kbe/src/lib/python/Doc/library/asyncio-eventloop.rst
.. currentmodule:: asyncio
.. rubric:: Preface
The event loop is the core of every asyncio application. Event loops run asynchronous tasks and callbacks, perform network IO operations, and run subprocesses.
Application developers should typically use the high-level asyncio functions,
such as :func:asyncio.run, and should rarely need to reference the loop
object or call its methods. This section is intended mostly for authors
of lower-level code, libraries, and frameworks, who need finer control over
the event loop behavior.
.. rubric:: Obtaining the Event Loop
The following low-level functions can be used to get, set, or create an event loop:
.. function:: get_running_loop()
Return the running event loop in the current OS thread.
If there is no running event loop a :exc:RuntimeError is raised.
This function can only be called from a coroutine or a callback.
.. versionadded:: 3.7
.. function:: get_event_loop()
Get the current event loop. If there is no current event loop set
in the current OS thread and :func:set_event_loop has not yet
been called, asyncio will create a new event loop and set it as the
current one.
Because this function has rather complex behavior (especially
when custom event loop policies are in use), using the
:func:get_running_loop function is preferred to :func:get_event_loop
in coroutines and callbacks.
Consider also using the :func:asyncio.run function instead of using
lower level functions to manually create and close an event loop.
.. function:: set_event_loop(loop)
Set loop as a current event loop for the current OS thread.
.. function:: new_event_loop()
Create a new event loop object.
Note that the behaviour of :func:get_event_loop, :func:set_event_loop,
and :func:new_event_loop functions can be altered by
:ref:setting a custom event loop policy <asyncio-policies>.
.. rubric:: Contents
This documentation page contains the following sections:
The Event Loop Methods_ section is the reference documentation of
the event loop APIs;
The Callback Handles_ section documents the :class:Handle and
:class:TimerHandle instances which are returned from scheduling
methods such as :meth:loop.call_soon and :meth:loop.call_later;
The Server Objects_ section documents types returned from
event loop methods like :meth:loop.create_server;
The Event Loop Implementations_ section documents the
:class:SelectorEventLoop and :class:ProactorEventLoop classes;
The Examples_ section showcases how to work with some event
loop APIs.
.. _asyncio-event-loop:
Event loops have low-level APIs for the following:
.. contents:: :depth: 1 :local:
Running and stopping the loop ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
.. method:: loop.run_until_complete(future)
Run until the future (an instance of :class:Future) has
completed.
If the argument is a :ref:coroutine object <coroutine> it
is implicitly scheduled to run as a :class:asyncio.Task.
Return the Future's result or raise its exception.
.. method:: loop.run_forever()
Run the event loop until :meth:stop is called.
If :meth:stop is called before :meth:run_forever() is called,
the loop will poll the I/O selector once with a timeout of zero,
run all callbacks scheduled in response to I/O events (and
those that were already scheduled), and then exit.
If :meth:stop is called while :meth:run_forever is running,
the loop will run the current batch of callbacks and then exit.
Note that new callbacks scheduled by callbacks will not run in this
case; instead, they will run the next time :meth:run_forever or
:meth:run_until_complete is called.
.. method:: loop.stop()
Stop the event loop.
.. method:: loop.is_running()
Return True if the event loop is currently running.
.. method:: loop.is_closed()
Return True if the event loop was closed.
.. method:: loop.close()
Close the event loop.
The loop must not be running when this function is called. Any pending callbacks will be discarded.
This method clears all queues and shuts down the executor, but does not wait for the executor to finish.
This method is idempotent and irreversible. No other methods should be called after the event loop is closed.
.. coroutinemethod:: loop.shutdown_asyncgens()
Schedule all currently open :term:asynchronous generator objects to
close with an :meth:~agen.aclose() call. After calling this method,
the event loop will issue a warning if a new asynchronous generator
is iterated. This should be used to reliably finalize all scheduled
asynchronous generators.
Note that there is no need to call this function when
:func:asyncio.run is used.
Example::
try:
loop.run_forever()
finally:
loop.run_until_complete(loop.shutdown_asyncgens())
loop.close()
.. versionadded:: 3.6
Scheduling callbacks ^^^^^^^^^^^^^^^^^^^^
.. method:: loop.call_soon(callback, *args, context=None)
Schedule a callback to be called with args arguments at the next iteration of the event loop.
Callbacks are called in the order in which they are registered. Each callback will be called exactly once.
An optional keyword-only context argument allows specifying a
custom :class:contextvars.Context for the callback to run in.
The current context is used when no context is provided.
An instance of :class:asyncio.Handle is returned, which can be
used later to cancel the callback.
This method is not thread-safe.
.. method:: loop.call_soon_threadsafe(callback, *args, context=None)
A thread-safe variant of :meth:call_soon. Must be used to
schedule callbacks from another thread.
See the :ref:concurrency and multithreading <asyncio-multithreading>
section of the documentation.
.. versionchanged:: 3.7
The context keyword-only parameter was added. See :pep:567
for more details.
.. _asyncio-pass-keywords:
.. note::
Most :mod:asyncio scheduling functions don't allow passing
keyword arguments. To do that, use :func:functools.partial::
# will schedule "print("Hello", flush=True)"
loop.call_soon(
functools.partial(print, "Hello", flush=True))
Using partial objects is usually more convenient than using lambdas, as asyncio can render partial objects better in debug and error messages.
