python_doc_md/Intermediate/library/asyncio-eventloop.md

::: currentmodule asyncio :::

Event Loop {#asyncio-event-loop}

Source code: Lib/asyncio/events.py{.interpreted-text role="source"}, Lib/asyncio/base_events.py{.interpreted-text role="source"}

---

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 asyncio.run{.interpreted-text role="func"}, 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.

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 RuntimeError{.interpreted-text role="exc"} 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 get_running_loop{.interpreted-text role="func"} function is preferred to get_event_loop{.interpreted-text role="func"} in coroutines and callbacks.

As noted above, consider using the higher-level asyncio.run{.interpreted-text role="func"} function, instead of using these lower level functions to manually create and close an event loop.

::: versionchanged 3.14 Raises a RuntimeError{.interpreted-text role="exc"} if there is no current event loop. :::

:::: note ::: title Note :::

The !asyncio{.interpreted-text role="mod"} policy system is deprecated and will be removed in Python 3.16; from there on, this function will return the current running event loop if present else it will return the loop set by set_event_loop{.interpreted-text role="func"}. :::: ::::::

::: 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 get_event_loop{.interpreted-text role="func"}, set_event_loop{.interpreted-text role="func"}, and new_event_loop{.interpreted-text role="func"} functions can be altered by setting a custom event loop policy <asyncio-policies>{.interpreted-text role="ref"}.

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 Handle{.interpreted-text role="class"} and TimerHandle{.interpreted-text role="class"} instances which are returned from scheduling methods such as loop.call_soon{.interpreted-text role="meth"} and loop.call_later{.interpreted-text role="meth"};
• The Server Objects section documents types returned from event loop methods like loop.create_server{.interpreted-text role="meth"};
• The Event Loop Implementations section documents the SelectorEventLoop{.interpreted-text role="class"} and ProactorEventLoop{.interpreted-text role="class"} classes;
• The Examples section showcases how to work with some event loop APIs.

Event Loop Methods {#asyncio-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 Future{.interpreted-text role="class"}) has completed.

If the argument is a coroutine object <coroutine>{.interpreted-text role="ref"} it is implicitly scheduled to run as a asyncio.Task{.interpreted-text role="class"}.

Return the Future's result or raise its exception. :::

::: method loop.run_forever()

Run the event loop until stop{.interpreted-text role="meth"} is called.

If stop{.interpreted-text role="meth"} is called before run_forever{.interpreted-text role="meth"} 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 stop{.interpreted-text role="meth"} is called while run_forever{.interpreted-text role="meth"} 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 run_forever{.interpreted-text role="meth"} or run_until_complete{.interpreted-text role="meth"} 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. :::

:::: {.method async=""} loop.shutdown_asyncgens()

Schedule all currently open asynchronous generator{.interpreted-text role="term"} objects to close with an ~agen.aclose{.interpreted-text role="meth"} 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 asyncio.run{.interpreted-text role="func"} is used.

Example:

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

::: versionadded 3.6 ::: ::::

::::::: {.method async=""} loop.shutdown_default_executor(timeout=None)

Schedule the closure of the default executor and wait for it to join all of the threads in the ~concurrent.futures.ThreadPoolExecutor{.interpreted-text role="class"}. Once this method has been called, using the default executor with loop.run_in_executor{.interpreted-text role="meth"} will raise a RuntimeError{.interpreted-text role="exc"}.

The timeout parameter specifies the amount of time (in float{.interpreted-text role="class"} seconds) the executor will be given to finish joining. With the default, None, the executor is allowed an unlimited amount of time.

If the timeout is reached, a RuntimeWarning{.interpreted-text role="exc"} is emitted and the default executor is terminated without waiting for its threads to finish joining.

:::: note ::: title Note :::

Do not call this method when using asyncio.run{.interpreted-text role="func"}, as the latter handles default executor shutdown automatically. ::::

::: versionadded 3.9 :::

::: versionchanged 3.12 Added the timeout parameter. ::: :::::::

Scheduling callbacks

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

Schedule the callback callback{.interpreted-text role="term"} to be called with args arguments at the next iteration of the event loop.

Return an instance of asyncio.Handle{.interpreted-text role="class"}, which can be used later to cancel the callback.

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

The optional keyword-only context argument specifies a custom contextvars.Context{.interpreted-text role="class"} for the callback to run in. Callbacks use the current context when no context is provided.

Unlike call_soon_threadsafe{.interpreted-text role="meth"}, this method is not thread-safe. :::

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

A thread-safe variant of call_soon{.interpreted-text role="meth"}. When scheduling callbacks from another thread, this function must be used, since call_soon{.interpreted-text role="meth"} is not thread-safe.

This function is safe to be called from a reentrant context or signal handler, however, it is not safe or fruitful to use the returned handle in such contexts.

Raises RuntimeError{.interpreted-text role="exc"} 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 concurrency and multithreading <asyncio-multithreading>{.interpreted-text role="ref"} section of the documentation.

