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cpython/Doc/library/asyncio-task.rst
Thomas Grainger c9159b7436
expand the asyncio.run_coroutine_threadsafe recipes (#127576) 
Co-authored-by: Kumar Aditya <kumaraditya@python.org>
2024-12-29 11:52:29 +05:30

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.. currentmodule:: asyncio
====================
Coroutines and Tasks
====================
This section outlines high-level asyncio APIs to work with coroutines
and Tasks.
.. contents::
:depth: 1
:local:
.. _coroutine:
Coroutines
==========
**Source code:** :source:`Lib/asyncio/coroutines.py`
----------------------------------------------------
:term:`Coroutines <coroutine>` declared with the async/await syntax is the
preferred way of writing asyncio applications. For example, the following
snippet of code prints "hello", waits 1 second,
and then prints "world"::
>>> import asyncio
>>> async def main():
... print('hello')
... await asyncio.sleep(1)
... print('world')
>>> asyncio.run(main())
hello
world
Note that simply calling a coroutine will not schedule it to
be executed::
>>> main()
<coroutine object main at 0x1053bb7c8>
To actually run a coroutine, asyncio provides the following mechanisms:
* The :func:`asyncio.run` function to run the top-level
entry point "main()" function (see the above example.)
* Awaiting on a coroutine. The following snippet of code will
print "hello" after waiting for 1 second, and then print "world"
after waiting for *another* 2 seconds::
import asyncio
import time
async def say_after(delay, what):
await asyncio.sleep(delay)
print(what)
async def main():
print(f"started at {time.strftime('%X')}")
await say_after(1, 'hello')
await say_after(2, 'world')
print(f"finished at {time.strftime('%X')}")
asyncio.run(main())
Expected output::
started at 17:13:52
hello
world
finished at 17:13:55
* The :func:`asyncio.create_task` function to run coroutines
concurrently as asyncio :class:`Tasks <Task>`.
Let's modify the above example and run two ``say_after`` coroutines
*concurrently*::
async def main():
task1 = asyncio.create_task(
say_after(1, 'hello'))
task2 = asyncio.create_task(
say_after(2, 'world'))
print(f"started at {time.strftime('%X')}")
# Wait until both tasks are completed (should take
# around 2 seconds.)
await task1
await task2
print(f"finished at {time.strftime('%X')}")
Note that expected output now shows that the snippet runs
1 second faster than before::
started at 17:14:32
hello
world
finished at 17:14:34
* The :class:`asyncio.TaskGroup` class provides a more modern
alternative to :func:`create_task`.
Using this API, the last example becomes::
async def main():
async with asyncio.TaskGroup() as tg:
task1 = tg.create_task(
say_after(1, 'hello'))
task2 = tg.create_task(
say_after(2, 'world'))
print(f"started at {time.strftime('%X')}")
# The await is implicit when the context manager exits.
print(f"finished at {time.strftime('%X')}")
The timing and output should be the same as for the previous version.
.. versionadded:: 3.11
:class:`asyncio.TaskGroup`.
.. _asyncio-awaitables:
Awaitables
==========
We say that an object is an **awaitable** object if it can be used
in an :keyword:`await` expression. Many asyncio APIs are designed to
accept awaitables.
There are three main types of *awaitable* objects:
**coroutines**, **Tasks**, and **Futures**.
.. rubric:: Coroutines
Python coroutines are *awaitables* and therefore can be awaited from
other coroutines::
import asyncio
async def nested():
return 42
async def main():
# Nothing happens if we just call "nested()".
# A coroutine object is created but not awaited,
# so it *won't run at all*.
nested() # will raise a "RuntimeWarning".
# Let's do it differently now and await it:
print(await nested()) # will print "42".
asyncio.run(main())
.. important::
In this documentation the term "coroutine" can be used for
two closely related concepts:
* a *coroutine function*: an :keyword:`async def` function;
* a *coroutine object*: an object returned by calling a
*coroutine function*.
.. rubric:: Tasks
*Tasks* are used to schedule coroutines *concurrently*.
When a coroutine is wrapped into a *Task* with functions like
:func:`asyncio.create_task` the coroutine is automatically
scheduled to run soon::
import asyncio
async def nested():
return 42
async def main():
# Schedule nested() to run soon concurrently
# with "main()".
task = asyncio.create_task(nested())
# "task" can now be used to cancel "nested()", or
# can simply be awaited to wait until it is complete:
await task
asyncio.run(main())
.. rubric:: Futures
A :class:`Future` is a special **low-level** awaitable object that
represents an **eventual result** of an asynchronous operation.
When a Future object is *awaited* it means that the coroutine will
wait until the Future is resolved in some other place.
Future objects in asyncio are needed to allow callback-based code
to be used with async/await.
Normally **there is no need** to create Future objects at the
application level code.
Future objects, sometimes exposed by libraries and some asyncio
APIs, can be awaited::
async def main():
await function_that_returns_a_future_object()
# this is also valid:
await asyncio.gather(
function_that_returns_a_future_object(),
some_python_coroutine()
)
A good example of a low-level function that returns a Future object
is :meth:`loop.run_in_executor`.