.. _asyncio-delayed-calls:
Scheduling delayed callbacks ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Event loop provides mechanisms to schedule callback functions to be called at some point in the future. Event loop uses monotonic clocks to track time.
.. method:: loop.call_later(delay, callback, *args, context=None)
Schedule callback to be called after the given delay number of seconds (can be either an int or a float).
An instance of :class:asyncio.TimerHandle is returned which can
be used to cancel the callback.
callback will be called exactly once. If two callbacks are scheduled for exactly the same time, the order in which they are called is undefined.
The optional positional args will be passed to the callback when
it is called. If you want the callback to be called with keyword
arguments use :func:functools.partial.
An optional keyword-only context argument allows specifying a
custom :class:contextvars.Context for the callback to run in.
The current context is used when no context is provided.
.. versionchanged:: 3.7
The context keyword-only parameter was added. See :pep:567
for more details.
.. versionchanged:: 3.7.1 In Python 3.7.0 and earlier with the default event loop implementation, the delay could not exceed one day. This has been fixed in Python 3.7.1.
.. method:: loop.call_at(when, callback, *args, context=None)
Schedule callback to be called at the given absolute timestamp
when (an int or a float), using the same time reference as
:meth:loop.time.
This method's behavior is the same as :meth:call_later.
An instance of :class:asyncio.TimerHandle is returned which can
be used to cancel the callback.
.. versionchanged:: 3.7
The context keyword-only parameter was added. See :pep:567
for more details.
.. versionchanged:: 3.7.1 In Python 3.7.0 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 3.7.1.
.. method:: loop.time()
Return the current time, as a :class:float value, according to
the event loop's internal monotonic clock.
.. note::
Timeouts (relative delay or absolute when) should not exceed one day.
.. seealso::
The :func:asyncio.sleep function.
Creating Futures and Tasks ^^^^^^^^^^^^^^^^^^^^^^^^^^
.. method:: loop.create_future()
Create an :class:asyncio.Future object attached to the event loop.
This is the preferred way to create Futures in asyncio. This lets third-party event loops provide alternative implementations of the Future object (with better performance or instrumentation).
.. versionadded:: 3.5.2
.. method:: loop.create_task(coro)
Schedule the execution of a :ref:coroutine.
Return a :class:Task object.
Third-party event loops can use their own subclass of :class:Task
for interoperability. In this case, the result type is a subclass
of :class:Task.
.. method:: loop.set_task_factory(factory)
Set a task factory that will be used by
:meth:loop.create_task.
If factory is None the default task factory will be set.
Otherwise, factory must be a callable with the signature matching
(loop, coro), where loop is a reference to the active
event loop, and coro is a coroutine object. The callable
must return a :class:asyncio.Future-compatible object.
.. method:: loop.get_task_factory()
Return a task factory or None if the default one is in use.
Opening network connections ^^^^^^^^^^^^^^^^^^^^^^^^^^^
.. coroutinemethod:: loop.create_connection(protocol_factory,
host=None, port=None, *, ssl=None,
family=0, proto=0, flags=0, sock=None,
local_addr=None, server_hostname=None,
ssl_handshake_timeout=None)
Open a streaming transport connection to a given address specified by host and port.
The socket family can be either :py:data:~socket.AF_INET or
:py:data:~socket.AF_INET6 depending on host (or the family
argument, if provided).
The socket type will be :py:data:~socket.SOCK_STREAM.
protocol_factory must be a callable returning an
:ref:asyncio protocol <asyncio-protocol> implementation.
This method will try to establish the connection in the background.
When successful, it returns a (transport, protocol) pair.
The chronological synopsis of the underlying operation is as follows:
#. The connection is established and a :ref:transport <asyncio-transport>
is created for it.
#. protocol_factory is called without arguments and is expected to
return a :ref:protocol <asyncio-protocol> instance.
#. The protocol instance is coupled with the transport by calling its
:meth:~BaseProtocol.connection_made method.
#. A (transport, protocol) tuple is returned on success.
The created transport is an implementation-dependent bidirectional stream.
Other arguments:
ssl: if given and not false, a SSL/TLS transport is created
(by default a plain TCP transport is created). If ssl is
a :class:ssl.SSLContext object, this context is used to create
the transport; if ssl is :const:True, a default context returned
from :func:ssl.create_default_context is used.
.. seealso:: :ref:SSL/TLS security considerations <ssl-security>
server_hostname sets or overrides the hostname that the target
server's certificate will be matched against. Should only be passed
if ssl is not None. By default the value of the host argument
is used. If host is empty, there is no default and you must pass a
value for server_hostname. If server_hostname is an empty
string, hostname matching is disabled (which is a serious security
risk, allowing for potential man-in-the-middle attacks).
family, proto, flags are the optional address family, protocol
and flags to be passed through to getaddrinfo() for host resolution.
If given, these should all be integers from the corresponding
:mod:socket module constants.
sock, if given, should be an existing, already connected
:class:socket.socket object to be used by the transport.
If sock is given, none of host, port, family, proto, flags
and local_addr should be specified.
local_addr, if given, is a (local_host, local_port) tuple used
to bind the socket to locally. The local_host and local_port
are looked up using getaddrinfo(), similarly to host and port.
ssl_handshake_timeout is (for 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).
.. versionadded:: 3.7
The *ssl_handshake_timeout* parameter.
.. versionchanged:: 3.6
The socket option :py:data:`~socket.TCP_NODELAY` is set by default
for all TCP connections.
