Event Loop¶
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 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 toget_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)¶ Set loop as a current event loop for the current OS thread.
-
asyncio.
new_event_loop
()¶ Create 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.
-
coroutine
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.
Scheduling callbacks¶
-
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
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.
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. 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 3.7.1: 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.
-
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 3.7.1: 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.
-
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).
New in version 3.5.2.
-
loop.
create_task
(coro)¶ 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 ofTask
.
-
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 aasyncio.Future
-compatible object.
-
loop.
get_task_factory
()¶ Return a task factory or
None
if the default one is in use.
Opening network connections¶
-
coroutine
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
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.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 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 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).
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.
-
coroutine
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
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 to locally. The local_host and local_port are looked up usinggetaddrinfo()
.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_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 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 beNone
).
On Windows, with
ProactorEventLoop
, this method is not supported.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.
-
coroutine
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 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.
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.
Creating network servers¶
-
coroutine
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
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.
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.
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).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.
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 ofStreamReader
andStreamWriter
that can be used in an async/await code.
-
coroutine
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 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.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.
-
coroutine
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 ifNone
(default).
Returns a
(transport, protocol)
pair.New in version 3.7: The ssl_handshake_timeout parameter.
New in version 3.5.3.
Transferring files¶
-
coroutine
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
.New in version 3.7.
TLS Upgrade¶
-
coroutine
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()
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).
New in version 3.7.
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.
-
loop.
remove_reader
(fd)¶ Stop monitoring the fd file descriptor for read availability.
-
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)¶ Stop monitoring the fd file descriptor for write availability.
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.
-
coroutine
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.
-
coroutine
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.
-
coroutine
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 anasync def
method.
-
coroutine
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
-
coroutine
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
-
coroutine
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.
New in version 3.7.
DNS¶
-
coroutine
loop.
getaddrinfo
(host, port, *, family=0, type=0, proto=0, flags=0)¶ Asynchronous version of
socket.getaddrinfo()
.
-
coroutine
loop.
getnameinfo
(sockaddr, flags=0)¶ Asynchronous version of
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.
Working with pipes¶
-
coroutine
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.
-
coroutine
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.
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.
The executor argument should be an
concurrent.futures.Executor
instance. The default executor is used if executor isNone
.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 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 should be an instance ofThreadPoolExecutor
.Deprecated since version 3.7: 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
.
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).
-
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.
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
-X dev
command line option can now also be used to enable the debug mode.
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
The default asyncio event loop on Windows does not support subprocesses. See Subprocess Support on Windows for details.
-
coroutine
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: either a file-like object representing a pipe to be connected to the subprocess’s standard input stream using
connect_write_pipe()
, or thesubprocess.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
connect_read_pipe()
, or thesubprocess.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
connect_read_pipe()
, or one ofsubprocess.PIPE
(default) orsubprocess.STDOUT
constants.By default a new pipe will be created and connected. When
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
subprocess.Popen
without interpretation, except for bufsize, universal_newlines and shell, which should not be specified at all.
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.
-
coroutine
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.
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()
.-
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.
-
-
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()
.New 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 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.
-
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.
-
get_loop
()¶ Return the event loop associated with the server object.
New in version 3.7.
-
coroutine
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()
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.New in version 3.7.
-
coroutine
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.
-
sockets
¶ List of
socket.socket
objects the server is listening on, orNone
if the server is closed.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 SelectorEventLoop
on all platforms.
-
class
asyncio.
SelectorEventLoop
¶ An 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.
-
class
asyncio.
ProactorEventLoop
¶ An event loop for Windows that uses “I/O Completion Ports” (IOCP).
Availability: Windows.
An example how to use
ProactorEventLoop
on Windows:import asyncio import sys if sys.platform == 'win32': loop = asyncio.ProactorEventLoop() asyncio.set_event_loop(loop)
-
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.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()
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.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()
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.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
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())