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.
If there is no running event loop a RuntimeError is raised.
This function can only be called from a coroutine or a callback.
New in version 3.7.
asyncio.get_event_loop()Get the current event loop.
If there is no current event loop set in the current OS thread,
the OS thread is main, and 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
get_running_loop() function is preferred to get_event_loop()
in coroutines and callbacks.
Consider also using the asyncio.run() function instead of using
lower level functions to manually create and close an event loop.
asyncio.set_event_loop(loop)asyncio.new_event_loop()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:
Handle and TimerHandle instances which are returned from scheduling methods such as loop.call_soon() and loop.call_later();loop.create_server();SelectorEventLoop and ProactorEventLoop classes;
Event loops have low-level APIs for the following:
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 before 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 stop() is called while 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 run_forever() or
run_until_complete() is called.
loop.stop()loop.is_running()True if the event loop is currently running.loop.is_closed()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.
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()
New in version 3.6.
loop.shutdown_default_executor()Schedule the closure of the default executor and wait for it to join all of
the threads in the ThreadPoolExecutor. After calling this method, a
RuntimeError will be raised if loop.run_in_executor() is called
while using the default executor.
Note that there is no need to call this function when
asyncio.run() is used.
New in version 3.9.
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.
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 contextvars.Context for the callback to run in.
The current context is used when no context is provided.
An instance of asyncio.Handle is returned, which can be
used later to cancel the callback.
This method is not thread-safe.
loop.call_soon_threadsafe(callback, *args, context=None)A thread-safe variant of call_soon(). Must be used to
schedule callbacks from another thread.
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.
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. If you want the callback to be called with keyword
arguments use functools.partial().
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.
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.
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()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.
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).
New in version 3.5.2.
loop.create_task(coro, *, name=None)Schedule the execution of a Coroutines.
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
of Task.
If the name argument is provided and not None, it is set as
the name of the task using Task.set_name().
Changed in version 3.8: Added the name parameter.
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), where loop is a reference to the active
event loop, and coro is a coroutine object. The callable
must return a asyncio.Future-compatible object.
loop.get_task_factory()None if the default one is in use.
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, happy_eyeballs_delay=None, interleave=None)Open a streaming transport connection to a given address specified by host and port.
The socket family can be either AF_INET or
AF_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 is True, a default context returned
from ssl.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 by getaddrinfo(). If a positive integer
is specified, the addresses are interleaved by address family, and the
given integer is interpreted as “First Address Family Count” as defined
in RFC 8305. The default is 0 if happy_eyeballs_delay is not
specified, and 1 if it is.
sock, if given, should be an existing, already connected
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.
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).
New 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://tools.ietf.org/html/rfc6555
New in version 3.7: The ssl_handshake_timeout parameter.
Changed in version 3.6: The socket option TCP_NODELAY is set by default
for all TCP connections.
Changed in version 3.5: Added support for SSL/TLS in ProactorEventLoop.
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.
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)Note
The parameter reuse_address is no longer supported, as using
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,
SO_REUSEPORT is used instead, which specifically
prevents processes with differing UIDs from assigning sockets to the same
socket address.
Create a datagram connection.
The socket family can be either AF_INET,
AF_INET6, or AF_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 to locally. The local_host and local_port
are looked up using 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 getaddrinfo().
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.
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 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 be None).
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,
Changed in version 3.8.1: The reuse_address parameter is no longer supported due to security concerns.
Changed in version 3.8: Added support for Windows.
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 AF_UNIX; socket
type will be 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,
str, bytes, and Path paths are
supported.
See the documentation of the loop.create_connection() method
for information about arguments to this method.
Availability: Unix.
New in version 3.7: The ssl_handshake_timeout parameter.
Changed in version 3.7: The path parameter can now be a path-like object.
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 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).
family can be set to either socket.AF_INET or
AF_INET6 to force the socket to use IPv4 or IPv6.
If not set, the family will be determined from host name
(defaults to AF_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.
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 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 Server.start_serving() or
Server.serve_forever() to make the server to start accepting
connections.
New in version 3.7: Added ssl_handshake_timeout and start_serving parameters.
Changed in version 3.6: The socket option TCP_NODELAY is set by default
for all TCP 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.
See also
The start_server() function is a higher-level alternative API
that returns a pair of StreamReader and StreamWriter
that can be used in an async/await code.
loop.create_unix_server(protocol_factory, path=None, *, sock=None, backlog=100, ssl=None, ssl_handshake_timeout=None, start_serving=True)Similar to loop.create_server() but works with the
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,
str, bytes, and Path paths
are supported.
See the documentation of the loop.create_server() method
for information about arguments to this method.
Availability: Unix.
New in version 3.7: The ssl_handshake_timeout and start_serving parameters.
Changed in version 3.7: The path parameter can now be a Path object.
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 protocol implementation.
sock is a preexisting socket object returned from
socket.accept.
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 if None (default).
Returns a (transport, protocol) pair.
New in version 3.7: The ssl_handshake_timeout parameter.
New in version 3.5.3.
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 is False.
New in version 3.7.
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
create_server() and
create_connection() return.
sslcontext: a configured instance of SSLContext.
server_side pass True when a server-side connection is being
upgraded (like the one created by 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).