.. versionchanged:: 3.5
Added support for SSL/TLS in :class:`ProactorEventLoop`.
.. seealso::
The :func:`open_connection` function is a high-level alternative
API. It returns a pair of (:class:`StreamReader`, :class:`StreamWriter`)
that can be used directly in async/await code.
.. coroutinemethod:: loop.create_datagram_endpoint(protocol_factory,
local_addr=None, remote_addr=None, *,
family=0, proto=0, flags=0,
reuse_address=None, reuse_port=None,
allow_broadcast=None, sock=None)
Create a datagram connection.
The socket family can be either :py:data:~socket.AF_INET,
:py:data:~socket.AF_INET6, or :py:data:~socket.AF_UNIX,
depending on host (or the family argument, if provided).
The socket type will be :py:data:~socket.SOCK_DGRAM.
protocol_factory must be a callable returning a
:ref:protocol <asyncio-protocol> implementation.
A tuple of (transport, protocol) is returned on success.
Other arguments:
local_addr, if given, is a (local_host, local_port) tuple used
to bind the socket to locally. The local_host and local_port
are looked up using :meth:getaddrinfo.
remote_addr, if given, is a (remote_host, remote_port) tuple used
to connect the socket to a remote address. The remote_host and
remote_port are looked up using :meth:getaddrinfo.
family, proto, flags are the optional address family, protocol
and flags to be passed through to :meth:getaddrinfo for host
resolution. If given, these should all be integers from the
corresponding :mod:socket module constants.
reuse_address tells the kernel to reuse a local socket in
TIME_WAIT state, without waiting for its natural timeout to
expire. If not specified will automatically be set to True on
Unix.
reuse_port tells the kernel to allow this endpoint to be bound to the
same port as other existing endpoints are bound to, so long as they all
set this flag when being created. This option is not supported on Windows
and some Unixes. If the :py:data:~socket.SO_REUSEPORT constant is not
defined then this capability is unsupported.
allow_broadcast tells the kernel to allow this endpoint to send messages to the broadcast address.
sock can optionally be specified in order to use a preexisting,
already connected, :class:socket.socket object to be used by the
transport. If specified, local_addr and remote_addr should be omitted
(must be :const:None).
On Windows, with :class:ProactorEventLoop, this method is not supported.
See :ref:UDP echo client protocol <asyncio-udp-echo-client-protocol> and
:ref:UDP echo server protocol <asyncio-udp-echo-server-protocol> examples.
.. versionchanged:: 3.4.4 The family, proto, flags, reuse_address, *reuse_port, allow_broadcast, and sock parameters were added.
.. coroutinemethod:: loop.create_unix_connection(protocol_factory,
path=None, *, ssl=None, sock=None,
server_hostname=None, ssl_handshake_timeout=None)
Create a Unix connection.
The socket family will be :py:data:~socket.AF_UNIX; socket
type will be :py:data:~socket.SOCK_STREAM.
A tuple of (transport, protocol) is returned on success.
path is the name of a Unix domain socket and is required,
unless a sock parameter is specified. Abstract Unix sockets,
:class:str, :class:bytes, and :class:~pathlib.Path paths are
supported.
See the documentation of the :meth:loop.create_connection method
for information about arguments to this method.
.. availability:: Unix.
.. versionadded:: 3.7
The *ssl_handshake_timeout* parameter.
.. versionchanged:: 3.7
The *path* parameter can now be a :term:`path-like object`.
Creating network servers ^^^^^^^^^^^^^^^^^^^^^^^^
.. coroutinemethod:: loop.create_server(protocol_factory,
host=None, port=None, *,
family=socket.AF_UNSPEC,
flags=socket.AI_PASSIVE,
sock=None, backlog=100, ssl=None,
reuse_address=None, reuse_port=None,
ssl_handshake_timeout=None, start_serving=True)
Create a TCP server (socket type :data:~socket.SOCK_STREAM) listening
on port of the host address.
Returns a :class:Server object.
Arguments:
protocol_factory must be a callable returning a
:ref:protocol <asyncio-protocol> implementation.
The host parameter can be set to several types which determine where the server would be listening:
If host is a string, the TCP server is bound to a single network interface specified by host.
If host is a sequence of strings, the TCP server is bound to all network interfaces specified by the sequence.
If host is an empty string or None, all interfaces are
assumed and a list of multiple sockets will be returned (most likely
one for IPv4 and another one for IPv6).
family can be set to either :data:socket.AF_INET or
:data:~socket.AF_INET6 to force the socket to use IPv4 or IPv6.
If not set, the family will be determined from host name
(defaults to :data:~socket.AF_UNSPEC).
flags is a bitmask for :meth:getaddrinfo.
sock can optionally be specified in order to use a preexisting socket object. If specified, host and port must not be specified.
backlog is the maximum number of queued connections passed to
:meth:~socket.socket.listen (defaults to 100).
ssl can be set to an :class:~ssl.SSLContext instance to enable
TLS over the accepted connections.
reuse_address tells the kernel to reuse a local socket in
TIME_WAIT state, without waiting for its natural timeout to
expire. If not specified will automatically be set to True on
Unix.
reuse_port tells the kernel to allow this endpoint to be bound to the same port as other existing endpoints are bound to, so long as they all set this flag when being created. This option is not supported on Windows.
ssl_handshake_timeout is (for a TLS server) the time in seconds to wait
for the TLS handshake to complete before aborting the connection.
