xref: /third_party/python/Doc/library/asyncio-task.rst (revision 7db96d56)

.. 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()

        # 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 newer alternative
      that allows for convenient waiting for a group of related tasks.

   .. 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`.

Example::

    async def main():
        async with asyncio.TaskGroup() as tg:
            task1 = tg.create_task(some_coro(...))
            task2 = tg.create_task(another_coro(...))
        print("Both tasks have completed now.")

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.


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.


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 more modern way to create and run tasks concurrently and
      wait for their completion is :class:`asyncio.TaskGroup`.

   .. _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.


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
========

.. coroutinefunction:: timeout(delay)

    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 an :exc:`asyncio.TimeoutError` which can be caught and handled.

    .. note::

      The :func:`asyncio.timeout` context manager is what transforms
      the :exc:`asyncio.CancelledError` into an :exc:`asyncio.TimeoutError`,
      which means the :exc:`asyncio.TimeoutError` can only be caught
      *outside* of the context manager.

    Example of catching :exc:`asyncio.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

.. coroutinefunction:: 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.

   .. versionchanged:: 3.10
      Removed the *loop* parameter.

   .. _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.


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 and generators yielding tasks are not accepted.

   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:

   .. tabularcolumns:: |l|L|

   +-----------------------------+----------------------------------------+
   | Constant                    | Description                            |
   +=============================+========================================+
   | :const:`FIRST_COMPLETED`    | The function will return when any      |
   |                             | future finishes or is cancelled.       |
   +-----------------------------+----------------------------------------+
   | :const:`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`.                |
   +-----------------------------+----------------------------------------+
   | :const:`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.

.. function:: as_completed(aws, *, timeout=None)

   Run :ref:`awaitable objects <asyncio-awaitables>` in the *aws*
   iterable concurrently. Generators yielding tasks are not accepted
   as *aws* iterable. Return an iterator of coroutines.
   Each coroutine returned can be awaited to get the earliest next
   result from the iterable of the remaining awaitables.

   Raises :exc:`TimeoutError` if the timeout occurs before
   all Futures are done.

   Example::

       for coro in as_completed(aws):
           earliest_result = await coro
           # ...

   .. 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.


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::

     # 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


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)

   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.

   .. 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.

   .. 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
      a :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`.

      .. versionadded:: 3.8

   .. 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, suppresing cancellation by
      catching :exc:`CancelledError`, it needs to call this method to remove
      the cancellation state.

   .. 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