Base Event Loop¶
Source code: :source:`Lib/asyncio/events.py`
The event loop is the central execution device provided by asyncio
.
It provides multiple facilities, including:
- Registering, executing and cancelling delayed calls (timeouts).
- Creating client and server transports for various kinds of communication.
- Launching subprocesses and the associated transports for communication with an external program.
- Delegating costly function calls to a pool of threads.
-
class
asyncio.
BaseEventLoop
¶ This class is an implementation detail. It is a subclass of
AbstractEventLoop
and may be a base class of concrete event loop implementations found inasyncio
. It should not be used directly; useAbstractEventLoop
instead.BaseEventLoop
should not be subclassed by third-party code; the internal interface is not stable.
-
class
asyncio.
AbstractEventLoop
¶ Abstract base class of event loops.
This class is not thread safe.
Run an event loop¶
-
AbstractEventLoop.
run_forever
()¶ Run until
stop()
is called. Ifstop()
is called beforerun_forever()
is called, this polls the I/O selector once with a timeout of zero, runs all callbacks scheduled in response to I/O events (and those that were already scheduled), and then exits. Ifstop()
is called whilerun_forever()
is running, this will run the current batch of callbacks and then exit. Note that callbacks scheduled by callbacks will not run in that case; they will run the next timerun_forever()
is called.Changed in version 3.5.1.
-
AbstractEventLoop.
run_until_complete
(future)¶ Run until the
Future
is done.If the argument is a coroutine object, it is wrapped by
ensure_future()
.Return the Future’s result, or raise its exception.
-
AbstractEventLoop.
is_running
()¶ Returns running status of event loop.
-
AbstractEventLoop.
stop
()¶ Stop running the event loop.
This causes
run_forever()
to exit at the next suitable opportunity (see there for more details).Changed in version 3.5.1.
-
AbstractEventLoop.
is_closed
()¶ Returns
True
if the event loop was closed.New in version 3.4.2.
-
AbstractEventLoop.
close
()¶ Close the event loop. The loop must not be running. Pending callbacks will be lost.
This clears the queues and shuts down the executor, but does not wait for the executor to finish.
This is idempotent and irreversible. No other methods should be called after this one.
Calls¶
Most asyncio
functions don’t accept keywords. If you want to pass
keywords to your callback, use functools.partial()
. For example,
loop.call_soon(functools.partial(print, "Hello", flush=True))
will call
print("Hello", flush=True)
.
Note
functools.partial()
is better than lambda
functions, because
asyncio
can inspect functools.partial()
object to display
parameters in debug mode, whereas lambda
functions have a poor
representation.
-
AbstractEventLoop.
call_soon
(callback, *args, context=None)¶ Arrange for a callback to be called as soon as possible. The callback is called after
call_soon()
returns, when control returns to the event loop.This operates as a FIFO queue, callbacks are called in the order in which they are registered. Each callback will be called exactly once.
Any positional arguments after the callback will be passed to the callback when it is called.
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 to cancel the callback.Use functools.partial to pass keywords to the callback.
Changed in version 3.7: The context keyword-only parameter was added. See PEP 567 for more details.
-
AbstractEventLoop.
call_soon_threadsafe
(callback, *args, context=None)¶ Like
call_soon()
, but thread safe.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.
Delayed calls¶
The event loop has its own internal clock for computing timeouts.
Which clock is used depends on the (platform-specific) event loop
implementation; ideally it is a monotonic clock. This will generally be
a different clock than time.time()
.
Note
Timeouts (relative delay or absolute when) should not exceed one day.
-
AbstractEventLoop.
call_later
(delay, callback, *args, context=None)¶ Arrange for the callback to be called after the given delay seconds (either an int or float).
An instance of
asyncio.TimerHandle
is returned, which can be used to cancel the callback.callback will be called exactly once per call to
call_later()
. If two callbacks are scheduled for exactly the same time, it is undefined which will be called first.The optional positional args will be passed to the callback when it is called. If you want the callback to be called with some named arguments, use a closure or
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.Use functools.partial to pass keywords to the callback.
Changed in version 3.7: The context keyword-only parameter was added. See PEP 567 for more details.