60.0 seconds if None (default).
start_serving set to True (the default) causes the created server
to start accepting connections immediately. When set to False,
the user should await on :meth:Server.start_serving or
:meth:Server.serve_forever to make the server to start accepting
connections.
.. versionadded:: 3.7
Added *ssl_handshake_timeout* and *start_serving* parameters.
.. versionchanged:: 3.6
The socket option :py:data:`~socket.TCP_NODELAY` is set by default
for all TCP connections.
.. versionchanged:: 3.5
Added support for SSL/TLS in :class:`ProactorEventLoop`.
.. versionchanged:: 3.5.1
The *host* parameter can be a sequence of strings.
.. seealso::
The :func:`start_server` function is a higher-level alternative API
that returns a pair of :class:`StreamReader` and :class:`StreamWriter`
that can be used in an async/await code.
.. coroutinemethod:: loop.create_unix_server(protocol_factory, path=None,
*, sock=None, backlog=100, ssl=None,
ssl_handshake_timeout=None, start_serving=True)
Similar to :meth:loop.create_server but works with the
:py:data:~socket.AF_UNIX socket family.
path is the name of a Unix domain socket, and is required,
unless a sock argument is provided. Abstract Unix sockets,
:class:str, :class:bytes, and :class:~pathlib.Path paths
are supported.
See the documentation of the :meth:loop.create_server method
for information about arguments to this method.
.. availability:: Unix.
.. versionadded:: 3.7
The *ssl_handshake_timeout* and *start_serving* parameters.
.. versionchanged:: 3.7
The *path* parameter can now be a :class:`~pathlib.Path` object.
.. coroutinemethod:: loop.connect_accepted_socket(protocol_factory,
sock, *, ssl=None, ssl_handshake_timeout=None)
Wrap an already accepted connection into a transport/protocol pair.
This method can be used by servers that accept connections outside of asyncio but that use asyncio to handle them.
Parameters:
protocol_factory must be a callable returning a
:ref:protocol <asyncio-protocol> implementation.
sock is a preexisting socket object returned from
:meth:socket.accept <socket.socket.accept>.
ssl can be set to an :class:~ssl.SSLContext to enable SSL over
the accepted connections.
ssl_handshake_timeout is (for an SSL connection) the time in seconds to
wait for the SSL handshake to complete before aborting the connection.
60.0 seconds if None (default).
Returns a (transport, protocol) pair.
.. versionadded:: 3.7
The *ssl_handshake_timeout* parameter.
.. versionadded:: 3.5.3
Transferring files ^^^^^^^^^^^^^^^^^^
.. coroutinemethod:: loop.sendfile(transport, file,
offset=0, count=None, *, fallback=True)
Send a file over a transport. Return the total number of bytes sent.
The method uses high-performance :meth:os.sendfile if available.
file must be a regular file object opened in binary mode.
offset tells from where to start reading the file. If specified,
count is the total number of bytes to transmit as opposed to
sending the file until EOF is reached. File position is always updated,
even when this method raises an error, and
:meth:file.tell() <io.IOBase.tell> can be used to obtain the actual
number of bytes sent.
fallback set to True makes asyncio to manually read and send
the file when the platform does not support the sendfile system call
(e.g. Windows or SSL socket on Unix).
Raise :exc:SendfileNotAvailableError if the system does not support
the sendfile syscall and fallback is False.
.. versionadded:: 3.7
TLS Upgrade ^^^^^^^^^^^
.. coroutinemethod:: loop.start_tls(transport, protocol,
sslcontext, *, server_side=False,
server_hostname=None, ssl_handshake_timeout=None)
Upgrade an existing transport-based connection to TLS.
Return a new transport instance, that the protocol must start using immediately after the await. The transport instance passed to the start_tls method should never be used again.
Parameters:
transport and protocol instances that methods like
:meth:~loop.create_server and
:meth:~loop.create_connection return.
sslcontext: a configured instance of :class:~ssl.SSLContext.
server_side pass True when a server-side connection is being
upgraded (like the one created by :meth:~loop.create_server).
server_hostname: sets or overrides the host name that the target server's certificate will be matched against.
ssl_handshake_timeout is (for 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).
.. versionadded:: 3.7
Watching file descriptors ^^^^^^^^^^^^^^^^^^^^^^^^^
.. method:: loop.add_reader(fd, callback, *args)
Start monitoring the fd file descriptor for read availability and invoke callback with the specified arguments once fd is available for reading.
.. method:: loop.remove_reader(fd)
Stop monitoring the fd file descriptor for read availability.
.. method:: loop.add_writer(fd, callback, *args)
Start monitoring the fd file descriptor for write availability and invoke callback with the specified arguments once fd is available for writing.
Use :func:functools.partial :ref:to pass keyword arguments <asyncio-pass-keywords> to callback.
.. method:: loop.remove_writer(fd)
Stop monitoring the fd file descriptor for write availability.
See also :ref:Platform Support <asyncio-platform-support> section
for some limitations of these methods.
Working with socket objects directly ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
In general, protocol implementations that use transport-based APIs
such as :meth:loop.create_connection and :meth:loop.create_server
are faster than implementations that work with sockets directly.
However, there are some use cases when performance is not critical, and
working with :class:~socket.socket objects directly is more
convenient.
.. coroutinemethod:: loop.sock_recv(sock, nbytes)
Receive up to nbytes from sock. Asynchronous version of
:meth:socket.recv() <socket.socket.recv>.