Creating Tasks
==============
**Source code:** :source:`Lib/asyncio/tasks.py`
-----------------------------------------------
.. function:: create_task(coro, *, name=None, context=None)
Wrap the *coro* :ref:`coroutine <coroutine>` into a :class:`Task`
and schedule its execution. Return the Task object.
If *name* is not ``None``, it is set as the name of the task using
:meth:`Task.set_name`.
An optional keyword-only *context* argument allows specifying a
custom :class:`contextvars.Context` for the *coro* to run in.
The current context copy is created when no *context* is provided.
The task is executed in the loop returned by :func:`get_running_loop`,
:exc:`RuntimeError` is raised if there is no running loop in
current thread.
.. note::
:meth:`asyncio.TaskGroup.create_task` is a new alternative
leveraging structural concurrency; it allows for waiting
for a group of related tasks with strong safety guarantees.
.. important::
Save a reference to the result of this function, to avoid
a task disappearing mid-execution. The event loop only keeps
weak references to tasks. A task that isn't referenced elsewhere
may get garbage collected at any time, even before it's done.
For reliable "fire-and-forget" background tasks, gather them in
a collection::
background_tasks = set()
for i in range(10):
task = asyncio.create_task(some_coro(param=i))
# Add task to the set. This creates a strong reference.
background_tasks.add(task)
# To prevent keeping references to finished tasks forever,
# make each task remove its own reference from the set after
# completion:
task.add_done_callback(background_tasks.discard)
.. versionadded:: 3.7
.. versionchanged:: 3.8
Added the *name* parameter.
.. versionchanged:: 3.11
Added the *context* parameter.
Task Cancellation
=================
Tasks can easily and safely be cancelled.
When a task is cancelled, :exc:`asyncio.CancelledError` will be raised
in the task at the next opportunity.
It is recommended that coroutines use ``try/finally`` blocks to robustly
perform clean-up logic. In case :exc:`asyncio.CancelledError`
is explicitly caught, it should generally be propagated when
clean-up is complete. :exc:`asyncio.CancelledError` directly subclasses
:exc:`BaseException` so most code will not need to be aware of it.
The asyncio components that enable structured concurrency, like
:class:`asyncio.TaskGroup` and :func:`asyncio.timeout`,
are implemented using cancellation internally and might misbehave if
a coroutine swallows :exc:`asyncio.CancelledError`. Similarly, user code
should not generally call :meth:`uncancel <asyncio.Task.uncancel>`.
However, in cases when suppressing :exc:`asyncio.CancelledError` is
truly desired, it is necessary to also call ``uncancel()`` to completely
remove the cancellation state.
.. _taskgroups:
Task Groups
===========
Task groups combine a task creation API with a convenient
and reliable way to wait for all tasks in the group to finish.
.. class:: TaskGroup()
An :ref:`asynchronous context manager <async-context-managers>`
holding a group of tasks.
Tasks can be added to the group using :meth:`create_task`.
All tasks are awaited when the context manager exits.
.. versionadded:: 3.11
.. method:: create_task(coro, *, name=None, context=None)
Create a task in this task group.
The signature matches that of :func:`asyncio.create_task`.
If the task group is inactive (e.g. not yet entered,
already finished, or in the process of shutting down),
we will close the given ``coro``.
.. versionchanged:: 3.13
Close the given coroutine if the task group is not active.
Example::
async def main():
async with asyncio.TaskGroup() as tg:
task1 = tg.create_task(some_coro(...))
task2 = tg.create_task(another_coro(...))
print(f"Both tasks have completed now: {task1.result()}, {task2.result()}")
The ``async with`` statement will wait for all tasks in the group to finish.
While waiting, new tasks may still be added to the group
(for example, by passing ``tg`` into one of the coroutines
and calling ``tg.create_task()`` in that coroutine).
Once the last task has finished and the ``async with`` block is exited,
no new tasks may be added to the group.
The first time any of the tasks belonging to the group fails
with an exception other than :exc:`asyncio.CancelledError`,
the remaining tasks in the group are cancelled.
No further tasks can then be added to the group.
At this point, if the body of the ``async with`` statement is still active
(i.e., :meth:`~object.__aexit__` hasn't been called yet),
the task directly containing the ``async with`` statement is also cancelled.
The resulting :exc:`asyncio.CancelledError` will interrupt an ``await``,
but it will not bubble out of the containing ``async with`` statement.
Once all tasks have finished, if any tasks have failed
with an exception other than :exc:`asyncio.CancelledError`,
those exceptions are combined in an
:exc:`ExceptionGroup` or :exc:`BaseExceptionGroup`
(as appropriate; see their documentation)
which is then raised.
Two base exceptions are treated specially:
If any task fails with :exc:`KeyboardInterrupt` or :exc:`SystemExit`,
the task group still cancels the remaining tasks and waits for them,
but then the initial :exc:`KeyboardInterrupt` or :exc:`SystemExit`
is re-raised instead of :exc:`ExceptionGroup` or :exc:`BaseExceptionGroup`.
If the body of the ``async with`` statement exits with an exception
(so :meth:`~object.__aexit__` is called with an exception set),
this is treated the same as if one of the tasks failed:
the remaining tasks are cancelled and then waited for,
and non-cancellation exceptions are grouped into an
exception group and raised.
The exception passed into :meth:`~object.__aexit__`,
unless it is :exc:`asyncio.CancelledError`,
is also included in the exception group.