::: versionchanged 3.7 The context keyword-only parameter was added. See 567{.interpreted-text role="pep"} for more details. ::: ::::

::::: {#asyncio-pass-keywords} :::: note ::: title Note :::

Most asyncio{.interpreted-text role="mod"} scheduling functions don't allow passing keyword arguments. To do that, use functools.partial{.interpreted-text role="func"}:

# 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. :::: :::::

Scheduling delayed callbacks {#asyncio-delayed-calls}

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 asyncio.TimerHandle{.interpreted-text role="class"} 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. Use functools.partial{.interpreted-text role="func"} to pass keyword arguments <asyncio-pass-keywords>{.interpreted-text role="ref"} to callback.

An optional keyword-only context argument allows specifying a custom contextvars.Context{.interpreted-text role="class"} for the callback to run in. The current context is used when no context is provided.

:::: note ::: title Note :::

For performance, callbacks scheduled with loop.call_later{.interpreted-text role="meth"} may run up to one clock-resolution early (see time.get_clock_info('monotonic').resolution). ::::

::: versionchanged 3.7 The context keyword-only parameter was added. See 567{.interpreted-text role="pep"} 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 loop.time{.interpreted-text role="meth"}.

This method's behavior is the same as call_later{.interpreted-text role="meth"}.

An instance of asyncio.TimerHandle{.interpreted-text role="class"} is returned which can be used to cancel the callback.

:::: note ::: title Note :::

For performance, callbacks scheduled with loop.call_at{.interpreted-text role="meth"} may run up to one clock-resolution early (see time.get_clock_info('monotonic').resolution). ::::

::: versionchanged 3.7 The context keyword-only parameter was added. See 567{.interpreted-text role="pep"} 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 float{.interpreted-text role="class"} value, according to the event loop's internal monotonic clock. :::

:::: note ::: title 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 asyncio.sleep{.interpreted-text role="func"} function. :::

Creating Futures and Tasks

:::: method loop.create_future()

Create an asyncio.Future{.interpreted-text role="class"} 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, eager_start=None,*kwargs)

Schedule the execution of coroutine <coroutine>{.interpreted-text role="ref"} coro. Return a Task{.interpreted-text role="class"} object.

Third-party event loops can use their own subclass of Task{.interpreted-text role="class"} for interoperability. In this case, the result type is a subclass of Task{.interpreted-text role="class"}.

The full function signature is largely the same as that of the Task{.interpreted-text role="class"} constructor (or factory) - all of the keyword arguments to this function are passed through to that interface.

If the name argument is provided and not None, it is set as the name of the task using Task.set_name{.interpreted-text role="meth"}.

An optional keyword-only context argument allows specifying a custom contextvars.Context{.interpreted-text role="class"} for the coro to run in. The current context copy is created when no context is provided.

An optional keyword-only eager_start argument allows specifying if the task should execute eagerly during the call to create_task, or be scheduled later. If eager_start is not passed the mode set by loop.set_task_factory{.interpreted-text role="meth"} will be used.

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

::: versionchanged 3.11 Added the context parameter. :::

::: versionchanged 3.13.3 Added kwargs which passes on arbitrary extra parameters, including name and context. :::

::: versionchanged 3.13.4 Rolled back the change that passes on name and context (if it is None), while still passing on other arbitrary keyword arguments (to avoid breaking backwards compatibility with 3.13.3). :::

::: versionchanged 3.14 All kwargs are now passed on. The eager_start parameter works with eager task factories. ::: ::::::::

:::::: method loop.set_task_factory(factory)

Set a task factory that will be used by loop.create_task{.interpreted-text role="meth"}.

If factory is None the default task factory will be set. Otherwise, factory must be a callable with the signature matching (loop, coro, **kwargs), where loop is a reference to the active event loop, and coro is a coroutine object. The callable must pass on all kwargs, and return a asyncio.Task{.interpreted-text role="class"}-compatible object.

::: versionchanged 3.13.3 Required that all kwargs are passed on to asyncio.Task{.interpreted-text role="class"}. :::

:::: versionchanged 3.13.4 name is no longer passed to task factories. context is no longer passed to task factories if it is None.

::: versionchanged 3.14 name and context are now unconditionally passed on to task factories again. ::: :::: ::::::

::: method loop.get_task_factory()

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

Opening network connections

:::::::::: {.method async=""} 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, all_errors=False)

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

The socket family can be either ~socket.AF_INET{.interpreted-text role="py:const"} or ~socket.AF_INET6{.interpreted-text role="py:const"} depending on host (or the family argument, if provided).

The socket type will be ~socket.SOCK_STREAM{.interpreted-text role="py:const"}.

protocol_factory must be a callable returning an asyncio protocol <asyncio-protocol>{.interpreted-text role="ref"} 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:

1. The connection is established and a transport <asyncio-transport>{.interpreted-text role="ref"} is created for it.
2. protocol_factory is called without arguments and is expected to return a protocol <asyncio-protocol>{.interpreted-text role="ref"} instance.
3. The protocol instance is coupled with the transport by calling its ~BaseProtocol.connection_made{.interpreted-text role="meth"} method.
4. 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 ssl.SSLContext{.interpreted-text role="class"} object, this context is used to create the transport; if ssl is True{.interpreted-text role="const"}, a default context returned from ssl.create_default_context{.interpreted-text role="func"} is used.