Return the received data as a bytes object.
sock must be a non-blocking socket.
.. versionchanged:: 3.7
Even though this method was always documented as a coroutine
method, releases before Python 3.7 returned a :class:Future.
Since Python 3.7 this is an async def method.
.. coroutinemethod:: loop.sock_recv_into(sock, buf)
Receive data from sock into the buf buffer. Modeled after the blocking
:meth:socket.recv_into() <socket.socket.recv_into> method.
Return the number of bytes written to the buffer.
sock must be a non-blocking socket.
.. versionadded:: 3.7
.. coroutinemethod:: loop.sock_sendall(sock, data)
Send data to the sock socket. Asynchronous version of
:meth:socket.sendall() <socket.socket.sendall>.
This method continues to send to the socket until either all data
in data has been sent or an error occurs. None is returned
on success. On error, an exception is raised. Additionally, there is no way
to determine how much data, if any, was successfully processed by the
receiving end of the connection.
sock must be a non-blocking socket.
.. versionchanged:: 3.7
Even though the method was always documented as a coroutine
method, before Python 3.7 it returned an :class:Future.
Since Python 3.7, this is an async def method.
.. coroutinemethod:: loop.sock_connect(sock, address)
Connect sock to a remote socket at address.
Asynchronous version of :meth:socket.connect() <socket.socket.connect>.
sock must be a non-blocking socket.
.. versionchanged:: 3.5.2
address no longer needs to be resolved. sock_connect
will try to check if the address is already resolved by calling
:func:socket.inet_pton. If not,
:meth:loop.getaddrinfo will be used to resolve the
address.
.. seealso::
:meth:`loop.create_connection`
and :func:`asyncio.open_connection() <open_connection>`.
.. coroutinemethod:: loop.sock_accept(sock)
Accept a connection. Modeled after the blocking
:meth:socket.accept() <socket.socket.accept> method.
The socket must be bound to an address and listening
for connections. The return value is a pair (conn, address) where conn
is a new socket object usable to send and receive data on the connection,
and address is the address bound to the socket on the other end of the
connection.
sock must be a non-blocking socket.
.. versionchanged:: 3.7
Even though the method was always documented as a coroutine
method, before Python 3.7 it returned a :class:Future.
Since Python 3.7, this is an async def method.
.. seealso::
:meth:`loop.create_server` and :func:`start_server`.
.. coroutinemethod:: loop.sock_sendfile(sock, file, offset=0, count=None,
*, fallback=True)
Send a file using high-performance :mod:os.sendfile if possible.
Return the total number of bytes sent.
Asynchronous version of :meth:socket.sendfile() <socket.socket.sendfile>.
sock must be a non-blocking :const:socket.SOCK_STREAM
:class:~socket.socket.
file must be a regular file object open in binary mode.
offset tells from where to start reading the file. If specified,
count is the total number of bytes to transmit as opposed to
sending the file until EOF is reached. File position is always updated,
even when this method raises an error, and
:meth:file.tell() <io.IOBase.tell> can be used to obtain the actual
number of bytes sent.
fallback, when set to True, makes asyncio manually read and send
the file when the platform does not support the sendfile syscall
(e.g. Windows or SSL socket on Unix).
Raise :exc:SendfileNotAvailableError if the system does not support
sendfile syscall and fallback is False.
sock must be a non-blocking socket.
.. versionadded:: 3.7
DNS ^^^
.. coroutinemethod:: loop.getaddrinfo(host, port, *, family=0,
type=0, proto=0, flags=0)
Asynchronous version of :meth:socket.getaddrinfo.
.. coroutinemethod:: loop.getnameinfo(sockaddr, flags=0)
Asynchronous version of :meth:socket.getnameinfo.
.. versionchanged:: 3.7
Both getaddrinfo and getnameinfo methods were always documented
to return a coroutine, but prior to Python 3.7 they were, in fact,
returning :class:asyncio.Future objects. Starting with Python 3.7
both methods are coroutines.
Working with pipes ^^^^^^^^^^^^^^^^^^
.. coroutinemethod:: loop.connect_read_pipe(protocol_factory, pipe)
Register the read end of pipe in the event loop.
protocol_factory must be a callable returning an
:ref:asyncio protocol <asyncio-protocol> implementation.
pipe is a :term:file-like object <file object>.
Return pair (transport, protocol), where transport supports
the :class:ReadTransport interface and protocol is an object
instantiated by the protocol_factory.
With :class:SelectorEventLoop event loop, the pipe is set to
non-blocking mode.
.. coroutinemethod:: loop.connect_write_pipe(protocol_factory, pipe)
Register the write end of pipe in the event loop.
protocol_factory must be a callable returning an
:ref:asyncio protocol <asyncio-protocol> implementation.
pipe is :term:file-like object <file object>.
Return pair (transport, protocol), where transport supports
:class:WriteTransport interface and protocol is an object
instantiated by the protocol_factory.
With :class:SelectorEventLoop event loop, the pipe is set to
non-blocking mode.
.. note::
:class:SelectorEventLoop does not support the above methods on
Windows. Use :class:ProactorEventLoop instead for Windows.
.. seealso::
The :meth:loop.subprocess_exec and
:meth:loop.subprocess_shell methods.
Unix signals ^^^^^^^^^^^^
.. method:: loop.add_signal_handler(signum, callback, *args)
Set callback as the handler for the signum signal.
