Event Loop

Event Loop¶
Source code: Lib/asyncio/events.py, Lib/asyncio/base_events.py
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()
, 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:
- asyncio.get_running_loop()¶
Return the running event loop in the current OS thread.
Raise a
RuntimeError
if there is no running event loop.This function can only be called from a coroutine or a callback.
Added in version 3.7.
- asyncio.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()
function is preferred toget_event_loop()
in coroutines and callbacks.As noted above, consider using the higher-level
asyncio.run()
function, instead of using these lower level functions to manually create and close an event loop.Changed in version 3.14: Raises a
RuntimeError
if there is no current event loop.Note
The
asyncio
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 byset_event_loop()
.
- asyncio.set_event_loop(loop)¶
Set loop as the current event loop for the current OS thread.
- asyncio.new_event_loop()¶
Create and return a new event loop object.
Note that the behaviour of get_event_loop()
, set_event_loop()
,
and new_event_loop()
functions can be altered by
setting a custom event loop policy.
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
andTimerHandle
instances which are returned from scheduling methods such asloop.call_soon()
andloop.call_later()
;The Server Objects section documents types returned from event loop methods like
loop.create_server()
;The Event Loop Implementations section documents the
SelectorEventLoop
andProactorEventLoop
classes;The Examples section showcases how to work with some event loop APIs.
Event Loop Methods¶
Event loops have low-level APIs for the following:
Running and stopping the loop¶
- loop.run_until_complete(future)¶
Run until the future (an instance of
Future
) has completed.If the argument is a coroutine object it is implicitly scheduled to run as a
asyncio.Task
.Return the Future’s result or raise its exception.
- loop.run_forever()¶
Run the event loop until
stop()
is called.If
stop()
is called beforerun_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
stop()
is called whilerun_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 timerun_forever()
orrun_until_complete()
is called.
- loop.stop()¶
Stop the event loop.
- loop.is_running()¶
Return
True
if the event loop is currently running.
- loop.is_closed()¶
Return
True
if the event loop was closed.
- 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.
- async loop.shutdown_asyncgens()¶
Schedule all currently open asynchronous generator objects to close with an
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
asyncio.run()
is used.Example:
try: loop.run_forever() finally: loop.run_until_complete(loop.shutdown_asyncgens()) loop.close()
Added in version 3.6.
- 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
ThreadPoolExecutor
. Once this method has been called, using the default executor withloop.run_in_executor()
will raise aRuntimeError
.The timeout parameter specifies the amount of time (in
float
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
is emitted and the default executor is terminated without waiting for its threads to finish joining.Note
Do not call this method when using
asyncio.run()
, as the latter handles default executor shutdown automatically.Added in version 3.9.
Changed in version 3.12: Added the timeout parameter.
Scheduling callbacks¶
- loop.call_soon(callback, *args, context=None)¶
Schedule the callback callback to be called with args arguments at the next iteration of the event loop.
Return an instance of
asyncio.Handle
, 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
for the callback to run in. Callbacks use the current context when no context is provided.Unlike
call_soon_threadsafe()
, this method is not thread-safe.
- loop.call_soon_threadsafe(callback, *args, context=None)¶
A thread-safe variant of
call_soon()
. When scheduling callbacks from another thread, this function must be used, sincecall_soon()
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
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 section of the documentation.
Changed in version 3.7: The context keyword-only parameter was added. See PEP 567 for more details.
Note
Most asyncio
scheduling functions don’t allow passing
keyword arguments. To do that, use 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.
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.
- 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
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()
to pass keyword arguments to callback.An optional keyword-only context argument allows specifying a custom
contextvars.Context
for the callback to run in. The current context is used when no context is provided.Note
For performance, callbacks scheduled with
loop.call_later()
may run up to one clock-resolution early (seetime.get_clock_info('monotonic').resolution
).Changed in version 3.7: The context keyword-only parameter was added. See PEP 567 for more details.
Changed in version 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.
