supported platforms, *reuse_port* can be used as a replacement for
similar functionality. With *reuse_port*,
:py:data:`~sockets.SO_REUSEPORT` is used instead, which specifically
prevents processes with differing UIDs from assigning sockets to the same
socket address.
.. versionchanged:: 3.8
Added support for Windows.
.. versionchanged:: 3.11
The *reuse_address* parameter, disabled since Python 3.9.0, 3.8.1,
3.7.6 and 3.6.10, has been entirely removed.
.. coroutinemethod:: loop.create_unix_connection(protocol_factory, \
path=None, *, ssl=None, sock=None, \
server_hostname=None, ssl_handshake_timeout=None, \
ssl_shutdown_timeout=None)
Create a 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.
.. versionchanged:: 3.7
Added the *ssl_handshake_timeout* parameter.
The *path* parameter can now be a :term:`path-like object`.
.. versionchanged:: 3.11
Added the *ssl_shutdown_timeout* parameter.
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, \
ssl_shutdown_timeout=None, \
start_serving=True)
Create a TCP server (socket type :data:`~socket.SOCK_STREAM`) listening
on *port* of the *host* address.
Returns a :class:`Server` object.
Arguments:
* *protocol_factory* must be a callable returning a
:ref:`protocol <asyncio-protocol>` implementation.
* The *host* parameter can be set to several types which determine where
the server would be listening:
- If *host* is a string, the TCP server is bound to a single network
interface specified by *host*.
- If *host* is a sequence of strings, the TCP server is bound to all
network interfaces specified by 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).
* The *port* parameter can be set to specify which port the server should
listen on. If ``0`` or ``None`` (the default), a random unused port will
be selected (note that if *host* resolves to multiple network interfaces,
a different random port will be selected for each interface).
* *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* must not be specified.
.. note::
The *sock* argument transfers ownership of the socket to the
server created. To close the socket, call the server's
:meth:`~asyncio.Server.close` method.
* *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` instance to enable
TLS 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 a TLS server) the time in seconds to wait
for the TLS handshake to complete before aborting the connection.
``60.0`` seconds if ``None`` (default).
* *ssl_shutdown_timeout* is the time in seconds to wait for the SSL shutdown
to complete before aborting the connection. ``30.0`` 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.
.. versionchanged:: 3.5
Added support for SSL/TLS in :class:`ProactorEventLoop`.
.. versionchanged:: 3.5.1
The *host* parameter can be a sequence of strings.
.. versionchanged:: 3.6
Added *ssl_handshake_timeout* and *start_serving* parameters.
The socket option :py:data:`~socket.TCP_NODELAY` is set by default
for all TCP connections.
.. versionchanged:: 3.11
Added the *ssl_shutdown_timeout* parameter.
.. 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, \
ssl_shutdown_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.
See the documentation of the :meth:`loop.create_server` method
for information about arguments to this method.
.. availability:: Unix.
.. versionchanged:: 3.7
Added the *ssl_handshake_timeout* and *start_serving* parameters.
The *path* parameter can now be a :class:`~pathlib.Path` object.
.. versionchanged:: 3.11
Added the *ssl_shutdown_timeout* parameter.
.. coroutinemethod:: loop.connect_accepted_socket(protocol_factory, \
sock, *, ssl=None, ssl_handshake_timeout=None, \
ssl_shutdown_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:
* *protocol_factory* must be a callable returning a
:ref:`protocol <asyncio-protocol>` implementation.
* *sock* is a preexisting socket object returned from
:meth:`socket.accept <socket.socket.accept>`.
.. note::
The *sock* argument transfers ownership of the socket to the
transport created. To close the socket, call the transport's
:meth:`~asyncio.BaseTransport.close` method.
* *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).
* *ssl_shutdown_timeout* is the time in seconds to wait for the SSL shutdown
to complete before aborting the connection. ``30.0`` seconds if ``None``
(default).
Returns a ``(transport, protocol)`` pair.
.. versionadded:: 3.5.3
.. versionchanged:: 3.7
Added the *ssl_handshake_timeout* parameter.
.. versionchanged:: 3.11
Added the *ssl_shutdown_timeout* parameter.
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 always updated,
even when this method raises an error, and
:meth:`file.tell() <io.IOBase.tell>` can be used to obtain the actual
number of bytes 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
the *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, \
ssl_shutdown_timeout=None)
Upgrade an existing transport-based connection to TLS.