The callback will be invoked by loop, along with other queued callbacks
and runnable coroutines of that event loop. Unlike signal handlers
registered using :func:signal.signal, a callback registered with this
function is allowed to interact with the event loop.
Raise :exc:ValueError if the signal number is invalid or uncatchable.
Raise :exc:RuntimeError if there is a problem setting up the handler.
Use :func:functools.partial :ref:to pass keyword arguments <asyncio-pass-keywords> to callback.
Like :func:signal.signal, this function must be invoked in the main
thread.
.. method:: loop.remove_signal_handler(sig)
Remove the handler for the sig signal.
Return True if the signal handler was removed, or False if
no handler was set for the given signal.
.. availability:: Unix.
.. seealso::
The :mod:signal module.
Executing code in thread or process pools ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
.. awaitablemethod:: loop.run_in_executor(executor, func, *args)
Arrange for func to be called in the specified executor.
The executor argument should be an :class:concurrent.futures.Executor
instance. The default executor is used if executor is None.
Example::
import asyncio
import concurrent.futures
def blocking_io():
# File operations (such as logging) can block the
# event loop: run them in a thread pool.
with open('/dev/urandom', 'rb') as f:
return f.read(100)
def cpu_bound():
# CPU-bound operations will block the event loop:
# in general it is preferable to run them in a
# process pool.
return sum(i * i for i in range(10 ** 7))
async def main():
loop = asyncio.get_running_loop()
## Options:
# 1. Run in the default loop's executor:
result = await loop.run_in_executor(
None, blocking_io)
print('default thread pool', result)
# 2. Run in a custom thread pool:
with concurrent.futures.ThreadPoolExecutor() as pool:
result = await loop.run_in_executor(
pool, blocking_io)
print('custom thread pool', result)
# 3. Run in a custom process pool:
with concurrent.futures.ProcessPoolExecutor() as pool:
result = await loop.run_in_executor(
pool, cpu_bound)
print('custom process pool', result)
asyncio.run(main())
This method returns a :class:asyncio.Future object.
Use :func:functools.partial :ref:to pass keyword arguments <asyncio-pass-keywords> to func.
.. versionchanged:: 3.5.3
:meth:loop.run_in_executor no longer configures the
max_workers of the thread pool executor it creates, instead
leaving it up to the thread pool executor
(:class:~concurrent.futures.ThreadPoolExecutor) to set the
default.
.. method:: loop.set_default_executor(executor)
Set executor as the default executor used by :meth:run_in_executor.
executor should be an instance of
:class:~concurrent.futures.ThreadPoolExecutor.
.. deprecated:: 3.7
Using an executor that is not an instance of
:class:~concurrent.futures.ThreadPoolExecutor is deprecated and
will trigger an error in Python 3.9.
executor must be an instance of
:class:concurrent.futures.ThreadPoolExecutor.
Error Handling API ^^^^^^^^^^^^^^^^^^
Allows customizing how exceptions are handled in the event loop.
.. method:: loop.set_exception_handler(handler)
Set handler as the new event loop exception handler.
If handler is None, the default exception handler will
be set. Otherwise, handler must be a callable with the signature
matching (loop, context), where loop
is a reference to the active event loop, and context
is a dict object containing the details of the exception
(see :meth:call_exception_handler documentation for details
about context).
.. method:: loop.get_exception_handler()
Return the current exception handler, or None if no custom
exception handler was set.
.. versionadded:: 3.5.2
.. method:: loop.default_exception_handler(context)
Default exception handler.
This is called when an exception occurs and no exception handler is set. This can be called by a custom exception handler that wants to defer to the default handler behavior.
context parameter has the same meaning as in
:meth:call_exception_handler.
.. method:: loop.call_exception_handler(context)
Call the current event loop exception handler.
context is a dict object containing the following keys
(new keys may be introduced in future Python versions):
asyncio.Future instance;asyncio.Handle instance;Protocol <asyncio-protocol> instance;Transport <asyncio-transport> instance;socket.socket instance... note::
This method should not be overloaded in subclassed
event loops. For custom exception handling, use
the :meth:`set_exception_handler()` method.
Enabling debug mode ^^^^^^^^^^^^^^^^^^^
.. method:: loop.get_debug()
Get the debug mode (:class:bool) of the event loop.
The default value is True if the environment variable
:envvar:PYTHONASYNCIODEBUG is set to a non-empty string, False
otherwise.
.. method:: loop.set_debug(enabled: bool)
Set the debug mode of the event loop.
.. versionchanged:: 3.7
The new ``-X dev`` command line option can now also be used
to enable the debug mode.
.. seealso::
The :ref:debug mode of asyncio <asyncio-debug-mode>.
Running Subprocesses ^^^^^^^^^^^^^^^^^^^^
Methods described in this subsections are low-level. In regular
async/await code consider using the high-level
:func:asyncio.create_subprocess_shell and
:func:asyncio.create_subprocess_exec convenience functions instead.
.. note::
The default asyncio event loop on Windows does not support
subprocesses. See :ref:Subprocess Support on Windows <asyncio-windows-subprocess> for details.
.. coroutinemethod:: loop.subprocess_exec(protocol_factory, *args,
stdin=subprocess.PIPE, stdout=subprocess.PIPE,
stderr=subprocess.PIPE, **kwargs)
Create a subprocess from one or more string arguments specified by args.
args must be a list of strings represented by:
str;bytes, encoded to the
:ref:filesystem encoding <filesystem-encoding>.The first string specifies the program executable,
and the remaining strings specify the arguments. Together, string
arguments form the argv of the program.