The same special case is made for
:exc:`KeyboardInterrupt` and :exc:`SystemExit` as in the previous paragraph.
Task groups are careful not to mix up the internal cancellation used to
"wake up" their :meth:`~object.__aexit__` with cancellation requests
for the task in which they are running made by other parties.
In particular, when one task group is syntactically nested in another,
and both experience an exception in one of their child tasks simultaneously,
the inner task group will process its exceptions, and then the outer task group
will receive another cancellation and process its own exceptions.
In the case where a task group is cancelled externally and also must
raise an :exc:`ExceptionGroup`, it will call the parent task's
:meth:`~asyncio.Task.cancel` method. This ensures that a
:exc:`asyncio.CancelledError` will be raised at the next
:keyword:`await`, so the cancellation is not lost.
Task groups preserve the cancellation count
reported by :meth:`asyncio.Task.cancelling`.
.. versionchanged:: 3.13
Improved handling of simultaneous internal and external cancellations
and correct preservation of cancellation counts.
Terminating a Task Group
------------------------
While terminating a task group is not natively supported by the standard
library, termination can be achieved by adding an exception-raising task
to the task group and ignoring the raised exception:
.. code-block:: python
import asyncio
from asyncio import TaskGroup
class TerminateTaskGroup(Exception):
"""Exception raised to terminate a task group."""
async def force_terminate_task_group():
"""Used to force termination of a task group."""
raise TerminateTaskGroup()
async def job(task_id, sleep_time):
print(f'Task {task_id}: start')
await asyncio.sleep(sleep_time)
print(f'Task {task_id}: done')
async def main():
try:
async with TaskGroup() as group:
# spawn some tasks
group.create_task(job(1, 0.5))
group.create_task(job(2, 1.5))
# sleep for 1 second
await asyncio.sleep(1)
# add an exception-raising task to force the group to terminate
group.create_task(force_terminate_task_group())
except* TerminateTaskGroup:
pass
asyncio.run(main())
Expected output:
.. code-block:: text
Task 1: start
Task 2: start
Task 1: done
Sleeping
========
.. coroutinefunction:: sleep(delay, result=None)
Block for *delay* seconds.
If *result* is provided, it is returned to the caller
when the coroutine completes.
``sleep()`` always suspends the current task, allowing other tasks
to run.
Setting the delay to 0 provides an optimized path to allow other
tasks to run. This can be used by long-running functions to avoid
blocking the event loop for the full duration of the function call.
.. _asyncio_example_sleep:
Example of coroutine displaying the current date every second
for 5 seconds::
import asyncio
import datetime
async def display_date():
loop = asyncio.get_running_loop()
end_time = loop.time() + 5.0
while True:
print(datetime.datetime.now())
if (loop.time() + 1.0) >= end_time:
break
await asyncio.sleep(1)
asyncio.run(display_date())
.. versionchanged:: 3.10
Removed the *loop* parameter.
.. versionchanged:: 3.13
Raises :exc:`ValueError` if *delay* is :data:`~math.nan`.
Running Tasks Concurrently
==========================
.. awaitablefunction:: gather(*aws, return_exceptions=False)
Run :ref:`awaitable objects <asyncio-awaitables>` in the *aws*
sequence *concurrently*.
If any awaitable in *aws* is a coroutine, it is automatically
scheduled as a Task.
If all awaitables are completed successfully, the result is an
aggregate list of returned values. The order of result values
corresponds to the order of awaitables in *aws*.
If *return_exceptions* is ``False`` (default), the first
raised exception is immediately propagated to the task that
awaits on ``gather()``. Other awaitables in the *aws* sequence
**won't be cancelled** and will continue to run.
If *return_exceptions* is ``True``, exceptions are treated the
same as successful results, and aggregated in the result list.
If ``gather()`` is *cancelled*, all submitted awaitables
(that have not completed yet) are also *cancelled*.
If any Task or Future from the *aws* sequence is *cancelled*, it is
treated as if it raised :exc:`CancelledError` -- the ``gather()``
call is **not** cancelled in this case. This is to prevent the
cancellation of one submitted Task/Future to cause other
Tasks/Futures to be cancelled.
.. note::
A new alternative to create and run tasks concurrently and
wait for their completion is :class:`asyncio.TaskGroup`. *TaskGroup*
provides stronger safety guarantees than *gather* for scheduling a nesting of subtasks:
if a task (or a subtask, a task scheduled by a task)
raises an exception, *TaskGroup* will, while *gather* will not,
cancel the remaining scheduled tasks).
.. _asyncio_example_gather:
Example::
import asyncio
async def factorial(name, number):
f = 1
for i in range(2, number + 1):
print(f"Task {name}: Compute factorial({number}), currently i={i}...")
await asyncio.sleep(1)
f *= i
print(f"Task {name}: factorial({number}) = {f}")
return f
async def main():
# Schedule three calls *concurrently*:
L = await asyncio.gather(
factorial("A", 2),
factorial("B", 3),
factorial("C", 4),
)
print(L)
asyncio.run(main())
# Expected output:
#
# Task A: Compute factorial(2), currently i=2...
# Task B: Compute factorial(3), currently i=2...
# Task C: Compute factorial(4), currently i=2...
# Task A: factorial(2) = 2
# Task B: Compute factorial(3), currently i=3...
# Task C: Compute factorial(4), currently i=3...
# Task B: factorial(3) = 6
# Task C: Compute factorial(4), currently i=4...
# Task C: factorial(4) = 24
# [2, 6, 24]
.. note::
If *return_exceptions* is false, cancelling gather() after it
has been marked done won't cancel any submitted awaitables.