  ::: seealso SSL/TLS security considerations <ssl-security>{.interpreted-text role="ref"} :::

• 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 socket{.interpreted-text role="mod"} 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 8305{.interpreted-text role="rfc"}. 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 getaddrinfo{.interpreted-text role="meth"}. 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 8305{.interpreted-text role="rfc"}. 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 socket.socket{.interpreted-text role="class"} 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 ::: title Note :::

  The sock argument transfers ownership of the socket to the transport created. To close the socket, call the transport's ~asyncio.BaseTransport.close{.interpreted-text role="meth"} 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).

• all_errors determines what exceptions are raised when a connection cannot be created. By default, only a single Exception is raised: the first exception if there is only one or all errors have same message, or a single OSError with the error messages combined. When all_errors is True, an ExceptionGroup will be raised containing all exceptions (even if there is only one).

::: versionchanged 3.5

Added support for SSL/TLS in ProactorEventLoop{.interpreted-text role="class"}. :::

::: versionchanged 3.6

The socket option socket.TCP_NODELAY <socket-unix-constants>{.interpreted-text role="ref"} 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://datatracker.ietf.org/doc/html/rfc6555 :::

::: versionchanged 3.11

Added the ssl_shutdown_timeout parameter. :::

::: versionchanged 3.12 all_errors was added. :::

::: seealso The open_connection{.interpreted-text role="func"} function is a high-level alternative API. It returns a pair of (StreamReader{.interpreted-text role="class"}, StreamWriter{.interpreted-text role="class"}) that can be used directly in async/await code. ::: ::::::::::

::::::: {.method async=""} 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 ~socket.AF_INET{.interpreted-text role="py:const"}, ~socket.AF_INET6{.interpreted-text role="py:const"}, or ~socket.AF_UNIX{.interpreted-text role="py:const"}, depending on host (or the family argument, if provided).

The socket type will be ~socket.SOCK_DGRAM{.interpreted-text role="py:const"}.

protocol_factory must be a callable returning a protocol <asyncio-protocol>{.interpreted-text role="ref"} 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 getaddrinfo{.interpreted-text role="meth"}.

  :::: note ::: title Note :::

  On Windows, when using the proactor event loop with local_addr=None, an OSError{.interpreted-text role="exc"} with !errno.WSAEINVAL{.interpreted-text role="attr"} will be raised when running it. ::::

• 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 getaddrinfo{.interpreted-text role="meth"}.

• family, proto, flags are the optional address family, protocol and flags to be passed through to getaddrinfo{.interpreted-text role="meth"} for host resolution. If given, these should all be integers from the corresponding socket{.interpreted-text role="mod"} 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 socket.SO_REUSEPORT <socket-unix-constants>{.interpreted-text role="ref"} 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, socket.socket{.interpreted-text role="class"} object to be used by the transport. If specified, local_addr and remote_addr should be omitted (must be None{.interpreted-text role="const"}).

  :::: note ::: title Note :::

  The sock argument transfers ownership of the socket to the transport created. To close the socket, call the transport's ~asyncio.BaseTransport.close{.interpreted-text role="meth"} method. ::::

See UDP echo client protocol <asyncio-udp-echo-client-protocol>{.interpreted-text role="ref"} and UDP echo server protocol <asyncio-udp-echo-server-protocol>{.interpreted-text role="ref"} examples.

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

::: versionchanged 3.8 Added support for Windows. :::

::: versionchanged 3.8.1 The reuse_address parameter is no longer supported, as using socket.SO_REUSEADDR <socket-unix-constants>{.interpreted-text role="ref"} 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, socket.SO_REUSEPORT <socket-unix-constants>{.interpreted-text role="ref"} is used instead, which specifically prevents processes with differing UIDs from assigning sockets to the same socket address. :::

::: versionchanged 3.11 The reuse_address parameter, disabled since Python 3.8.1, 3.7.6 and 3.6.10, has been entirely removed. ::: :::::::

:::::: {.method async=""} 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 ~socket.AF_UNIX{.interpreted-text role="py:const"}; socket type will be ~socket.SOCK_STREAM{.interpreted-text role="py:const"}.

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, str{.interpreted-text role="class"}, bytes{.interpreted-text role="class"}, and ~pathlib.Path{.interpreted-text role="class"} paths are supported.

See the documentation of the loop.create_connection{.interpreted-text role="meth"} 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 path-like object{.interpreted-text role="term"}. :::

::: versionchanged 3.11

Added the ssl_shutdown_timeout parameter. ::: ::::::

Creating network servers

:::::::::: {#loop_create_server} ::::::::: {.method async=""} 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, keep_alive=None, ssl_handshake_timeout=None, ssl_shutdown_timeout=None, start_serving=True)

Create a TCP server (socket type ~socket.SOCK_STREAM{.interpreted-text role="const"}) listening on port of the host address.

Returns a Server{.interpreted-text role="class"} object.