This is similar to the standard library :class:subprocess.Popen
class called with shell=False and the list of strings passed as
the first argument; however, where :class:~subprocess.Popen takes
a single argument which is list of strings, subprocess_exec
takes multiple string arguments.
The protocol_factory must be a callable returning a subclass of the
:class:asyncio.SubprocessProtocol class.
Other parameters:
stdin: either a file-like object representing a pipe to be
connected to the subprocess's standard input stream using
:meth:~loop.connect_write_pipe, or the
:const:subprocess.PIPE constant (default). By default a new
pipe will be created and connected.
stdout: either a file-like object representing the pipe to be
connected to the subprocess's standard output stream using
:meth:~loop.connect_read_pipe, or the
:const:subprocess.PIPE constant (default). By default a new pipe
will be created and connected.
stderr: either a file-like object representing the pipe to be
connected to the subprocess's standard error stream using
:meth:~loop.connect_read_pipe, or one of
:const:subprocess.PIPE (default) or :const:subprocess.STDOUT
constants.
By default a new pipe will be created and connected. When
:const:subprocess.STDOUT is specified, the subprocess' standard
error stream will be connected to the same pipe as the standard
output stream.
All other keyword arguments are passed to :class:subprocess.Popen
without interpretation, except for bufsize, universal_newlines
and shell, which should not be specified at all.
See the constructor of the :class:subprocess.Popen class
for documentation on other arguments.
Returns a pair of (transport, protocol), where transport
conforms to the :class:asyncio.SubprocessTransport base class and
protocol is an object instantiated by the protocol_factory.
.. coroutinemethod:: loop.subprocess_shell(protocol_factory, cmd, *,
stdin=subprocess.PIPE, stdout=subprocess.PIPE,
stderr=subprocess.PIPE, **kwargs)
Create a subprocess from cmd, which can be a :class:str or a
:class:bytes string encoded to the
:ref:filesystem encoding <filesystem-encoding>,
using the platform's "shell" syntax.
This is similar to the standard library :class:subprocess.Popen
class called with shell=True.
The protocol_factory must be a callable returning a subclass of the
:class:SubprocessProtocol class.
See :meth:~loop.subprocess_exec for more details about
the remaining arguments.
Returns a pair of (transport, protocol), where transport
conforms to the :class:SubprocessTransport base class and
protocol is an object instantiated by the protocol_factory.
.. note::
It is the application's responsibility to ensure that all whitespace
and special characters are quoted appropriately to avoid shell injection <https://en.wikipedia.org/wiki/Shell_injection#Shell_injection>_
vulnerabilities. The :func:shlex.quote function can be used to
properly escape whitespace and special characters in strings that
are going to be used to construct shell commands.
.. class:: Handle
A callback wrapper object returned by :meth:loop.call_soon,
:meth:loop.call_soon_threadsafe.
.. method:: cancel()
Cancel the callback. If the callback has already been canceled
or executed, this method has no effect.
.. method:: cancelled()
Return ``True`` if the callback was cancelled.
.. versionadded:: 3.7
.. class:: TimerHandle
A callback wrapper object returned by :meth:loop.call_later,
and :meth:loop.call_at.
This class is a subclass of :class:Handle.
.. method:: when()
Return a scheduled callback time as :class:`float` seconds.
The time is an absolute timestamp, using the same time
reference as :meth:`loop.time`.
.. versionadded:: 3.7
Server objects are created by :meth:loop.create_server,
:meth:loop.create_unix_server, :func:start_server,
and :func:start_unix_server functions.
Do not instantiate the class directly.
.. class:: Server
Server objects are asynchronous context managers. When used in an
async with statement, it's guaranteed that the Server object is
closed and not accepting new connections when the async with
statement is completed::
srv = await loop.create_server(...)
async with srv:
# some code
# At this point, srv is closed and no longer accepts new connections.
.. versionchanged:: 3.7 Server object is an asynchronous context manager since Python 3.7.
.. method:: close()
Stop serving: close listening sockets and set the :attr:`sockets`
attribute to ``None``.
The sockets that represent existing incoming client connections
are left open.
The server is closed asynchronously, use the :meth:`wait_closed`
coroutine to wait until the server is closed.
.. method:: get_loop()
Return the event loop associated with the server object.
.. versionadded:: 3.7
.. coroutinemethod:: start_serving()
Start accepting connections.
This method is idempotent, so it can be called when
the server is already being serving.
The *start_serving* keyword-only parameter to
:meth:`loop.create_server` and
:meth:`asyncio.start_server` allows creating a Server object
that is not accepting connections initially. In this case
``Server.start_serving()``, or :meth:`Server.serve_forever` can be used
to make the Server start accepting connections.
.. versionadded:: 3.7
.. coroutinemethod:: serve_forever()
Start accepting connections until the coroutine is cancelled.
Cancellation of ``serve_forever`` task causes the server
to be closed.
This method can be called if the server is already accepting
connections. Only one ``serve_forever`` task can exist per
one *Server* object.
Example::
async def client_connected(reader, writer):
# Communicate with the client with
# reader/writer streams. For example:
await reader.readline()
async def main(host, port):
srv = await asyncio.start_server(
client_connected, host, port)
await srv.serve_forever()
asyncio.run(main('127.0.0.1', 0))
.. versionadded:: 3.7
.. method:: is_serving()
Return ``True`` if the server is accepting new connections.