- 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()
.This method’s behavior is the same as
call_later()
.An instance of
asyncio.TimerHandle
is returned which can be used to cancel the callback.Note
For performance, callbacks scheduled with
loop.call_at()
may run up to one clock-resolution early (seetime.get_clock_info('monotonic').resolution
).Changed in version 3.7: The context keyword-only parameter was added. See PEP 567 for more details.
Changed in version 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.
- loop.time()¶
Return the current time, as a
float
value, according to the event loop’s internal monotonic clock.
Note
Changed in version 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.
See also
The asyncio.sleep()
function.
Creating Futures and Tasks¶
- loop.create_future()¶
Create an
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).
Added in version 3.5.2.
- loop.create_task(coro, *, name=None, context=None, eager_start=None, **kwargs)¶
Schedule the execution of coroutine coro. Return a
Task
object.Third-party event loops can use their own subclass of
Task
for interoperability. In this case, the result type is a subclass ofTask
.The full function signature is largely the same as that of the
Task
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 usingTask.set_name()
.An optional keyword-only context argument allows specifying a custom
contextvars.Context
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()
will be used.Changed in version 3.8: Added the name parameter.
Changed in version 3.11: Added the context parameter.
Changed in version 3.13.3: Added
kwargs
which passes on arbitrary extra parameters, includingname
andcontext
.Changed in version 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).
Changed in version 3.14: All kwargs are now passed on. The eager_start parameter works with eager task factories.
- loop.set_task_factory(factory)¶
Set a task factory that will be used by
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, **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 aasyncio.Task
-compatible object.Changed in version 3.13.3: Required that all kwargs are passed on to
asyncio.Task
.Changed in version 3.13.4: name is no longer passed to task factories. context is no longer passed to task factories if it is
None
.Changed in version 3.14: name and context are now unconditionally passed on to task factories again.
- loop.get_task_factory()¶
Return a task factory or
None
if the default one is in use.
Opening network connections¶
- 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
AF_INET
orAF_INET6
depending on host (or the family argument, if provided).The socket type will be
SOCK_STREAM
.protocol_factory must be a callable returning an 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 transport is created for it.
protocol_factory is called without arguments and is expected to return a protocol instance.
The protocol instance is coupled with the transport by calling its
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
ssl.SSLContext
object, this context is used to create the transport; if ssl isTrue
, a default context returned fromssl.create_default_context()
is used.See also
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
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 bygetaddrinfo()
. 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 is0
if happy_eyeballs_delay is not specified, and1
if it is.sock, if given, should be an existing, already connected
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
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 usinggetaddrinfo()
, 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 ifNone
(default).ssl_shutdown_timeout is the time in seconds to wait for the SSL shutdown to complete before aborting the connection.
30.0
seconds ifNone
(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 singleOSError
with the error messages combined. Whenall_errors
isTrue
, anExceptionGroup
will be raised containing all exceptions (even if there is only one).
Changed in version 3.5: Added support for SSL/TLS in
ProactorEventLoop
.Changed in version 3.6: The socket option socket.TCP_NODELAY is set by default for all TCP connections.
Changed in version 3.7: Added the ssl_handshake_timeout parameter.
Changed in version 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
Changed in version 3.11: Added the ssl_shutdown_timeout parameter.
Changed in version 3.12: all_errors was added.
See also
The
open_connection()
function is a high-level alternative API. It returns a pair of (StreamReader
,StreamWriter
) that can be used directly in async/await code.
- 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
AF_INET
,AF_INET6
, orAF_UNIX
, depending on host (or the family argument, if provided).The socket type will be
SOCK_DGRAM
.protocol_factory must be a callable returning a 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 usinggetaddrinfo()
.Note
On Windows, when using the proactor event loop with
local_addr=None
, anOSError
witherrno.WSAEINVAL
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 usinggetaddrinfo()
.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 correspondingsocket
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 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
object to be used by the transport. If specified, local_addr and remote_addr should be omitted (must beNone
).Note
The sock argument transfers ownership of the socket to the transport created. To close the socket, call the transport’s
close()
method.
