.. currentmodule:: asyncio
This section outlines high-level asyncio APIs to work with coroutines and Tasks.
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_aftercoroutines 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`.
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.
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()
# 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.
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())
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`.
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.
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.
Task groups combine a task creation API with a convenient and reliable way to wait for all tasks in the group to finish.
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.
.. 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`.
.. 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.
.. 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
.. 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.
.. 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`.
.. 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).
.. 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
.. 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::
# 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) == 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
.. 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