New in version 3.7.
loop.add_reader(fd, callback, *args)loop.remove_reader(fd)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 functools.partial() to pass keyword arguments to callback.
loop.remove_writer(fd)See also Platform Support section for some limitations of these methods.
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.
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 an async def method.
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.
New in version 3.7.
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 an Future.
Since Python 3.7, this is an async def method.
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 calling
socket.inet_pton(). If not,
loop.getaddrinfo() will be used to resolve the
address.
See also
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 an async def method.
See also
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 is False.
sock must be a non-blocking socket.
New in version 3.7.
loop.getaddrinfo(host, port, *, family=0, type=0, proto=0, flags=0)socket.getaddrinfo().loop.getnameinfo(sockaddr, flags=0)socket.getnameinfo().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.
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
the ReadTransport interface and protocol is an object
instantiated by the protocol_factory.
With SelectorEventLoop event loop, the pipe is set to
non-blocking mode.
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 supports
WriteTransport 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.
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 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.
Raise RuntimeError 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, or False if
no handler was set for the given signal.
Availability: Unix.
loop.run_in_executor(executor, func, *args)Arrange for func to be called in the specified executor.
The executor argument should be an 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 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 the
max_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 should be an instance of
ThreadPoolExecutor.
Deprecated since version 3.8: Using an executor that is not an instance of
ThreadPoolExecutor is deprecated and
will trigger an error in Python 3.9.
executor must be an instance of
concurrent.futures.ThreadPoolExecutor.
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), where loop
is a reference to the active event loop, and context
is a dict object containing the details of the exception
(see call_exception_handler() documentation for details
about context).
loop.get_exception_handler()Return the current exception handler, or None if no custom
exception handler was set.
New 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;
‘handle’ (optional): asyncio.Handle instance;
‘protocol’ (optional): Protocol instance;
‘transport’ (optional): Transport instance;
‘socket’ (optional): socket.socket instance.
Note
This method should not be overloaded in subclassed
event loops. For custom exception handling, use
the set_exception_handler() method.
loop.get_debug()Get the debug mode (bool) of the event loop.
The default value is True if the environment variable
PYTHONASYNCIODEBUG 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.
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
The default asyncio event loop on Windows does not support subprocesses. See Subprocess Support on Windows for details.
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 with shell=False and the list of strings passed as
the first argument; however, where 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
asyncio.SubprocessProtocol class.
Other parameters:
stdin can be any of these:
a file-like object representing a pipe to be connected to the
subprocess’s standard input stream using
connect_write_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 process
the subprocess.DEVNULL constant which indicates that the
special os.devnull file will be used
stdout can be any of these:
a file-like object representing a pipe to be connected to the
subprocess’s standard output stream using
connect_write_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 process
the subprocess.DEVNULL constant which indicates that the
special os.devnull file will be used
stderr can be any of these:
a file-like object representing a pipe to be connected to the
subprocess’s standard error stream using
connect_write_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 process
the subprocess.DEVNULL constant which indicates that the
special os.devnull file will be used
the 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.
See the constructor of the subprocess.Popen class
for documentation on other arguments.
Returns a pair of (transport, protocol), where transport
conforms to the asyncio.SubprocessTransport base class and
protocol is an object instantiated by the protocol_factory.
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 a
bytes string encoded to the
filesystem encoding,
using the platform’s “shell” syntax.
This is similar to the standard library subprocess.Popen
class called with shell=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 the 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
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.
asyncio.HandleA callback wrapper object returned by loop.call_soon(),
loop.call_soon_threadsafe().
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.
New in version 3.7.
asyncio.TimerHandleA callback wrapper object returned by loop.call_later(),
and loop.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().
New in version 3.7.
Server objects are created by loop.create_server(),
loop.create_unix_server(), start_server(),
and start_unix_server() functions.
Do not instantiate the class directly.
asyncio.ServerServer 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.
Changed in version 3.7: Server object is an asynchronous context manager since Python 3.7.
close()Stop serving: close listening sockets and set the sockets
attribute to None.
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.
get_loop()Return the event loop associated with the server object.
New in version 3.7.
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
loop.create_server() and
asyncio.start_server() allows creating a Server object
that is not accepting connections initially. In this case
Server.start_serving(), or Server.serve_forever() can be used
to make the Server start accepting connections.
New in version 3.7.
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))
New in version 3.7.
is_serving()Return True if the server is accepting new connections.
New in version 3.7.
wait_closed()Wait until the close() method completes.
socketsList of socket.socket objects 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.
asyncio ships with two different event loop implementations:
SelectorEventLoop and ProactorEventLoop.
By default asyncio is configured to use SelectorEventLoop
on Unix and ProactorEventLoop on Windows.
asyncio.SelectorEventLoopAn event loop based on the 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.
asyncio.ProactorEventLoopAn event loop for Windows that uses “I/O Completion Ports” (IOCP).
Availability: Windows.
asyncio.AbstractEventLoopAbstract base class for asyncio-compliant event loops.
The Event Loop Methods 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 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().
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.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()
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.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()
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.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()
See also
loop.create_connection() method.asyncio.open_connection() function and streams.
(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())