See UDP echo client protocol and UDP echo server protocol examples.
Changed in version 3.4.4: The family, proto, flags, reuse_address, reuse_port, allow_broadcast, and sock parameters were added.
Changed in version 3.8: Added support for Windows.
Changed in version 3.8.1: The reuse_address parameter is no longer supported, as using socket.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, socket.SO_REUSEPORT is used instead, which specifically prevents processes with differing UIDs from assigning sockets to the same socket address.
Changed in version 3.11: The reuse_address parameter, disabled since Python 3.8.1, 3.7.6 and 3.6.10, has been entirely removed.
- 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
AF_UNIX
; socket type will beSOCK_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,
str
,bytes
, andPath
paths are supported.See the documentation of the
loop.create_connection()
method for information about arguments to this method.Availability: Unix.
Changed in version 3.7: Added the ssl_handshake_timeout parameter. The path parameter can now be a path-like object.
Changed in version 3.11: Added the ssl_shutdown_timeout parameter.
Creating network servers¶
- 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
SOCK_STREAM
) listening on port of the host address.Returns a
Server
object.Arguments:
protocol_factory must be a callable returning a 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
orNone
(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
orAF_INET6
to force the socket to use IPv4 or IPv6. If not set, the family will be determined from host name (defaults toAF_UNSPEC
).flags is a bitmask for
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
close()
method.backlog is the maximum number of queued connections passed to
listen()
(defaults to 100).ssl can be set to an
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 toTrue
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.
Changed in version 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 ifNone
(default).ssl_shutdown_timeout is the time in seconds to wait for the SSL shutdown to complete before aborting the connection.
30.0
seconds ifNone
(default).start_serving set to
True
(the default) causes the created server to start accepting connections immediately. When set toFalse
, the user should await onServer.start_serving()
orServer.serve_forever()
to make the server to start accepting connections.
Changed in version 3.5: Added support for SSL/TLS in
ProactorEventLoop
.Changed in version 3.5.1: The host parameter can be a sequence of strings.
Changed in version 3.6: Added ssl_handshake_timeout and start_serving parameters. The socket option socket.TCP_NODELAY is set by default for all TCP connections.
Changed in version 3.11: Added the ssl_shutdown_timeout parameter.
See also
The
start_server()
function is a higher-level alternative API that returns a pair ofStreamReader
andStreamWriter
that can be used in an async/await code.
- 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()
but works with theAF_UNIX
socket family.path is the name of a Unix domain socket, and is required, unless a sock argument is provided. Abstract Unix sockets,
str
,bytes
, andPath
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()
method for information about arguments to this method.Availability: Unix.
Changed in version 3.7: Added the ssl_handshake_timeout and start_serving parameters. The path parameter can now be a
Path
object.Changed in version 3.11: Added the ssl_shutdown_timeout parameter.
Changed in version 3.13: Added the cleanup_socket parameter.
- 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 implementation.
sock is a preexisting socket object returned from
socket.accept
.Note
The sock argument transfers ownership of the socket to the transport created. To close the socket, call the transport’s
close()
method.ssl can be set to an
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 ifNone
(default).ssl_shutdown_timeout is the time in seconds to wait for the SSL shutdown to complete before aborting the connection.
30.0
seconds ifNone
(default).
Returns a
(transport, protocol)
pair.Added in version 3.5.3.
Changed in version 3.7: Added the ssl_handshake_timeout parameter.
Changed in version 3.11: Added the ssl_shutdown_timeout parameter.
Transferring files¶
- 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()
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()
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
if the system does not support the sendfile syscall and fallback isFalse
.Added in version 3.7.
TLS Upgrade¶
- 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
create_server()
andcreate_connection()
return.sslcontext: a configured instance of
SSLContext
.server_side pass
True
when a server-side connection is being upgraded (like the one created bycreate_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 ifNone
(default).ssl_shutdown_timeout is the time in seconds to wait for the SSL shutdown to complete before aborting the connection.
30.0
seconds ifNone
(default).