For instance, gather can be marked done after propagating an
exception to the caller, therefore, calling ``gather.cancel()``
after catching an exception (raised by one of the awaitables) from
gather won't cancel any other awaitables.
.. versionchanged:: 3.7
If the *gather* itself is cancelled, the cancellation is
propagated regardless of *return_exceptions*.
.. versionchanged:: 3.10
Removed the *loop* parameter.
.. deprecated:: 3.10
Deprecation warning is emitted if no positional arguments are provided
or not all positional arguments are Future-like objects
and there is no running event loop.
.. _eager-task-factory:
Eager Task Factory
==================
.. function:: eager_task_factory(loop, coro, *, name=None, context=None)
A task factory for eager task execution.
When using this factory (via :meth:`loop.set_task_factory(asyncio.eager_task_factory) <loop.set_task_factory>`),
coroutines begin execution synchronously during :class:`Task` construction.
Tasks are only scheduled on the event loop if they block.
This can be a performance improvement as the overhead of loop scheduling
is avoided for coroutines that complete synchronously.
A common example where this is beneficial is coroutines which employ
caching or memoization to avoid actual I/O when possible.
.. note::
Immediate execution of the coroutine is a semantic change.
If the coroutine returns or raises, the task is never scheduled
to the event loop. If the coroutine execution blocks, the task is
scheduled to the event loop. This change may introduce behavior
changes to existing applications. For example,
the application's task execution order is likely to change.
.. versionadded:: 3.12
.. function:: create_eager_task_factory(custom_task_constructor)
Create an eager task factory, similar to :func:`eager_task_factory`,
using the provided *custom_task_constructor* when creating a new task instead
of the default :class:`Task`.
*custom_task_constructor* must be a *callable* with the signature matching
the signature of :class:`Task.__init__ <Task>`.
The callable must return a :class:`asyncio.Task`-compatible object.
This function returns a *callable* intended to be used as a task factory of an
event loop via :meth:`loop.set_task_factory(factory) <loop.set_task_factory>`).
.. versionadded:: 3.12
Shielding From Cancellation
===========================
.. awaitablefunction:: shield(aw)
Protect an :ref:`awaitable object <asyncio-awaitables>`
from being :meth:`cancelled <Task.cancel>`.
If *aw* is a coroutine it is automatically scheduled as a Task.
The statement::
task = asyncio.create_task(something())
res = await shield(task)
is equivalent to::
res = await something()
*except* that if the coroutine containing it is cancelled, the
Task running in ``something()`` is not cancelled. From the point
of view of ``something()``, the cancellation did not happen.
Although its caller is still cancelled, so the "await" expression
still raises a :exc:`CancelledError`.
If ``something()`` is cancelled by other means (i.e. from within
itself) that would also cancel ``shield()``.
If it is desired to completely ignore cancellation (not recommended)
the ``shield()`` function should be combined with a try/except
clause, as follows::
task = asyncio.create_task(something())
try:
res = await shield(task)
except CancelledError:
res = None
.. important::
Save a reference to tasks passed to this function, to avoid
a task disappearing mid-execution. The event loop only keeps
weak references to tasks. A task that isn't referenced elsewhere
may get garbage collected at any time, even before it's done.
.. versionchanged:: 3.10
Removed the *loop* parameter.
.. deprecated:: 3.10
Deprecation warning is emitted if *aw* is not Future-like object
and there is no running event loop.
Timeouts
========
.. function:: timeout(delay)
Return an :ref:`asynchronous context manager <async-context-managers>`
that can be used to limit the amount of time spent waiting on
something.
*delay* can either be ``None``, or a float/int number of
seconds to wait. If *delay* is ``None``, no time limit will
be applied; this can be useful if the delay is unknown when
the context manager is created.
In either case, the context manager can be rescheduled after
creation using :meth:`Timeout.reschedule`.
Example::
async def main():
async with asyncio.timeout(10):
await long_running_task()
If ``long_running_task`` takes more than 10 seconds to complete,
the context manager will cancel the current task and handle
the resulting :exc:`asyncio.CancelledError` internally, transforming it
into a :exc:`TimeoutError` which can be caught and handled.
.. note::
The :func:`asyncio.timeout` context manager is what transforms
the :exc:`asyncio.CancelledError` into a :exc:`TimeoutError`,
which means the :exc:`TimeoutError` can only be caught
*outside* of the context manager.
Example of catching :exc:`TimeoutError`::
async def main():
try:
async with asyncio.timeout(10):
await long_running_task()
except TimeoutError:
print("The long operation timed out, but we've handled it.")
print("This statement will run regardless.")
The context manager produced by :func:`asyncio.timeout` can be
rescheduled to a different deadline and inspected.
.. class:: Timeout(when)
An :ref:`asynchronous context manager <async-context-managers>`
for cancelling overdue coroutines.
``when`` should be an absolute time at which the context should time out,
as measured by the event loop's clock:
- If ``when`` is ``None``, the timeout will never trigger.