-
AbstractEventLoop.
call_at
(when, callback, *args, context=None)¶ Arrange for the callback to be called at the given absolute timestamp when (an int or float), using the same time reference as
AbstractEventLoop.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.Use functools.partial to pass keywords to the callback.
Changed in version 3.7: The context keyword-only parameter was added. See PEP 567 for more details.
-
AbstractEventLoop.
time
()¶ Return the current time, as a
float
value, according to the event loop’s internal clock.
See also
The asyncio.sleep()
function.
Futures¶
-
AbstractEventLoop.
create_future
()¶ Create an
asyncio.Future
object attached to the loop.This is a preferred way to create futures in asyncio, as event loop implementations can provide alternative implementations of the Future class (with better performance or instrumentation).
New in version 3.5.2.
Tasks¶
-
AbstractEventLoop.
create_task
(coro)¶ Schedule the execution of a coroutine object: wrap it in a future. 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
.New in version 3.4.2.
-
AbstractEventLoop.
set_task_factory
(factory)¶ Set a task factory that will be used by
AbstractEventLoop.create_task()
.If factory is
None
the default task factory will be set.If factory is a callable, it should have a signature matching
(loop, coro)
, where loop will be a reference to the active event loop, coro will be a coroutine object. The callable must return anasyncio.Future
compatible object.New in version 3.4.4.
-
AbstractEventLoop.
get_task_factory
()¶ Return a task factory, or
None
if the default one is in use.New in version 3.4.4.
Creating connections¶
Creating listening connections¶
File Transferring¶
TLS Upgrade¶
Watch file descriptors¶
On Windows with SelectorEventLoop
, only socket handles are supported
(ex: pipe file descriptors are not supported).
On Windows with ProactorEventLoop
, these methods are not supported.
-
AbstractEventLoop.
add_reader
(fd, callback, *args)¶ Start watching the file descriptor for read availability and then call the callback with specified arguments.
-
AbstractEventLoop.
remove_reader
(fd)¶ Stop watching the file descriptor for read availability.
-
AbstractEventLoop.
add_writer
(fd, callback, *args)¶ Start watching the file descriptor for write availability and then call the callback with specified arguments.
-
AbstractEventLoop.
remove_writer
(fd)¶ Stop watching the file descriptor for write availability.
The watch a file descriptor for read events
example uses the low-level AbstractEventLoop.add_reader()
method to register
the file descriptor of a socket.
Low-level socket operations¶
Resolve host name¶
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.
Connect pipes¶
On Windows with SelectorEventLoop
, these methods are not supported.
Use ProactorEventLoop
to support pipes on Windows.
See also
The AbstractEventLoop.subprocess_exec()
and
AbstractEventLoop.subprocess_shell()
methods.
UNIX signals¶
Availability: UNIX only.
-
AbstractEventLoop.
add_signal_handler
(signum, callback, *args)¶ Add a handler for a signal.
Raise
ValueError
if the signal number is invalid or uncatchable. RaiseRuntimeError
if there is a problem setting up the handler.
-
AbstractEventLoop.
remove_signal_handler
(sig)¶ Remove a handler for a signal.
Return
True
if a signal handler was removed,False
if not.
See also
The signal
module.
Executor¶
Call a function in an Executor
(pool of threads or
pool of processes). By default, an event loop uses a thread pool executor
(ThreadPoolExecutor
).
-
AbstractEventLoop.
run_in_executor
(executor, func, *args)¶ Arrange for a func to be called in the specified executor.
The executor argument should be an
Executor
instance. The default executor is used if executor isNone
.Use functools.partial to pass keywords to the *func*.
This method returns a
asyncio.Future
object.Changed in version 3.5.3:
BaseEventLoop.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.
-
AbstractEventLoop.
set_default_executor
(executor)¶ Set the default executor used by
run_in_executor()
.
Error Handling API¶
Allows customizing how exceptions are handled in the event loop.
-
AbstractEventLoop.
set_exception_handler
(handler)¶ Set handler as the new event loop exception handler.
If handler is
None
, the default exception handler will be set.If handler is a callable object, it should have a matching signature to
(loop, context)
, whereloop
will be a reference to the active event loop,context
will be adict
object (seecall_exception_handler()
documentation for details about context).