Added in version 3.7.
Changed in version 3.11: Added the ssl_shutdown_timeout parameter.
Watching file descriptors¶
- 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.
- loop.remove_reader(fd)¶
Stop monitoring the fd file descriptor for read availability. Returns
True
if fd was previously being monitored for reads.
- 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()
to pass keyword arguments to callback.
- 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 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()
and 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 socket
objects directly is more
convenient.
- async loop.sock_recv(sock, nbytes)¶
Receive up to nbytes from sock. Asynchronous version of
socket.recv()
.Return the received data as a bytes object.
sock must be a non-blocking socket.
Changed in version 3.7: Even though this method was always documented as a coroutine method, releases before Python 3.7 returned a
Future
. Since Python 3.7 this is anasync def
method.
- async loop.sock_recv_into(sock, buf)¶
Receive data from sock into the buf buffer. Modeled after the blocking
socket.recv_into()
method.Return the number of bytes written to the buffer.
sock must be a non-blocking socket.
Added in version 3.7.
- async loop.sock_recvfrom(sock, bufsize)¶
Receive a datagram of up to bufsize from sock. Asynchronous version of
socket.recvfrom()
.Return a tuple of (received data, remote address).
sock must be a non-blocking socket.
Added in version 3.11.
- 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()
.Return a tuple of (number of bytes received, remote address).
sock must be a non-blocking socket.
Added in version 3.11.
- async loop.sock_sendall(sock, data)¶
Send data to the sock socket. Asynchronous version of
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.
Changed in version 3.7: Even though the method was always documented as a coroutine method, before Python 3.7 it returned a
Future
. Since Python 3.7, this is anasync def
method.
- async loop.sock_sendto(sock, data, address)¶
Send a datagram from sock to address. Asynchronous version of
socket.sendto()
.Return the number of bytes sent.
sock must be a non-blocking socket.
Added in version 3.11.
- async loop.sock_connect(sock, address)¶
Connect sock to a remote socket at address.
Asynchronous version of
socket.connect()
.sock must be a non-blocking socket.
Changed in version 3.5.2:
address
no longer needs to be resolved.sock_connect
will try to check if the address is already resolved by callingsocket.inet_pton()
. If not,loop.getaddrinfo()
will be used to resolve the address.See also
- async loop.sock_accept(sock)¶
Accept a connection. Modeled after the blocking
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.
Changed in version 3.7: Even though the method was always documented as a coroutine method, before Python 3.7 it returned a
Future
. Since Python 3.7, this is anasync def
method.See also
- async loop.sock_sendfile(sock, file, offset=0, count=None, *, fallback=True)¶
Send a file using high-performance
os.sendfile
if possible. Return the total number of bytes sent.Asynchronous version of
socket.sendfile()
.sock must be a non-blocking
socket.SOCK_STREAM
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
file.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
SendfileNotAvailableError
if the system does not support sendfile syscall and fallback isFalse
.sock must be a non-blocking socket.
Added in version 3.7.
DNS¶
- async loop.getaddrinfo(host, port, *, family=0, type=0, proto=0, flags=0)¶
Asynchronous version of
socket.getaddrinfo()
.
- async loop.getnameinfo(sockaddr, flags=0)¶
Asynchronous version of
socket.getnameinfo()
.
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.
Changed in version 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
objects. Starting with Python 3.7
both methods are coroutines.
Working with pipes¶
- 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 implementation.
pipe is a file-like object.
Return pair
(transport, protocol)
, where transport supports theReadTransport
interface and protocol is an object instantiated by the protocol_factory.With
SelectorEventLoop
event loop, the pipe is set to non-blocking mode.
- 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 implementation.
pipe is file-like object.
Return pair
(transport, protocol)
, where transport supportsWriteTransport
interface and protocol is an object instantiated by the protocol_factory.With
SelectorEventLoop
event loop, the pipe is set to non-blocking mode.
Note
SelectorEventLoop
does not support the above methods on
Windows. Use ProactorEventLoop
instead for Windows.