- If ``when < loop.time()``, the timeout will trigger on the next
iteration of the event loop.
.. method:: when() -> float | None
Return the current deadline, or ``None`` if the current
deadline is not set.
.. method:: reschedule(when: float | None)
Reschedule the timeout.
.. method:: expired() -> bool
Return whether the context manager has exceeded its deadline
(expired).
Example::
async def main():
try:
# We do not know the timeout when starting, so we pass ``None``.
async with asyncio.timeout(None) as cm:
# We know the timeout now, so we reschedule it.
new_deadline = get_running_loop().time() + 10
cm.reschedule(new_deadline)
await long_running_task()
except TimeoutError:
pass
if cm.expired():
print("Looks like we haven't finished on time.")
Timeout context managers can be safely nested.
.. versionadded:: 3.11
.. function:: timeout_at(when)
Similar to :func:`asyncio.timeout`, except *when* is the absolute time
to stop waiting, or ``None``.
Example::
async def main():
loop = get_running_loop()
deadline = loop.time() + 20
try:
async with asyncio.timeout_at(deadline):
await long_running_task()
except TimeoutError:
print("The long operation timed out, but we've handled it.")
print("This statement will run regardless.")
.. versionadded:: 3.11
.. coroutinefunction:: wait_for(aw, timeout)
Wait for the *aw* :ref:`awaitable <asyncio-awaitables>`
to complete with a timeout.
If *aw* is a coroutine it is automatically scheduled as a Task.
*timeout* can either be ``None`` or a float or int number of seconds
to wait for. If *timeout* is ``None``, block until the future
completes.
If a timeout occurs, it cancels the task and raises
:exc:`TimeoutError`.
To avoid the task :meth:`cancellation <Task.cancel>`,
wrap it in :func:`shield`.
The function will wait until the future is actually cancelled,
so the total wait time may exceed the *timeout*. If an exception
happens during cancellation, it is propagated.
If the wait is cancelled, the future *aw* is also cancelled.
.. _asyncio_example_waitfor:
Example::
async def eternity():
# Sleep for one hour
await asyncio.sleep(3600)
print('yay!')
async def main():
# Wait for at most 1 second
try:
await asyncio.wait_for(eternity(), timeout=1.0)
except TimeoutError:
print('timeout!')
asyncio.run(main())
# Expected output:
#
# timeout!
.. versionchanged:: 3.7
When *aw* is cancelled due to a timeout, ``wait_for`` waits
for *aw* to be cancelled. Previously, it raised
:exc:`TimeoutError` immediately.
.. versionchanged:: 3.10
Removed the *loop* parameter.
.. versionchanged:: 3.11
Raises :exc:`TimeoutError` instead of :exc:`asyncio.TimeoutError`.
Waiting Primitives
==================
.. coroutinefunction:: wait(aws, *, timeout=None, return_when=ALL_COMPLETED)
Run :class:`~asyncio.Future` and :class:`~asyncio.Task` instances in the *aws*
iterable concurrently and block until the condition specified
by *return_when*.
The *aws* iterable must not be empty.
Returns two sets of Tasks/Futures: ``(done, pending)``.
Usage::
done, pending = await asyncio.wait(aws)
*timeout* (a float or int), if specified, can be used to control
the maximum number of seconds to wait before returning.
Note that this function does not raise :exc:`TimeoutError`.
Futures or Tasks that aren't done when the timeout occurs are simply
returned in the second set.
*return_when* indicates when this function should return. It must
be one of the following constants:
.. list-table::
:header-rows: 1
* - Constant
- Description
* - .. data:: FIRST_COMPLETED
- The function will return when any future finishes or is cancelled.
* - .. data:: FIRST_EXCEPTION
- The function will return when any future finishes by raising an
exception. If no future raises an exception
then it is equivalent to :const:`ALL_COMPLETED`.
* - .. data:: ALL_COMPLETED
- The function will return when all futures finish or are cancelled.
Unlike :func:`~asyncio.wait_for`, ``wait()`` does not cancel the
futures when a timeout occurs.
.. versionchanged:: 3.10
Removed the *loop* parameter.
.. versionchanged:: 3.11
Passing coroutine objects to ``wait()`` directly is forbidden.
.. versionchanged:: 3.12
Added support for generators yielding tasks.
.. function:: as_completed(aws, *, timeout=None)
Run :ref:`awaitable objects <asyncio-awaitables>` in the *aws* iterable
concurrently. The returned object can be iterated to obtain the results
of the awaitables as they finish.
The object returned by ``as_completed()`` can be iterated as an
:term:`asynchronous iterator` or a plain :term:`iterator`. When asynchronous
iteration is used, the originally-supplied awaitables are yielded if they
are tasks or futures. This makes it easy to correlate previously-scheduled
tasks with their results. Example::
ipv4_connect = create_task(open_connection("127.0.0.1", 80))
ipv6_connect = create_task(open_connection("::1", 80))
tasks = [ipv4_connect, ipv6_connect]
async for earliest_connect in as_completed(tasks):
# earliest_connect is done. The result can be obtained by
# awaiting it or calling earliest_connect.result()
reader, writer = await earliest_connect
if earliest_connect is ipv6_connect:
print("IPv6 connection established.")
else:
print("IPv4 connection established.")