Create a TLS coder/decoder instance and insert it between the *transport*
and the *protocol*. The coder/decoder implements both *transport*-facing
protocol and *protocol*-facing transport.
Return the created two-interface instance. After *await*, the *protocol*
must stop using the original *transport* and communicate with the returned
object only because the coder caches *protocol*-side data and sporadically
exchanges extra TLS session packets with *transport*.
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 a TLS connection) the time in seconds to
wait for the TLS handshake to complete before aborting the connection.
``60.0`` seconds if ``None`` (default).
* *ssl_shutdown_timeout* is the time in seconds to wait for the SSL shutdown
to complete before aborting the connection. ``30.0`` seconds if ``None``
(default).
.. versionadded:: 3.7
.. versionchanged:: 3.11
Added the *ssl_shutdown_timeout* parameter.
Watching file descriptors
^^^^^^^^^^^^^^^^^^^^^^^^^
.. method:: loop.add_reader(fd, callback, *args)
Start monitoring the *fd* file descriptor for read availability and
invoke *callback* with the specified arguments once *fd* is available for
reading.
.. method:: loop.remove_reader(fd)
Stop monitoring the *fd* file descriptor for read availability. Returns
``True`` if *fd* was previously being monitored for reads.
.. method:: loop.add_writer(fd, callback, *args)
Start monitoring the *fd* file descriptor for write availability and
invoke *callback* with the specified arguments once *fd* is available for
writing.
Use :func:`functools.partial` :ref:`to pass keyword arguments
<asyncio-pass-keywords>` to *callback*.
.. method:: loop.remove_writer(fd)
Stop monitoring the *fd* file descriptor for write availability. Returns
``True`` if *fd* was previously being monitored for writes.
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 up to *nbytes* from *sock*. Asynchronous version of
:meth:`socket.recv() <socket.socket.recv>`.
Return the received data as a bytes object.
*sock* must be a non-blocking socket.
.. 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 from *sock* into the *buf* buffer. Modeled after the blocking
:meth:`socket.recv_into() <socket.socket.recv_into>` method.
Return the number of bytes written to the buffer.
*sock* must be a non-blocking socket.
.. versionadded:: 3.7
.. coroutinemethod:: loop.sock_recvfrom(sock, bufsize)
Receive a datagram of up to *bufsize* from *sock*. Asynchronous version of
:meth:`socket.recvfrom() <socket.socket.recvfrom>`.
Return a tuple of (received data, remote address).
*sock* must be a non-blocking socket.
.. versionadded:: 3.11
.. coroutinemethod:: loop.sock_recvfrom_into(sock, buf, nbytes=0)
Receive a datagram of up to *nbytes* from *sock* into *buf*.
Asynchronous version of
:meth:`socket.recvfrom_into() <socket.socket.recvfrom_into>`.
Return a tuple of (number of bytes received, remote address).
*sock* must be a non-blocking socket.
.. versionadded:: 3.11
.. coroutinemethod:: loop.sock_sendall(sock, data)
Send *data* to the *sock* socket. Asynchronous version of
:meth:`socket.sendall() <socket.socket.sendall>`.
This method continues to send 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.
*sock* must be a non-blocking socket.
.. 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.
.. coroutinemethod:: loop.sock_sendto(sock, data, address)
Send a datagram from *sock* to *address*.
Asynchronous version of
:meth:`socket.sendto() <socket.socket.sendto>`.
Return the number of bytes sent.
*sock* must be a non-blocking socket.
.. versionadded:: 3.11
.. coroutinemethod:: loop.sock_connect(sock, address)
Connect *sock* to a remote socket at *address*.
Asynchronous version of :meth:`socket.connect() <socket.socket.connect>`.
*sock* must be a non-blocking socket.
.. 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.
*sock* must be a non-blocking socket.
.. 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 sent.
Asynchronous version of :meth:`socket.sendfile() <socket.socket.sendfile>`.
*sock* must be a non-blocking :const:`socket.SOCK_STREAM`
:class:`~socket.socket`.
*file* must be a regular file object open 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 always updated,
even when this method raises an error, and
:meth:`file.tell() <io.IOBase.tell>` can be used to obtain the actual
number of bytes 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``.
*sock* must be a non-blocking socket.
.. 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 the read end of *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 and *protocol* is an object
instantiated by the *protocol_factory*.