Arguments:

• protocol_factory must be a callable returning a protocol <asyncio-protocol>{.interpreted-text role="ref"} 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 socket.AF_INET{.interpreted-text role="const"} or ~socket.AF_INET6{.interpreted-text role="const"} to force the socket to use IPv4 or IPv6. If not set, the family will be determined from host name (defaults to ~socket.AF_UNSPEC{.interpreted-text role="const"}).

• flags is a bitmask for getaddrinfo{.interpreted-text role="meth"}.

• sock can optionally be specified in order to use a preexisting socket object. If specified, host and port must not be specified.

  :::: note ::: title Note :::

  The sock argument transfers ownership of the socket to the server created. To close the socket, call the server's ~asyncio.Server.close{.interpreted-text role="meth"} method. ::::

• backlog is the maximum number of queued connections passed to ~socket.socket.listen{.interpreted-text role="meth"} (defaults to 100).

• ssl can be set to an ~ssl.SSLContext{.interpreted-text role="class"} 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.

• keep_alive set to True keeps connections active by enabling the periodic transmission of messages.

::: versionchanged 3.13

Added the keep_alive parameter. :::

• 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 Server.start_serving{.interpreted-text role="meth"} or Server.serve_forever{.interpreted-text role="meth"} to make the server to start accepting connections.

::: versionchanged 3.5

Added support for SSL/TLS in ProactorEventLoop{.interpreted-text role="class"}. :::

::: 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 socket.TCP_NODELAY <socket-unix-constants>{.interpreted-text role="ref"} is set by default for all TCP connections. :::

::: versionchanged 3.11

Added the ssl_shutdown_timeout parameter. :::

::: seealso The start_server{.interpreted-text role="func"} function is a higher-level alternative API that returns a pair of StreamReader{.interpreted-text role="class"} and StreamWriter{.interpreted-text role="class"} that can be used in an async/await code. ::: ::::::::: ::::::::::

::::::: {.method async=""} 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, cleanup_socket=True)

Similar to loop.create_server{.interpreted-text role="meth"} but works with the ~socket.AF_UNIX{.interpreted-text role="py:const"} socket family.

path is the name of a Unix domain socket, and is required, unless a sock argument is provided. Abstract Unix sockets, str{.interpreted-text role="class"}, bytes{.interpreted-text role="class"}, and ~pathlib.Path{.interpreted-text role="class"} paths are supported.

If cleanup_socket is true then the Unix socket will automatically be removed from the filesystem when the server is closed, unless the socket has been replaced after the server has been created.

See the documentation of the loop.create_server{.interpreted-text role="meth"} 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 ~pathlib.Path{.interpreted-text role="class"} object. :::

::: versionchanged 3.11

Added the ssl_shutdown_timeout parameter. :::

::: versionchanged 3.13

Added the cleanup_socket parameter. ::: :::::::

:::::: {.method async=""} 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 protocol <asyncio-protocol>{.interpreted-text role="ref"} implementation.

• sock is a preexisting socket object returned from socket.accept <socket.socket.accept>{.interpreted-text role="meth"}.

  :::: note ::: title Note :::

  The sock argument transfers ownership of the socket to the transport created. To close the socket, call the transport's ~asyncio.BaseTransport.close{.interpreted-text role="meth"} method. ::::

• ssl can be set to an ~ssl.SSLContext{.interpreted-text role="class"} 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

:::: {.method async=""} 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 os.sendfile{.interpreted-text role="meth"} 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 file.tell() <io.IOBase.tell>{.interpreted-text role="meth"} 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 SendfileNotAvailableError{.interpreted-text role="exc"} if the system does not support the sendfile syscall and fallback is False.

::: versionadded 3.7 ::: ::::

TLS Upgrade

::::: {.method async=""} 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.

In some situations (e.g. when the passed transport is already closing) this may return None.

Parameters:

• transport and protocol instances that methods like ~loop.create_server{.interpreted-text role="meth"} and ~loop.create_connection{.interpreted-text role="meth"} return.
• sslcontext: a configured instance of ~ssl.SSLContext{.interpreted-text role="class"}.
• server_side pass True when a server-side connection is being upgraded (like the one created by ~loop.create_server{.interpreted-text role="meth"}).
• 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.

Any preexisting callback registered for fd is cancelled and replaced by callback. :::

::: 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 args once fd is available for writing.

Any preexisting callback registered for fd is cancelled and replaced by callback.

Use functools.partial{.interpreted-text role="func"} to pass keyword arguments <asyncio-pass-keywords>{.interpreted-text role="ref"} 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 Platform Support <asyncio-platform-support>{.interpreted-text role="ref"} section for some limitations of these methods.

Working with socket objects directly

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

:::: {.method async=""} loop.sock_recv(sock, nbytes)

Receive up to nbytes from sock. Asynchronous version of socket.recv() <socket.socket.recv>{.interpreted-text role="meth"}.

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 Future{.interpreted-text role="class"}. Since Python 3.7 this is an async def method. ::: ::::

:::: {.method async=""} loop.sock_recv_into(sock, buf)

Receive data from sock into the buf buffer. Modeled after the blocking socket.recv_into() <socket.socket.recv_into>{.interpreted-text role="meth"} method.