During asynchronous iteration, implicitly-created tasks will be yielded for
supplied awaitables that aren't tasks or futures.
When used as a plain iterator, each iteration yields a new coroutine that
returns the result or raises the exception of the next completed awaitable.
This pattern is compatible with Python versions older than 3.13::
ipv4_connect = create_task(open_connection("127.0.0.1", 80))
ipv6_connect = create_task(open_connection("::1", 80))
tasks = [ipv4_connect, ipv6_connect]
for next_connect in as_completed(tasks):
# next_connect is not one of the original task objects. It must be
# awaited to obtain the result value or raise the exception of the
# awaitable that finishes next.
reader, writer = await next_connect
A :exc:`TimeoutError` is raised if the timeout occurs before all awaitables
are done. This is raised by the ``async for`` loop during asynchronous
iteration or by the coroutines yielded during plain iteration.
.. versionchanged:: 3.10
Removed the *loop* parameter.
.. deprecated:: 3.10
Deprecation warning is emitted if not all awaitable objects in the *aws*
iterable are Future-like objects and there is no running event loop.
.. versionchanged:: 3.12
Added support for generators yielding tasks.
.. versionchanged:: 3.13
The result can now be used as either an :term:`asynchronous iterator`
or as a plain :term:`iterator` (previously it was only a plain iterator).
Running in Threads
==================
.. coroutinefunction:: to_thread(func, /, *args, **kwargs)
Asynchronously run function *func* in a separate thread.
Any \*args and \*\*kwargs supplied for this function are directly passed
to *func*. Also, the current :class:`contextvars.Context` is propagated,
allowing context variables from the event loop thread to be accessed in the
separate thread.
Return a coroutine that can be awaited to get the eventual result of *func*.
This coroutine function is primarily intended to be used for executing
IO-bound functions/methods that would otherwise block the event loop if
they were run in the main thread. For example::
def blocking_io():
print(f"start blocking_io at {time.strftime('%X')}")
# Note that time.sleep() can be replaced with any blocking
# IO-bound operation, such as file operations.
time.sleep(1)
print(f"blocking_io complete at {time.strftime('%X')}")
async def main():
print(f"started main at {time.strftime('%X')}")
await asyncio.gather(
asyncio.to_thread(blocking_io),
asyncio.sleep(1))
print(f"finished main at {time.strftime('%X')}")
asyncio.run(main())
# Expected output:
#
# started main at 19:50:53
# start blocking_io at 19:50:53
# blocking_io complete at 19:50:54
# finished main at 19:50:54
Directly calling ``blocking_io()`` in any coroutine would block the event loop
for its duration, resulting in an additional 1 second of run time. Instead,
by using ``asyncio.to_thread()``, we can run it in a separate thread without
blocking the event loop.
.. note::
Due to the :term:`GIL`, ``asyncio.to_thread()`` can typically only be used
to make IO-bound functions non-blocking. However, for extension modules
that release the GIL or alternative Python implementations that don't
have one, ``asyncio.to_thread()`` can also be used for CPU-bound functions.
.. versionadded:: 3.9
Scheduling From Other Threads
=============================
.. function:: run_coroutine_threadsafe(coro, loop)
Submit a coroutine to the given event loop. Thread-safe.
Return a :class:`concurrent.futures.Future` to wait for the result
from another OS thread.
This function is meant to be called from a different OS thread
than the one where the event loop is running. Example::
def in_thread(loop: asyncio.AbstractEventLoop) -> None:
# Run some blocking IO
pathlib.Path("example.txt").write_text("hello world", encoding="utf8")
# Create a coroutine
coro = asyncio.sleep(1, result=3)
# Submit the coroutine to a given loop
future = asyncio.run_coroutine_threadsafe(coro, loop)
# Wait for the result with an optional timeout argument
assert future.result(timeout=2) == 3
async def amain() -> None:
# Get the running loop
loop = asyncio.get_running_loop()
# Run something in a thread
await asyncio.to_thread(in_thread, loop)
It's also possible to run the other way around. Example::
@contextlib.contextmanager
def loop_in_thread() -> Generator[asyncio.AbstractEventLoop]:
loop_fut = concurrent.futures.Future[asyncio.AbstractEventLoop]()
stop_event = asyncio.Event()
async def main() -> None:
loop_fut.set_result(asyncio.get_running_loop())
await stop_event.wait()
with concurrent.futures.ThreadPoolExecutor(1) as tpe:
complete_fut = tpe.submit(asyncio.run, main())
for fut in concurrent.futures.as_completed((loop_fut, complete_fut)):
if fut is loop_fut:
loop = loop_fut.result()
try:
yield loop
finally:
loop.call_soon_threadsafe(stop_event.set)
else:
fut.result()
# Create a loop in another thread
with loop_in_thread() as loop:
# Create a coroutine
coro = asyncio.sleep(1, result=3)
# Submit the coroutine to a given loop
future = asyncio.run_coroutine_threadsafe(coro, loop)
# Wait for the result with an optional timeout argument
assert future.result(timeout=2) == 3
If an exception is raised in the coroutine, the returned Future
will be notified. It can also be used to cancel the task in
the event loop::
try:
result = future.result(timeout)
except TimeoutError:
print('The coroutine took too long, cancelling the task...')
future.cancel()
except Exception as exc:
print(f'The coroutine raised an exception: {exc!r}')
else:
print(f'The coroutine returned: {result!r}')
See the :ref:`concurrency and multithreading <asyncio-multithreading>`
section of the documentation.