With :class:`SelectorEventLoop` event loop, the *pipe* is set to
non-blocking mode.
.. coroutinemethod:: loop.connect_write_pipe(protocol_factory, pipe)
Register the write end of *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 and *protocol* is an object
instantiated by the *protocol_factory*.
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)
Set *callback* as the handler for the *signum* signal.
The callback will be invoked by *loop*, along with other queued callbacks
and runnable coroutines of that event loop. Unlike signal handlers
registered using :func:`signal.signal`, a callback registered with this
function is allowed to interact with the event loop.
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 keyword arguments
<asyncio-pass-keywords>` to *callback*.
Like :func:`signal.signal`, this function must be invoked in the main
thread.
.. method:: loop.remove_signal_handler(sig)
Remove the handler for the *sig* signal.
Return ``True`` if the signal handler was removed, or ``False`` if
no handler was set for the given signal.
.. availability:: Unix.
.. seealso::
The :mod:`signal` module.
Executing code in thread or process pools
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
.. awaitablemethod:: loop.run_in_executor(executor, func, *args)
Arrange for *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``.
Example::
import asyncio
import concurrent.futures
def blocking_io():
# File operations (such as logging) can block the
# event loop: run them in a thread pool.
with open('/dev/urandom', 'rb') as f:
return f.read(100)
def cpu_bound():
# CPU-bound operations will block the event loop:
# in general it is preferable to run them in a
# process pool.
return sum(i * i for i in range(10 ** 7))
async def main():
loop = asyncio.get_running_loop()
## Options:
# 1. Run in the default loop's executor:
result = await loop.run_in_executor(
None, blocking_io)
print('default thread pool', result)
# 2. Run in a custom thread pool:
with concurrent.futures.ThreadPoolExecutor() as pool:
result = await loop.run_in_executor(
pool, blocking_io)
print('custom thread pool', result)
# 3. Run in a custom process pool:
with concurrent.futures.ProcessPoolExecutor() as pool:
result = await loop.run_in_executor(
pool, cpu_bound)
print('custom process pool', result)
if __name__ == '__main__':
asyncio.run(main())
Note that the entry point guard (``if __name__ == '__main__'``)
is required for option 3 due to the peculiarities of :mod:`multiprocessing`,
which is used by :class:`~concurrent.futures.ProcessPoolExecutor`.
See :ref:`Safe importing of main module <multiprocessing-safe-main-import>`.
This method returns a :class:`asyncio.Future` object.
Use :func:`functools.partial` :ref:`to pass keyword arguments
<asyncio-pass-keywords>` to *func*.
.. 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* must be an instance of
:class:`~concurrent.futures.ThreadPoolExecutor`.
.. versionchanged:: 3.11
*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. Otherwise, *handler* must be a callable with the signature
matching ``(loop, context)``, where ``loop``
is a reference to the active event loop, and ``context``
is a ``dict`` object containing the details of the exception
(see :meth:`call_exception_handler` documentation for details
about context).
.. method:: loop.get_exception_handler()
Return the current exception handler, or ``None`` if no custom
exception handler was set.
.. 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 in future Python versions):
* 'message': Error message;
* 'exception' (optional): Exception object;
* 'future' (optional): :class:`asyncio.Future` instance;
* 'task' (optional): :class:`asyncio.Task` 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;
* 'asyncgen' (optional): Asynchronous generator that caused
the exception.
.. note::
This method should not be overloaded in subclassed
event loops. For custom exception handling, use
the :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.
.. versionchanged:: 3.7
The new :ref:`Python Development Mode <devmode>` can now also be used
to enable the debug mode.
.. seealso::
The :ref:`debug mode of asyncio <asyncio-debug-mode>`.
Running Subprocesses
^^^^^^^^^^^^^^^^^^^^
Methods described in this subsections are low-level. In regular
async/await code consider using the high-level
:func:`asyncio.create_subprocess_shell` and
:func:`asyncio.create_subprocess_exec` convenience functions instead.
.. note::
On Windows, the default event loop :class:`ProactorEventLoop` supports
subprocesses, whereas :class:`SelectorEventLoop` does not. 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 executable,
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 be a callable returning a subclass of the
:class:`asyncio.SubprocessProtocol` class.