Return the number of bytes written to the buffer.

sock must be a non-blocking socket.

::: versionadded 3.7 ::: ::::

:::: {.method async=""} loop.sock_recvfrom(sock, bufsize)

Receive a datagram of up to bufsize from sock. Asynchronous version of socket.recvfrom() <socket.socket.recvfrom>{.interpreted-text role="meth"}.

Return a tuple of (received data, remote address).

sock must be a non-blocking socket.

::: versionadded 3.11 ::: ::::

:::: {.method async=""} loop.sock_recvfrom_into(sock, buf, nbytes=0)

Receive a datagram of up to nbytes from sock into buf. Asynchronous version of socket.recvfrom_into() <socket.socket.recvfrom_into>{.interpreted-text role="meth"}.

Return a tuple of (number of bytes received, remote address).

sock must be a non-blocking socket.

::: versionadded 3.11 ::: ::::

:::: {.method async=""} loop.sock_sendall(sock, data)

Send data to the sock socket. Asynchronous version of socket.sendall() <socket.socket.sendall>{.interpreted-text role="meth"}.

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 Future{.interpreted-text role="class"}. Since Python 3.7, this is an async def method. ::: ::::

:::: {.method async=""} loop.sock_sendto(sock, data, address)

Send a datagram from sock to address. Asynchronous version of socket.sendto() <socket.socket.sendto>{.interpreted-text role="meth"}.

Return the number of bytes sent.

sock must be a non-blocking socket.

::: versionadded 3.11 ::: ::::

::::: {.method async=""} loop.sock_connect(sock, address)

Connect sock to a remote socket at address.

Asynchronous version of socket.connect() <socket.socket.connect>{.interpreted-text role="meth"}.

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 socket.inet_pton{.interpreted-text role="func"}. If not, loop.getaddrinfo{.interpreted-text role="meth"} will be used to resolve the address. :::

::: seealso loop.create_connection{.interpreted-text role="meth"} and asyncio.open_connection() <open_connection>{.interpreted-text role="func"}. ::: :::::

::::: {.method async=""} loop.sock_accept(sock)

Accept a connection. Modeled after the blocking socket.accept() <socket.socket.accept>{.interpreted-text role="meth"} 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 Future{.interpreted-text role="class"}. Since Python 3.7, this is an async def method. :::

::: seealso loop.create_server{.interpreted-text role="meth"} and start_server{.interpreted-text role="func"}. ::: :::::

:::: {.method async=""} loop.sock_sendfile(sock, file, offset=0, count=None, *, fallback=True)

Send a file using high-performance os.sendfile{.interpreted-text role="mod"} if possible. Return the total number of bytes sent.

Asynchronous version of socket.sendfile() <socket.socket.sendfile>{.interpreted-text role="meth"}.

sock must be a non-blocking socket.SOCK_STREAM{.interpreted-text role="const"} ~socket.socket{.interpreted-text role="class"}.

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 file.tell() <io.IOBase.tell>{.interpreted-text role="meth"} 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 SendfileNotAvailableError{.interpreted-text role="exc"} if the system does not support sendfile syscall and fallback is False.

sock must be a non-blocking socket.

::: versionadded 3.7 ::: ::::

DNS

::: {.method async=""} loop.getaddrinfo(host, port, *, family=0, type=0, proto=0, flags=0)

Asynchronous version of socket.getaddrinfo{.interpreted-text role="meth"}. :::

::: {.method async=""} loop.getnameinfo(sockaddr, flags=0)

Asynchronous version of socket.getnameinfo{.interpreted-text role="meth"}. :::

:::: note ::: title Note :::

Both getaddrinfo and getnameinfo internally utilize their synchronous versions through the loop's default thread pool executor. When this executor is saturated, these methods may experience delays, which higher-level networking libraries may report as increased timeouts. To mitigate this, consider using a custom executor for other user tasks, or setting a default executor with a larger number of workers. ::::

::: 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 asyncio.Future{.interpreted-text role="class"} objects. Starting with Python 3.7 both methods are coroutines. :::

Working with pipes

::: {.method async=""} 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 asyncio protocol <asyncio-protocol>{.interpreted-text role="ref"} implementation.

pipe is a file-like object <file object>{.interpreted-text role="term"}.

Return pair (transport, protocol), where transport supports the ReadTransport{.interpreted-text role="class"} interface and protocol is an object instantiated by the protocol_factory.

With SelectorEventLoop{.interpreted-text role="class"} event loop, the pipe is set to non-blocking mode. :::

::: {.method async=""} 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 asyncio protocol <asyncio-protocol>{.interpreted-text role="ref"} implementation.

pipe is file-like object <file object>{.interpreted-text role="term"}.

Return pair (transport, protocol), where transport supports WriteTransport{.interpreted-text role="class"} interface and protocol is an object instantiated by the protocol_factory.