Unlike other asyncio functions this function requires the *loop*
argument to be passed explicitly.
.. versionadded:: 3.5.1
Introspection
=============
.. function:: current_task(loop=None)
Return the currently running :class:`Task` instance, or ``None`` if
no task is running.
If *loop* is ``None`` :func:`get_running_loop` is used to get
the current loop.
.. versionadded:: 3.7
.. function:: all_tasks(loop=None)
Return a set of not yet finished :class:`Task` objects run by
the loop.
If *loop* is ``None``, :func:`get_running_loop` is used for getting
current loop.
.. versionadded:: 3.7
.. function:: iscoroutine(obj)
Return ``True`` if *obj* is a coroutine object.
.. versionadded:: 3.4
Task Object
===========
.. class:: Task(coro, *, loop=None, name=None, context=None, eager_start=False)
A :class:`Future-like <Future>` object that runs a Python
:ref:`coroutine <coroutine>`. Not thread-safe.
Tasks are used to run coroutines in event loops.
If a coroutine awaits on a Future, the Task suspends
the execution of the coroutine and waits for the completion
of the Future. When the Future is *done*, the execution of
the wrapped coroutine resumes.
Event loops use cooperative scheduling: an event loop runs
one Task at a time. While a Task awaits for the completion of a
Future, the event loop runs other Tasks, callbacks, or performs
IO operations.
Use the high-level :func:`asyncio.create_task` function to create
Tasks, or the low-level :meth:`loop.create_task` or
:func:`ensure_future` functions. Manual instantiation of Tasks
is discouraged.
To cancel a running Task use the :meth:`cancel` method. Calling it
will cause the Task to throw a :exc:`CancelledError` exception into
the wrapped coroutine. If a coroutine is awaiting on a Future
object during cancellation, the Future object will be cancelled.
:meth:`cancelled` can be used to check if the Task was cancelled.
The method returns ``True`` if the wrapped coroutine did not
suppress the :exc:`CancelledError` exception and was actually
cancelled.
:class:`asyncio.Task` inherits from :class:`Future` all of its
APIs except :meth:`Future.set_result` and
:meth:`Future.set_exception`.
An optional keyword-only *context* argument allows specifying a
custom :class:`contextvars.Context` for the *coro* to run in.
If no *context* is provided, the Task copies the current context
and later runs its coroutine in the copied context.
An optional keyword-only *eager_start* argument allows eagerly starting
the execution of the :class:`asyncio.Task` at task creation time.
If set to ``True`` and the event loop is running, the task will start
executing the coroutine immediately, until the first time the coroutine
blocks. If the coroutine returns or raises without blocking, the task
will be finished eagerly and will skip scheduling to the event loop.
.. versionchanged:: 3.7
Added support for the :mod:`contextvars` module.
.. versionchanged:: 3.8
Added the *name* parameter.
.. deprecated:: 3.10
Deprecation warning is emitted if *loop* is not specified
and there is no running event loop.
.. versionchanged:: 3.11
Added the *context* parameter.
.. versionchanged:: 3.12
Added the *eager_start* parameter.
.. method:: done()
Return ``True`` if the Task is *done*.
A Task is *done* when the wrapped coroutine either returned
a value, raised an exception, or the Task was cancelled.
.. method:: result()
Return the result of the Task.
If the Task is *done*, the result of the wrapped coroutine
is returned (or if the coroutine raised an exception, that
exception is re-raised.)
If the Task has been *cancelled*, this method raises
a :exc:`CancelledError` exception.
If the Task's result isn't yet available, this method raises
an :exc:`InvalidStateError` exception.
.. method:: exception()
Return the exception of the Task.
If the wrapped coroutine raised an exception that exception
is returned. If the wrapped coroutine returned normally
this method returns ``None``.
If the Task has been *cancelled*, this method raises a
:exc:`CancelledError` exception.
If the Task isn't *done* yet, this method raises an
:exc:`InvalidStateError` exception.
.. method:: add_done_callback(callback, *, context=None)
Add a callback to be run when the Task is *done*.
This method should only be used in low-level callback-based code.
See the documentation of :meth:`Future.add_done_callback`
for more details.
.. method:: remove_done_callback(callback)
Remove *callback* from the callbacks list.
This method should only be used in low-level callback-based code.
See the documentation of :meth:`Future.remove_done_callback`
for more details.
.. method:: get_stack(*, limit=None)
Return the list of stack frames for this Task.
If the wrapped coroutine is not done, this returns the stack
where it is suspended. If the coroutine has completed
successfully or was cancelled, this returns an empty list.
If the coroutine was terminated by an exception, this returns
the list of traceback frames.
The frames are always ordered from oldest to newest.
Only one stack frame is returned for a suspended coroutine.
The optional *limit* argument sets the maximum number of frames
to return; by default all available frames are returned.
The ordering of the returned list differs depending on whether
a stack or a traceback is returned: the newest frames of a
stack are returned, but the oldest frames of a traceback are
returned. (This matches the behavior of the traceback module.)