Other parameters:
* *stdin* can be any of these:
* a file-like object representing a pipe to be connected to the
subprocess's standard input stream using
:meth:`~loop.connect_write_pipe`
* the :const:`subprocess.PIPE` constant (default) which will create a new
pipe and connect it,
* the value ``None`` which will make the subprocess inherit the file
descriptor from this process
* the :const:`subprocess.DEVNULL` constant which indicates that the
special :data:`os.devnull` file will be used
* *stdout* can be any of these:
* a file-like object representing a pipe to be connected to the
subprocess's standard output stream using
:meth:`~loop.connect_write_pipe`
* the :const:`subprocess.PIPE` constant (default) which will create a new
pipe and connect it,
* the value ``None`` which will make the subprocess inherit the file
descriptor from this process
* the :const:`subprocess.DEVNULL` constant which indicates that the
special :data:`os.devnull` file will be used
* *stderr* can be any of these:
* a file-like object representing a pipe to be connected to the
subprocess's standard error stream using
:meth:`~loop.connect_write_pipe`
* the :const:`subprocess.PIPE` constant (default) which will create a new
pipe and connect it,
* the value ``None`` which will make the subprocess inherit the file
descriptor from this process
* the :const:`subprocess.DEVNULL` constant which indicates that the
special :data:`os.devnull` file will be used
* the :const:`subprocess.STDOUT` constant which will connect the standard
error stream to the process' standard output stream
* All other keyword arguments are passed to :class:`subprocess.Popen`
without interpretation, except for *bufsize*, *universal_newlines*,
*shell*, *text*, *encoding* and *errors*, which should not be specified
at all.
The ``asyncio`` subprocess API does not support decoding the streams
as text. :func:`bytes.decode` can be used to convert the bytes returned
from the stream to text.
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 and
*protocol* is an object instantiated by the *protocol_factory*.
.. 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 be a callable returning 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 and
*protocol* is an object instantiated by the *protocol_factory*.
.. note::
It is the application's responsibility to ensure that all whitespace
and special characters 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 special characters 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 callback. If the callback has already been canceled
or executed, this method has no effect.
.. method:: cancelled()
Return ``True`` if the callback was cancelled.
.. versionadded:: 3.7
.. class:: TimerHandle
A callback wrapper object returned by :meth:`loop.call_later`,
and :meth:`loop.call_at`.
This class is a subclass of :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()
Return 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 serving.
The *start_serving* keyword-only parameter to
:meth:`loop.create_server` and
:meth:`asyncio.start_server` allows creating a Server object
that is not accepting connections initially. In this case
``Server.start_serving()``, or :meth:`Server.serve_forever` can be used
to make the Server 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 on.
.. versionchanged:: 3.7
Prior to Python 3.7 ``Server.sockets`` used to return an
internal list of server sockets directly. In 3.7 a copy
of that list is returned.
.. _asyncio-event-loops:
.. _asyncio-event-loop-implementations:
Event Loop Implementations
==========================
asyncio ships with two different event loop implementations:
:class:`SelectorEventLoop` and :class:`ProactorEventLoop`.
By default asyncio is configured to use :class:`SelectorEventLoop`
on Unix and :class:`ProactorEventLoop` on Windows.
.. 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 the
exact selector implementation to be used::
import asyncio
import selectors
class MyPolicy(asyncio.DefaultEventLoopPolicy):
def new_event_loop(self):
selector = selectors.SelectSelector()
return asyncio.SelectorEventLoop(selector)
asyncio.set_event_loop_policy(MyPolicy())
.. availability:: Unix, Windows.
.. class:: ProactorEventLoop
An event loop for Windows that uses "I/O Completion Ports" (IOCP).
.. availability:: Windows.
.. 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:`asyncio-event-loop-methods` 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 the 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 the high-level functions
like :func:`asyncio.run`.
.. _asyncio_example_lowlevel_helloworld:
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.new_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 <coroutine>`
example created with a coroutine and the :func:`run` function.
.. _asyncio_example_call_later:
Display the current date with call_later()
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
An example of a callback displaying the current date every second. The
callback uses the :meth:`loop.call_later` method to reschedule itself
after 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.new_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_example_sleep>` example
created with a coroutine and the :func:`run` function.
.. _asyncio_example_watch_fd:
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.new_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_example_create_connection>`
using transports, protocols, and the
:meth:`loop.create_connection` method.
* Another similar :ref:`example <asyncio_example_create_connection-streams>`
using the high-level :func:`asyncio.open_connection` function
and streams.
.. _asyncio_example_unix_signals:
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, loop):
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, loop))
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())
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