With SelectorEventLoop{.interpreted-text role="class"} event loop, the pipe is set to non-blocking mode. :::

:::: note ::: title Note :::

SelectorEventLoop{.interpreted-text role="class"} does not support the above methods on Windows. Use ProactorEventLoop{.interpreted-text role="class"} instead for Windows. ::::

::: seealso The loop.subprocess_exec{.interpreted-text role="meth"} and loop.subprocess_shell{.interpreted-text role="meth"} methods. :::

Unix signals

:::: {#loop_add_signal_handler} ::: method loop.add_signal_handler(signum, callback, *args)

Set callback as the handler for the signum signal, passing args as positional arguments.

The callback will be invoked by loop, along with other queued callbacks and runnable coroutines of that event loop. Unlike signal handlers registered using signal.signal{.interpreted-text role="func"}, a callback registered with this function is allowed to interact with the event loop.

Raise ValueError{.interpreted-text role="exc"} if the signal number is invalid or uncatchable. Raise RuntimeError{.interpreted-text role="exc"} if there is a problem setting up the handler.

Use functools.partial{.interpreted-text role="func"} to pass keyword arguments <asyncio-pass-keywords>{.interpreted-text role="ref"} to callback.

Like signal.signal{.interpreted-text role="func"}, 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 signal{.interpreted-text role="mod"} 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 passing args as positional arguments.

The executor argument should be an concurrent.futures.Executor{.interpreted-text role="class"} instance. The default executor is used if executor is None. The default executor can be set by loop.set_default_executor{.interpreted-text role="meth"}, otherwise, a concurrent.futures.ThreadPoolExecutor{.interpreted-text role="class"} will be lazy-initialized and used by run_in_executor{.interpreted-text role="func"} if needed.

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)

    # 4. Run in a custom interpreter pool:
    with concurrent.futures.InterpreterPoolExecutor() as pool:
        result = await loop.run_in_executor(
            pool, cpu_bound)
        print('custom interpreter 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 multiprocessing{.interpreted-text role="mod"}, which is used by ~concurrent.futures.ProcessPoolExecutor{.interpreted-text role="class"}. See Safe importing of main module <multiprocessing-safe-main-import>{.interpreted-text role="ref"}.

This method returns a asyncio.Future{.interpreted-text role="class"} object.

Use functools.partial{.interpreted-text role="func"} to pass keyword arguments <asyncio-pass-keywords>{.interpreted-text role="ref"} to func.

::: versionchanged 3.5.3 loop.run_in_executor{.interpreted-text role="meth"} no longer configures the max_workers of the thread pool executor it creates, instead leaving it up to the thread pool executor (~concurrent.futures.ThreadPoolExecutor{.interpreted-text role="class"}) to set the default. ::: ::::

:::: method loop.set_default_executor(executor)

Set executor as the default executor used by run_in_executor{.interpreted-text role="meth"}. executor must be an instance of ~concurrent.futures.ThreadPoolExecutor{.interpreted-text role="class"}, which includes ~concurrent.futures.InterpreterPoolExecutor{.interpreted-text role="class"}.

::: versionchanged 3.11 executor must be an instance of ~concurrent.futures.ThreadPoolExecutor{.interpreted-text role="class"}. ::: ::::

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 call_exception_handler{.interpreted-text role="meth"} documentation for details about context).

If the handler is called on behalf of a ~asyncio.Task{.interpreted-text role="class"} or ~asyncio.Handle{.interpreted-text role="class"}, it is run in the contextvars.Context{.interpreted-text role="class"} of that task or callback handle.

::: versionchanged 3.12

The handler may be called in the ~contextvars.Context{.interpreted-text role="class"} of the task or handle where the exception originated. ::: ::::

:::: 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 call_exception_handler{.interpreted-text role="meth"}. :::

::::: 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): asyncio.Future{.interpreted-text role="class"} instance;

• 'task' (optional): asyncio.Task{.interpreted-text role="class"} instance;

• 'handle' (optional): asyncio.Handle{.interpreted-text role="class"} instance;

• 'protocol' (optional): Protocol <asyncio-protocol>{.interpreted-text role="ref"} instance;

• 'transport' (optional): Transport <asyncio-transport>{.interpreted-text role="ref"} instance;

• 'socket' (optional): socket.socket{.interpreted-text role="class"} instance;

• 'source_traceback' (optional): Traceback of the source;

• 'handle_traceback' (optional): Traceback of the handle;

• 'asyncgen' (optional): Asynchronous generator that caused

  : the exception.

:::: note ::: title Note :::

This method should not be overloaded in subclassed event loops. For custom exception handling, use the set_exception_handler{.interpreted-text role="meth"} method. :::: :::::

Enabling debug mode

::: method loop.get_debug()

Get the debug mode (bool{.interpreted-text role="class"}) of the event loop.

The default value is True if the environment variable PYTHONASYNCIODEBUG{.interpreted-text role="envvar"} 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 Python Development Mode <devmode>{.interpreted-text role="ref"} can now also be used to enable the debug mode. ::: ::::

::: attribute loop.slow_callback_duration

This attribute can be used to set the minimum execution duration in seconds that is considered "slow". When debug mode is enabled, "slow" callbacks are logged.