See also
The loop.subprocess_exec()
and
loop.subprocess_shell()
methods.
Unix signals¶
- 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()
, a callback registered with this function is allowed to interact with the event loop.Raise
ValueError
if the signal number is invalid or uncatchable. RaiseRuntimeError
if there is a problem setting up the handler.Use
functools.partial()
to pass keyword arguments to callback.Like
signal.signal()
, this function must be invoked in the main thread.
- loop.remove_signal_handler(sig)¶
Remove the handler for the sig signal.
Return
True
if the signal handler was removed, orFalse
if no handler was set for the given signal.Availability: Unix.
See also
The signal
module.
Executing code in thread or process pools¶
- awaitable 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
instance. The default executor is used if executor isNone
. The default executor can be set byloop.set_default_executor()
, otherwise, aconcurrent.futures.ThreadPoolExecutor
will be lazy-initialized and used byrun_in_executor()
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 ofmultiprocessing
, which is used byProcessPoolExecutor
. See Safe importing of main module.This method returns a
asyncio.Future
object.Use
functools.partial()
to pass keyword arguments to func.Changed in version 3.5.3:
loop.run_in_executor()
no longer configures themax_workers
of the thread pool executor it creates, instead leaving it up to the thread pool executor (ThreadPoolExecutor
) to set the default.
- loop.set_default_executor(executor)¶
Set executor as the default executor used by
run_in_executor()
. executor must be an instance ofThreadPoolExecutor
, which includesInterpreterPoolExecutor
.Changed in version 3.11: executor must be an instance of
ThreadPoolExecutor
.
Error Handling API¶
Allows customizing how exceptions are handled in the event loop.
- 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)
, whereloop
is a reference to the active event loop, andcontext
is adict
object containing the details of the exception (seecall_exception_handler()
documentation for details about context).If the handler is called on behalf of a
Task
orHandle
, it is run in thecontextvars.Context
of that task or callback handle.Changed in version 3.12: The handler may be called in the
Context
of the task or handle where the exception originated.
- loop.get_exception_handler()¶
Return the current exception handler, or
None
if no custom exception handler was set.Added in version 3.5.2.
- 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()
.
- 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
instance;‘task’ (optional):
asyncio.Task
instance;‘handle’ (optional):
asyncio.Handle
instance;‘protocol’ (optional): Protocol instance;
‘transport’ (optional): Transport instance;
‘socket’ (optional):
socket.socket
instance;‘source_traceback’ (optional): Traceback of the source;
‘handle_traceback’ (optional): Traceback of the handle;
- ‘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
set_exception_handler()
method.
Enabling debug mode¶
- loop.get_debug()¶
Get the debug mode (
bool
) of the event loop.The default value is
True
if the environment variablePYTHONASYNCIODEBUG
is set to a non-empty string,False
otherwise.
- loop.set_debug(enabled: bool)¶
Set the debug mode of the event loop.
Changed in version 3.7: The new Python Development Mode can now also be used to enable the debug mode.
- 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.
See also
Running Subprocesses¶
Methods described in this subsections are low-level. In regular
async/await code consider using the high-level
asyncio.create_subprocess_shell()
and
asyncio.create_subprocess_exec()
convenience functions instead.
Note
On Windows, the default event loop ProactorEventLoop
supports
subprocesses, whereas SelectorEventLoop
does not. See
Subprocess Support on Windows for
details.
- 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
;or
bytes
, encoded to the 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
subprocess.Popen
class called withshell=False
and the list of strings passed as the first argument; however, wherePopen
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
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()
the
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 processthe
subprocess.DEVNULL
constant which indicates that the specialos.devnull
file will be used
stdout can be any of these:
a file-like object
the
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 processthe
subprocess.DEVNULL
constant which indicates that the specialos.devnull
file will be used
stderr can be any of these:
a file-like object
the
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 processthe
subprocess.DEVNULL
constant which indicates that the specialos.devnull
file will be usedthe
subprocess.STDOUT
constant which will connect the standard error stream to the process’ standard output stream
All other keyword arguments are passed to
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.bytes.decode()
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
connect_write_pipe()
orconnect_read_pipe()
for use with the event loop.See the constructor of the
subprocess.Popen
class for documentation on other arguments.Returns a pair of
(transport, protocol)
, where transport conforms to theasyncio.SubprocessTransport
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.