.. versionadded:: 3.7
.. coroutinemethod:: wait_closed()
Wait until the :meth:`close` method completes.
.. attribute:: sockets
List of :class:`socket.socket` objects the server is listening on,
or ``None`` if the server is closed.
.. versionchanged:: 3.7
Prior to Python 3.7 ``Server.sockets`` used to return an
internal list of server sockets directly. In 3.7 a copy
of that list is returned.
.. _asyncio-event-loops:
asyncio ships with two different event loop implementations:
:class:SelectorEventLoop and :class:ProactorEventLoop.
By default asyncio is configured to use :class:SelectorEventLoop
on all platforms.
.. class:: SelectorEventLoop
An event loop based on the :mod:selectors module.
Uses the most efficient selector available for the given platform. It is also possible to manually configure the exact selector implementation to be used::
import asyncio
import selectors
selector = selectors.SelectSelector()
loop = asyncio.SelectorEventLoop(selector)
asyncio.set_event_loop(loop)
.. availability:: Unix, Windows.
.. class:: ProactorEventLoop
An event loop for Windows that uses "I/O Completion Ports" (IOCP).
.. availability:: Windows.
An example how to use :class:ProactorEventLoop on Windows::
import asyncio
import sys
if sys.platform == 'win32':
loop = asyncio.ProactorEventLoop()
asyncio.set_event_loop(loop)
.. seealso::
`MSDN documentation on I/O Completion Ports
<https://docs.microsoft.com/en-ca/windows/desktop/FileIO/i-o-completion-ports>`_.
.. class:: AbstractEventLoop
Abstract base class for asyncio-compliant event loops.
The :ref:Event Loop Methods <asyncio-event-loop> section lists all
methods that an alternative implementation of AbstractEventLoop
should have defined.
Note that all examples in this section purposefully show how
to use the low-level event loop APIs, such as :meth:loop.run_forever
and :meth:loop.call_soon. Modern asyncio applications rarely
need to be written this way; consider using the high-level functions
like :func:asyncio.run.
.. _asyncio_example_lowlevel_helloworld:
Hello World with call_soon() ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
An example using the :meth:loop.call_soon method to schedule a
callback. The callback displays "Hello World" and then stops the
event loop::
import asyncio
def hello_world(loop):
"""A callback to print 'Hello World' and stop the event loop"""
print('Hello World')
loop.stop()
loop = asyncio.get_event_loop()
# Schedule a call to hello_world()
loop.call_soon(hello_world, loop)
# Blocking call interrupted by loop.stop()
try:
loop.run_forever()
finally:
loop.close()
.. seealso::
A similar :ref:Hello World <coroutine>
example created with a coroutine and the :func:run function.
.. _asyncio_example_call_later:
Display the current date with call_later() ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
An example of a callback displaying the current date every second. The
callback uses the :meth:loop.call_later method to reschedule itself
after 5 seconds, and then stops the event loop::
import asyncio
import datetime
def display_date(end_time, loop):
print(datetime.datetime.now())
if (loop.time() + 1.0) < end_time:
loop.call_later(1, display_date, end_time, loop)
else:
loop.stop()
loop = asyncio.get_event_loop()
# Schedule the first call to display_date()
end_time = loop.time() + 5.0
loop.call_soon(display_date, end_time, loop)
# Blocking call interrupted by loop.stop()
try:
loop.run_forever()
finally:
loop.close()
.. seealso::
A similar :ref:current date <asyncio_example_sleep> example
created with a coroutine and the :func:run function.
.. _asyncio_example_watch_fd:
Watch a file descriptor for read events ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Wait until a file descriptor received some data using the
:meth:loop.add_reader method and then close the event loop::
import asyncio
from socket import socketpair
# Create a pair of connected file descriptors
rsock, wsock = socketpair()
loop = asyncio.get_event_loop()
def reader():
data = rsock.recv(100)
print("Received:", data.decode())
# We are done: unregister the file descriptor
loop.remove_reader(rsock)
# Stop the event loop
loop.stop()
# Register the file descriptor for read event
loop.add_reader(rsock, reader)
# Simulate the reception of data from the network
loop.call_soon(wsock.send, 'abc'.encode())
try:
# Run the event loop
loop.run_forever()
finally:
# We are done. Close sockets and the event loop.
rsock.close()
wsock.close()
loop.close()
.. seealso::
A similar :ref:example <asyncio_example_create_connection>
using transports, protocols, and the
:meth:loop.create_connection method.
Another similar :ref:example <asyncio_example_create_connection-streams>
using the high-level :func:asyncio.open_connection function
and streams.
.. _asyncio_example_unix_signals:
Set signal handlers for SIGINT and SIGTERM ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
(This signals example only works on Unix.)
Register handlers for signals :py:data:SIGINT and :py:data:SIGTERM
using the :meth:loop.add_signal_handler method::
import asyncio
import functools
import os
import signal
def ask_exit(signame):
print("got signal %s: exit" % signame)
loop.stop()
async def main():
loop = asyncio.get_running_loop()
for signame in {'SIGINT', 'SIGTERM'}:
loop.add_signal_handler(
getattr(signal, signame),
functools.partial(ask_exit, signame))
await asyncio.sleep(3600)
print("Event loop running for 1 hour, press Ctrl+C to interrupt.")
print(f"pid {os.getpid()}: send SIGINT or SIGTERM to exit.")
asyncio.run(main())