Default value is 100 milliseconds. :::

::: seealso The debug mode of asyncio <asyncio-debug-mode>{.interpreted-text role="ref"}. :::

Running Subprocesses

Methods described in this subsections are low-level. In regular async/await code consider using the high-level asyncio.create_subprocess_shell{.interpreted-text role="func"} and asyncio.create_subprocess_exec{.interpreted-text role="func"} convenience functions instead.

:::: note ::: title Note :::

On Windows, the default event loop ProactorEventLoop{.interpreted-text role="class"} supports subprocesses, whereas SelectorEventLoop{.interpreted-text role="class"} does not. See Subprocess Support on Windows <asyncio-windows-subprocess>{.interpreted-text role="ref"} for details. ::::

:::: {#loop_subprocess_exec} ::: {.method async=""} 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:

• str{.interpreted-text role="class"};
• or bytes{.interpreted-text role="class"}, encoded to the filesystem encoding <filesystem-encoding>{.interpreted-text role="ref"}.

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 subprocess.Popen{.interpreted-text role="class"} class called with shell=False and the list of strings passed as the first argument; however, where ~subprocess.Popen{.interpreted-text role="class"} 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 asyncio.SubprocessProtocol{.interpreted-text role="class"} class.

Other parameters:

• stdin can be any of these:

  • a file-like object
  • an existing file descriptor (a positive integer), for example those created with os.pipe{.interpreted-text role="meth"}
  • the subprocess.PIPE{.interpreted-text role="const"} 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 subprocess.DEVNULL{.interpreted-text role="const"} constant which indicates that the special os.devnull{.interpreted-text role="data"} file will be used

• stdout can be any of these:

  • a file-like object
  • the subprocess.PIPE{.interpreted-text role="const"} 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 subprocess.DEVNULL{.interpreted-text role="const"} constant which indicates that the special os.devnull{.interpreted-text role="data"} file will be used

• stderr can be any of these:

  • a file-like object
  • the subprocess.PIPE{.interpreted-text role="const"} 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 subprocess.DEVNULL{.interpreted-text role="const"} constant which indicates that the special os.devnull{.interpreted-text role="data"} file will be used
  • the subprocess.STDOUT{.interpreted-text role="const"} constant which will connect the standard error stream to the process' standard output stream

• All other keyword arguments are passed to subprocess.Popen{.interpreted-text role="class"} 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. bytes.decode{.interpreted-text role="func"} can be used to convert the bytes returned from the stream to text.

If a file-like object passed as stdin, stdout or stderr represents a pipe, then the other side of this pipe should be registered with ~loop.connect_write_pipe{.interpreted-text role="meth"} or ~loop.connect_read_pipe{.interpreted-text role="meth"} for use with the event loop.

See the constructor of the subprocess.Popen{.interpreted-text role="class"} class for documentation on other arguments.

Returns a pair of (transport, protocol), where transport conforms to the asyncio.SubprocessTransport{.interpreted-text role="class"} base class and protocol is an object instantiated by the protocol_factory.

If the transport is closed or is garbage collected, the child process is killed if it is still running. ::: ::::

::: {.method async=""} 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 str{.interpreted-text role="class"} or a bytes{.interpreted-text role="class"} string encoded to the filesystem encoding <filesystem-encoding>{.interpreted-text role="ref"}, using the platform's "shell" syntax.

This is similar to the standard library subprocess.Popen{.interpreted-text role="class"} class called with shell=True.

The protocol_factory must be a callable returning a subclass of the SubprocessProtocol{.interpreted-text role="class"} class.

See ~loop.subprocess_exec{.interpreted-text role="meth"} for more details about the remaining arguments.

Returns a pair of (transport, protocol), where transport conforms to the SubprocessTransport{.interpreted-text role="class"} base class and protocol is an object instantiated by the protocol_factory.

If the transport is closed or is garbage collected, the child process is killed if it is still running. :::

:::: note ::: title Note :::

It is the application's responsibility to ensure that all whitespace and special characters are quoted appropriately to avoid shell injection vulnerabilities. The shlex.quote{.interpreted-text role="func"} 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

:::::::: Handle A callback wrapper object returned by loop.call_soon{.interpreted-text role="meth"}, loop.call_soon_threadsafe{.interpreted-text role="meth"}.

:::: method get_context()

Return the contextvars.Context{.interpreted-text role="class"} object associated with the handle.

::: versionadded 3.12 ::: ::::

::: 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 ::: :::: ::::::::

::::: TimerHandle A callback wrapper object returned by loop.call_later{.interpreted-text role="meth"}, and loop.call_at{.interpreted-text role="meth"}.

This class is a subclass of Handle{.interpreted-text role="class"}.

:::: method when()

Return a scheduled callback time as float{.interpreted-text role="class"} seconds.

The time is an absolute timestamp, using the same time reference as loop.time{.interpreted-text role="meth"}.

::: versionadded 3.7 ::: :::: :::::

Server Objects

Server objects are created by loop.create_server{.interpreted-text role="meth"}, loop.create_unix_server{.interpreted-text role="meth"}, start_server{.interpreted-text role="func"}, and start_unix_server{.interpreted-text role="func"} functions.

Do not instantiate the Server{.interpreted-text role="class"} class directly.