- 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
or abytes
string encoded to the filesystem encoding, using the platform’s “shell” syntax.This is similar to the standard library
subprocess.Popen
class called withshell=True
.The protocol_factory must be a callable returning a subclass of the
SubprocessProtocol
class.See
subprocess_exec()
for more details about the remaining arguments.Returns a pair of
(transport, protocol)
, where transport conforms to theSubprocessTransport
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
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()
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 asyncio.Handle¶
A callback wrapper object returned by
loop.call_soon()
,loop.call_soon_threadsafe()
.- get_context()¶
Return the
contextvars.Context
object associated with the handle.Added in version 3.12.
- cancel()¶
Cancel the callback. If the callback has already been canceled or executed, this method has no effect.
- cancelled()¶
Return
True
if the callback was cancelled.Added in version 3.7.
- class asyncio.TimerHandle¶
A callback wrapper object returned by
loop.call_later()
, andloop.call_at()
.This class is a subclass of
Handle
.- when()¶
Return a scheduled callback time as
float
seconds.The time is an absolute timestamp, using the same time reference as
loop.time()
.Added in version 3.7.
Server Objects¶
Server objects are created by loop.create_server()
,
loop.create_unix_server()
, start_server()
,
and start_unix_server()
functions.
Do not instantiate the Server
class directly.
- class asyncio.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 theasync 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.
Changed in version 3.7: Server object is an asynchronous context manager since Python 3.7.
Changed in version 3.11: This class was exposed publicly as
asyncio.Server
in Python 3.9.11, 3.10.3 and 3.11.- close()¶
Stop serving: close listening sockets and set the
sockets
attribute toNone
.The sockets that represent existing incoming client connections are left open.
The server is closed asynchronously; use the
wait_closed()
coroutine to wait until the server is closed (and no more connections are active).
- close_clients()¶
Close all existing incoming client connections.
Calls
close()
on all associated transports.close()
should be called beforeclose_clients()
when closing the server to avoid races with new clients connecting.Added in version 3.13.
- abort_clients()¶
Close all existing incoming client connections immediately, without waiting for pending operations to complete.
Calls
abort()
on all associated transports.close()
should be called beforeabort_clients()
when closing the server to avoid races with new clients connecting.Added in version 3.13.
- get_loop()¶
Return the event loop associated with the server object.
Added in version 3.7.
- 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()
andasyncio.start_server()
allows creating a Server object that is not accepting connections initially. In this caseServer.start_serving()
, orServer.serve_forever()
can be used to make the Server start accepting connections.Added in version 3.7.
- 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))
Added in version 3.7.
- is_serving()¶
Return
True
if the server is accepting new connections.Added in version 3.7.
- async wait_closed()¶
Wait until the
close()
method completes and all active connections have finished.
- sockets¶
List of socket-like objects,
asyncio.trsock.TransportSocket
, which the server is listening on.Changed in version 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 ships with two different event loop implementations:
SelectorEventLoop
and ProactorEventLoop
.
By default asyncio is configured to use EventLoop
.
- class asyncio.SelectorEventLoop¶
A subclass of
AbstractEventLoop
based on theselectors
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.
- class asyncio.ProactorEventLoop¶
A subclass of
AbstractEventLoop
for Windows that uses “I/O Completion Ports” (IOCP).Availability: Windows.
- class asyncio.EventLoop¶
An alias to the most efficient available subclass of
AbstractEventLoop
for the given platform.It is an alias to
SelectorEventLoop
on Unix andProactorEventLoop
on Windows.Added in version 3.13.
- class asyncio.AbstractEventLoop¶
Abstract base class for asyncio-compliant event loops.