.. method:: print_stack(*, limit=None, file=None)
Print the stack or traceback for this Task.
This produces output similar to that of the traceback module
for the frames retrieved by :meth:`get_stack`.
The *limit* argument is passed to :meth:`get_stack` directly.
The *file* argument is an I/O stream to which the output
is written; by default output is written to :data:`sys.stdout`.
.. method:: get_coro()
Return the coroutine object wrapped by the :class:`Task`.
.. note::
This will return ``None`` for Tasks which have already
completed eagerly. See the :ref:`Eager Task Factory <eager-task-factory>`.
.. versionadded:: 3.8
.. versionchanged:: 3.12
Newly added eager task execution means result may be ``None``.
.. method:: get_context()
Return the :class:`contextvars.Context` object
associated with the task.
.. versionadded:: 3.12
.. method:: get_name()
Return the name of the Task.
If no name has been explicitly assigned to the Task, the default
asyncio Task implementation generates a default name during
instantiation.
.. versionadded:: 3.8
.. method:: set_name(value)
Set the name of the Task.
The *value* argument can be any object, which is then
converted to a string.
In the default Task implementation, the name will be visible
in the :func:`repr` output of a task object.
.. versionadded:: 3.8
.. method:: cancel(msg=None)
Request the Task to be cancelled.
This arranges for a :exc:`CancelledError` exception to be thrown
into the wrapped coroutine on the next cycle of the event loop.
The coroutine then has a chance to clean up or even deny the
request by suppressing the exception with a :keyword:`try` ...
... ``except CancelledError`` ... :keyword:`finally` block.
Therefore, unlike :meth:`Future.cancel`, :meth:`Task.cancel` does
not guarantee that the Task will be cancelled, although
suppressing cancellation completely is not common and is actively
discouraged. Should the coroutine nevertheless decide to suppress
the cancellation, it needs to call :meth:`Task.uncancel` in addition
to catching the exception.
.. versionchanged:: 3.9
Added the *msg* parameter.
.. versionchanged:: 3.11
The ``msg`` parameter is propagated from cancelled task to its awaiter.
.. _asyncio_example_task_cancel:
The following example illustrates how coroutines can intercept
the cancellation request::
async def cancel_me():
print('cancel_me(): before sleep')
try:
# Wait for 1 hour
await asyncio.sleep(3600)
except asyncio.CancelledError:
print('cancel_me(): cancel sleep')
raise
finally:
print('cancel_me(): after sleep')
async def main():
# Create a "cancel_me" Task
task = asyncio.create_task(cancel_me())
# Wait for 1 second
await asyncio.sleep(1)
task.cancel()
try:
await task
except asyncio.CancelledError:
print("main(): cancel_me is cancelled now")
asyncio.run(main())
# Expected output:
#
# cancel_me(): before sleep
# cancel_me(): cancel sleep
# cancel_me(): after sleep
# main(): cancel_me is cancelled now
.. method:: cancelled()
Return ``True`` if the Task is *cancelled*.
The Task is *cancelled* when the cancellation was requested with
:meth:`cancel` and the wrapped coroutine propagated the
:exc:`CancelledError` exception thrown into it.
.. method:: uncancel()
Decrement the count of cancellation requests to this Task.
Returns the remaining number of cancellation requests.
Note that once execution of a cancelled task completed, further
calls to :meth:`uncancel` are ineffective.
.. versionadded:: 3.11
This method is used by asyncio's internals and isn't expected to be
used by end-user code. In particular, if a Task gets successfully
uncancelled, this allows for elements of structured concurrency like
:ref:`taskgroups` and :func:`asyncio.timeout` to continue running,
isolating cancellation to the respective structured block.
For example::
async def make_request_with_timeout():
try:
async with asyncio.timeout(1):
# Structured block affected by the timeout:
await make_request()
await make_another_request()
except TimeoutError:
log("There was a timeout")
# Outer code not affected by the timeout:
await unrelated_code()
While the block with ``make_request()`` and ``make_another_request()``
might get cancelled due to the timeout, ``unrelated_code()`` should
continue running even in case of the timeout. This is implemented
with :meth:`uncancel`. :class:`TaskGroup` context managers use
:func:`uncancel` in a similar fashion.
If end-user code is, for some reason, suppressing cancellation by
catching :exc:`CancelledError`, it needs to call this method to remove
the cancellation state.
When this method decrements the cancellation count to zero,
the method checks if a previous :meth:`cancel` call had arranged
for :exc:`CancelledError` to be thrown into the task.
If it hasn't been thrown yet, that arrangement will be
rescinded (by resetting the internal ``_must_cancel`` flag).
.. versionchanged:: 3.13
Changed to rescind pending cancellation requests upon reaching zero.
.. method:: cancelling()
Return the number of pending cancellation requests to this Task, i.e.,
the number of calls to :meth:`cancel` less the number of
:meth:`uncancel` calls.
Note that if this number is greater than zero but the Task is
still executing, :meth:`cancelled` will still return ``False``.
This is because this number can be lowered by calling :meth:`uncancel`,
which can lead to the task not being cancelled after all if the
cancellation requests go down to zero.
This method is used by asyncio's internals and isn't expected to be
used by end-user code. See :meth:`uncancel` for more details.
.. versionadded:: 3.11
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