::::::::::::::::::::: 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. :::

::: versionchanged 3.11 This class was exposed publicly as asyncio.Server in Python 3.9.11, 3.10.3 and 3.11. :::

::: method close()

Stop serving: close listening sockets and set the sockets{.interpreted-text role="attr"} attribute to None.

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

The server is closed asynchronously; use the wait_closed{.interpreted-text role="meth"} coroutine to wait until the server is closed (and no more connections are active). :::

:::: method close_clients()

Close all existing incoming client connections.

Calls ~asyncio.BaseTransport.close{.interpreted-text role="meth"} on all associated transports.

close{.interpreted-text role="meth"} should be called before close_clients{.interpreted-text role="meth"} when closing the server to avoid races with new clients connecting.

::: versionadded 3.13 ::: ::::

:::: method abort_clients()

Close all existing incoming client connections immediately, without waiting for pending operations to complete.

Calls ~asyncio.WriteTransport.abort{.interpreted-text role="meth"} on all associated transports.

close{.interpreted-text role="meth"} should be called before abort_clients{.interpreted-text role="meth"} when closing the server to avoid races with new clients connecting.

::: versionadded 3.13 ::: ::::

:::: method get_loop()

Return the event loop associated with the server object.

::: versionadded 3.7 ::: ::::

:::: {.method async=""} 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 loop.create_server{.interpreted-text role="meth"} and asyncio.start_server{.interpreted-text role="meth"} allows creating a Server object that is not accepting connections initially. In this case Server.start_serving(), or Server.serve_forever{.interpreted-text role="meth"} can be used to make the Server start accepting connections.

::: versionadded 3.7 ::: ::::

:::: {.method async=""} 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 ::: ::::

::: {.method async=""} wait_closed()

Wait until the close{.interpreted-text role="meth"} method completes and all active connections have finished. :::

:::: attribute sockets

List of socket-like objects, asyncio.trsock.TransportSocket, which 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. ::: :::: :::::::::::::::::::::

Event Loop Implementations[]{#asyncio-event-loops} {#asyncio-event-loop-implementations}

asyncio ships with two different event loop implementations: SelectorEventLoop{.interpreted-text role="class"} and ProactorEventLoop{.interpreted-text role="class"}.

By default asyncio is configured to use EventLoop{.interpreted-text role="class"}.

:::: SelectorEventLoop A subclass of AbstractEventLoop{.interpreted-text role="class"} based on the selectors{.interpreted-text role="mod"} 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

async def main():
   ...

loop_factory = lambda: asyncio.SelectorEventLoop(selectors.SelectSelector())
asyncio.run(main(), loop_factory=loop_factory)

::: availability Unix, Windows. ::: ::::

::::: ProactorEventLoop A subclass of AbstractEventLoop{.interpreted-text role="class"} for Windows that uses "I/O Completion Ports" (IOCP).

::: availability Windows. :::

::: seealso MSDN documentation on I/O Completion Ports. ::: :::::

:::: EventLoop An alias to the most efficient available subclass of AbstractEventLoop{.interpreted-text role="class"} for the given platform.

It is an alias to SelectorEventLoop{.interpreted-text role="class"} on Unix and ProactorEventLoop{.interpreted-text role="class"} on Windows.

::: versionadded 3.13 ::: ::::

::: AbstractEventLoop Abstract base class for asyncio-compliant event loops.

The asyncio-event-loop-methods{.interpreted-text role="ref"} 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 loop.run_forever{.interpreted-text role="meth"} and loop.call_soon{.interpreted-text role="meth"}. Modern asyncio applications rarely need to be written this way; consider using the high-level functions like asyncio.run{.interpreted-text role="func"}.

Hello World with call_soon() {#asyncio_example_lowlevel_helloworld}

An example using the loop.call_soon{.interpreted-text role="meth"} 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 Hello World <coroutine>{.interpreted-text role="ref"} example created with a coroutine and the run{.interpreted-text role="func"} function. :::

Display the current date with call_later() {#asyncio_example_call_later}

An example of a callback displaying the current date every second. The callback uses the loop.call_later{.interpreted-text role="meth"} 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 current date <asyncio_example_sleep>{.interpreted-text role="ref"} example created with a coroutine and the run{.interpreted-text role="func"} function. :::

Watch a file descriptor for read events {#asyncio_example_watch_fd}

Wait until a file descriptor received some data using the loop.add_reader{.interpreted-text role="meth"} 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 example <asyncio_example_create_connection>{.interpreted-text role="ref"} using transports, protocols, and the loop.create_connection{.interpreted-text role="meth"} method.
• Another similar example <asyncio_example_create_connection-streams>{.interpreted-text role="ref"} using the high-level asyncio.open_connection{.interpreted-text role="func"} function and streams. :::

Set signal handlers for SIGINT and SIGTERM {#asyncio_example_unix_signals}

(This signals example only works on Unix.)

Register handlers for signals ~signal.SIGINT{.interpreted-text role="const"} and ~signal.SIGTERM{.interpreted-text role="const"} using the loop.add_signal_handler{.interpreted-text role="meth"} 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())