The 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 loop.run_forever()
and loop.call_soon()
. Modern asyncio applications rarely
need to be written this way; consider using the high-level functions
like asyncio.run()
.
Hello World with call_soon()¶
An example using the 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()
See also
A similar Hello World
example created with a coroutine and the run()
function.
Display the current date with call_later()¶
An example of a callback displaying the current date every second. The
callback uses the 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()
See also
A similar current date example
created with a coroutine and the run()
function.
Watch a file descriptor for read events¶
Wait until a file descriptor received some data using the
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()
See also
A similar example using transports, protocols, and the
loop.create_connection()
method.Another similar example using the high-level
asyncio.open_connection()
function and streams.
Set signal handlers for SIGINT and SIGTERM¶
(This signals
example only works on Unix.)
Register handlers for signals SIGINT
and SIGTERM
using the 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())


Extending

Extending¶
The main direction for asyncio
extending is writing custom event loop
classes. Asyncio has helpers that could be used to simplify this task.
Note
Third-parties should reuse existing asyncio code with caution, a new Python version is free to break backward compatibility in internal part of API.
Writing a Custom Event Loop¶
asyncio.AbstractEventLoop
declares very many methods. Implementing all them
from scratch is a tedious job.
A loop can get many common methods implementation for free by inheriting from
asyncio.BaseEventLoop
.
In turn, the successor should implement a bunch of private methods declared but not
implemented in asyncio.BaseEventLoop
.
For example, loop.create_connection()
checks arguments, resolves DNS addresses, and
calls loop._make_socket_transport()
that should be implemented by inherited class.
The _make_socket_transport()
method is not documented and is considered as an
internal API.
Future and Task private constructors¶
asyncio.Future
and asyncio.Task
should be never created directly,
please use corresponding loop.create_future()
and loop.create_task()
,
or asyncio.create_task()
factories instead.
However, third-party event loops may reuse built-in future and task implementations for the sake of getting a complex and highly optimized code for free.
For this purpose the following, private constructors are listed:
- Future.__init__(*, loop=None)¶
Create a built-in future instance.
loop is an optional event loop instance.
- Task.__init__(coro, *, loop=None, name=None, context=None)¶
Create a built-in task instance.
loop is an optional event loop instance. The rest of arguments are described in
loop.create_task()
description.Changed in version 3.11: context argument is added.
Task lifetime support¶
A third party task implementation should call the following functions to keep a task
visible by asyncio.all_tasks()
and asyncio.current_task()
:
- asyncio._register_task(task)¶
Register a new task as managed by asyncio.
Call the function from a task constructor.
- asyncio._unregister_task(task)¶
Unregister a task from asyncio internal structures.
The function should be called when a task is about to finish.
- asyncio._enter_task(loop, task)¶
Switch the current task to the task argument.
Call the function just before executing a portion of embedded coroutine (
coroutine.send()
orcoroutine.throw()
).
- asyncio._leave_task(loop, task)¶
Switch the current task back from task to
None
.Call the function just after
coroutine.send()
orcoroutine.throw()
execution.


— Interface to Sun’s NIS (Yellow Pages)

nis
— Interface to Sun’s NIS (Yellow Pages)¶
Deprecated since version 3.11, removed in version 3.13.
This module is no longer part of the Python standard library. It was removed in Python 3.13 after being deprecated in Python 3.11. The removal was decided in PEP 594.
The last version of Python that provided the nis
module was
Python 3.12.


msilib
— Read and write Microsoft Installer files¶
Deprecated since version 3.11, removed in version 3.13.
This module is no longer part of the Python standard library. It was removed in Python 3.13 after being deprecated in Python 3.11. The removal was decided in PEP 594.
The last version of Python that provided the msilib
module was
Python 3.12.


— Mailcap file handling

mailcap
— Mailcap file handling¶
Deprecated since version 3.11, removed in version 3.13.
This module is no longer part of the Python standard library. It was removed in Python 3.13 after being deprecated in Python 3.11. The removal was decided in PEP 594.