-
AbstractEventLoop.
get_exception_handler
()¶ Return the exception handler, or
None
if the default one is in use.New in version 3.5.2.
-
AbstractEventLoop.
default_exception_handler
(context)¶ Default exception handler.
This is called when an exception occurs and no exception handler is set, and can be called by a custom exception handler that wants to defer to the default behavior.
context parameter has the same meaning as in
call_exception_handler()
.
-
AbstractEventLoop.
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 later):- ‘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
Note: this method should not be overloaded in subclassed event loops. For any custom exception handling, use
set_exception_handler()
method.
Debug mode¶
-
AbstractEventLoop.
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.New in version 3.4.2.
-
AbstractEventLoop.
set_debug
(enabled: bool)¶ Set the debug mode of the event loop.
New in version 3.4.2.
See also
Server¶
-
class
asyncio.
Server
¶ Server listening on sockets.
Object created by
AbstractEventLoop.create_server()
,AbstractEventLoop.create_unix_server()
,start_server()
, andstart_unix_server()
functions. Don’t instantiate the class directly.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
()¶ Gives the event loop associated with the server object.
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 to, orNone
if the server is closed.Changed in version 3.7: Prior to Python 3.7
Server.sockets
used to return the internal list of server’s sockets directly. In 3.7 a copy of that list is returned.
-
Handle¶
-
class
asyncio.
Handle
¶ A callback wrapper object returned by
AbstractEventLoop.call_soon()
,AbstractEventLoop.call_soon_threadsafe()
.-
cancel
()¶ Cancel the call. If the callback is already canceled or executed, this method has no effect.
-
cancelled
()¶ Return
True
if the call was cancelled.New in version 3.7.
-
-
class
asyncio.
TimerHandle
¶ A callback wrapper object returned by
AbstractEventLoop.call_later()
, andAbstractEventLoop.call_at()
.The class is inherited from
Handle
.-
when
()¶ Return a scheduled callback time as
float
seconds.The time is an absolute timestamp, using the same time reference as
AbstractEventLoop.time()
.New in version 3.7.
-
SendfileNotAvailableError¶
-
exception
asyncio.
SendfileNotAvailableError
¶ Sendfile syscall is not available, subclass of
RuntimeError
.Raised if the OS does not support sendfile syscall for given socket or file type.
Event loop examples¶
Hello World with call_soon()¶
Example using the AbstractEventLoop.call_soon()
method to schedule a
callback. The callback displays "Hello World"
and then stops the event
loop:
import asyncio
def hello_world(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()
loop.run_forever()
loop.close()
See also
The Hello World coroutine example uses a coroutine.
Display the current date with call_later()¶
Example of callback displaying the current date every second. The callback uses
the AbstractEventLoop.call_later()
method to reschedule itself during 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()
loop.run_forever()
loop.close()
See also
The coroutine displaying the current date example uses a coroutine.
Watch a file descriptor for read events¶
Wait until a file descriptor received some data using the
AbstractEventLoop.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())
# Run the event loop
loop.run_forever()
# We are done, close sockets and the event loop
rsock.close()
wsock.close()
loop.close()
See also
The register an open socket to wait for data using a protocol example uses a low-level protocol created by the
AbstractEventLoop.create_connection()
method.
The register an open socket to wait for data using streams example uses high-level streams
created by the open_connection()
function in a coroutine.
Set signal handlers for SIGINT and SIGTERM¶
Register handlers for signals SIGINT
and SIGTERM
using
the AbstractEventLoop.add_signal_handler()
method:
import asyncio
import functools
import os
import signal
def ask_exit(signame):
print("got signal %s: exit" % signame)
loop.stop()
loop = asyncio.get_event_loop()
for signame in ('SIGINT', 'SIGTERM'):
loop.add_signal_handler(getattr(signal, signame),
functools.partial(ask_exit, signame))
print("Event loop running forever, press Ctrl+C to interrupt.")
print("pid %s: send SIGINT or SIGTERM to exit." % os.getpid())
try:
loop.run_forever()
finally:
loop.close()
This example only works on UNIX.