The last version of Python that provided the mailcap
module was
Python 3.12.


Policies

Policies¶
Warning
Policies are deprecated and will be removed in Python 3.16.
Users are encouraged to use the asyncio.run()
function
or the asyncio.Runner
with loop_factory to use
the desired loop implementation.
An event loop policy is a global object used to get and set the current event loop, as well as create new event loops. The default policy can be replaced with built-in alternatives to use different event loop implementations, or substituted by a custom policy that can override these behaviors.
The policy object gets and sets a separate event loop per context. This is per-thread by default, though custom policies could define context differently.
Custom event loop policies can control the behavior of
get_event_loop()
, set_event_loop()
, and new_event_loop()
.
Policy objects should implement the APIs defined
in the AbstractEventLoopPolicy
abstract base class.
Getting and Setting the Policy¶
The following functions can be used to get and set the policy for the current process:
- asyncio.get_event_loop_policy()¶
Return the current process-wide policy.
Deprecated since version 3.14: The
get_event_loop_policy()
function is deprecated and will be removed in Python 3.16.
- asyncio.set_event_loop_policy(policy)¶
Set the current process-wide policy to policy.
If policy is set to
None
, the default policy is restored.Deprecated since version 3.14: The
set_event_loop_policy()
function is deprecated and will be removed in Python 3.16.
Policy Objects¶
The abstract event loop policy base class is defined as follows:
- class asyncio.AbstractEventLoopPolicy¶
An abstract base class for asyncio policies.
- get_event_loop()¶
Get the event loop for the current context.
Return an event loop object implementing the
AbstractEventLoop
interface.This method should never return
None
.Changed in version 3.6.
- set_event_loop(loop)¶
Set the event loop for the current context to loop.
- new_event_loop()¶
Create and return a new event loop object.
This method should never return
None
.
Deprecated since version 3.14: The
AbstractEventLoopPolicy
class is deprecated and will be removed in Python 3.16.
asyncio ships with the following built-in policies:
- class asyncio.DefaultEventLoopPolicy¶
The default asyncio policy. Uses
SelectorEventLoop
on Unix andProactorEventLoop
on Windows.There is no need to install the default policy manually. asyncio is configured to use the default policy automatically.
Changed in version 3.8: On Windows,
ProactorEventLoop
is now used by default.Changed in version 3.14: The
get_event_loop()
method of the default asyncio policy now raises aRuntimeError
if there is no set event loop.Deprecated since version 3.14: The
DefaultEventLoopPolicy
class is deprecated and will be removed in Python 3.16.
- class asyncio.WindowsSelectorEventLoopPolicy¶
An alternative event loop policy that uses the
SelectorEventLoop
event loop implementation.Availability: Windows.
Deprecated since version 3.14: The
WindowsSelectorEventLoopPolicy
class is deprecated and will be removed in Python 3.16.
- class asyncio.WindowsProactorEventLoopPolicy¶
An alternative event loop policy that uses the
ProactorEventLoop
event loop implementation.Availability: Windows.
Deprecated since version 3.14: The
WindowsProactorEventLoopPolicy
class is deprecated and will be removed in Python 3.16.
Custom Policies¶
To implement a new event loop policy, it is recommended to subclass
DefaultEventLoopPolicy
and override the methods for which
custom behavior is wanted, e.g.:
class MyEventLoopPolicy(asyncio.DefaultEventLoopPolicy):
def get_event_loop(self):
"""Get the event loop.
This may be None or an instance of EventLoop.
"""
loop = super().get_event_loop()
# Do something with loop ...
return loop
asyncio.set_event_loop_policy(MyEventLoopPolicy())


— Access the import internals

imp
— Access the import internals¶
Deprecated since version 3.4, removed in version 3.12.
This module is no longer part of the Python standard library. It was removed in Python 3.12 after being deprecated in Python 3.4.
The removal notice includes guidance for
migrating code from imp
to importlib
.
The last version of Python that provided the imp
module was
Python 3.11.