Doc/library/asyncio-task.rst

.. currentmodule:: asyncio


====================
Coroutines and Tasks
====================

This section outlines high-level asyncio APIs to work with coroutines
and Tasks.

.. contents::
   :depth: 1
   :local:


.. _coroutine:

Coroutines
==========

Coroutines declared with async/await syntax is the preferred way of
writing asyncio applications.  For example, the following snippet
of code (requires Python 3.7+) 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 three main 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('started at', time.strftime('%X'))

          await say_after(1, 'hello')
          await say_after(2, 'world')

          print('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('started at', time.strftime('%X'))

          # Wait until both tasks are completed (should take
          # around 2 seconds.)
          await task1
          await task2

          print('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


.. _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*.

asyncio also supports legacy :ref:`generator-based
<asyncio_generator_based_coro>` coroutines.


.. 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`.


Running an asyncio Program
==========================

.. function:: run(coro, \*, debug=False)

    This function runs the passed coroutine, taking care of
    managing the asyncio event loop and *finalizing asynchronous
    generators*.

    This function cannot be called when another asyncio event loop is
    running in the same thread.

    If *debug* is ``True``, the event loop will be run in debug mode.

    This function always creates a new event loop and closes it at
    the end.  It should be used as a main entry point for asyncio
    programs, and should ideally only be called once.

    .. versionadded:: 3.7
       **Important:** this function has been added to asyncio in
       Python 3.7 on a :term:`provisional basis <provisional api>`.


Creating Tasks
==============

.. function:: create_task(coro, \*, name=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`.

   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.

   This function has been **added in Python 3.7**.  Prior to
   Python 3.7, the low-level :func:`asyncio.ensure_future` function
   can be used instead::

       async def coro():
           ...

       # In Python 3.7+
       task = asyncio.create_task(coro())
       ...

       # This works in all Python versions but is less readable
       task = asyncio.ensure_future(coro())
       ...

   .. versionadded:: 3.7

   .. versionchanged:: 3.8
      Added the ``name`` parameter.


Sleeping
========

.. coroutinefunction:: sleep(delay, result=None, \*, loop=None)

   Block for *delay* seconds.

   If *result* is provided, it is returned to the caller
   when the coroutine completes.

   The *loop* argument is deprecated and scheduled for removal
   in Python 4.0.

   .. _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())


Running Tasks Concurrently
==========================

.. awaitablefunction:: gather(\*aws, loop=None, 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.

   .. _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({i})...")
              await asyncio.sleep(1)
              f *= i
          print(f"Task {name}: factorial({number}) = {f}")

      async def main():
          # Schedule three calls *concurrently*:
          await asyncio.gather(
              factorial("A", 2),
              factorial("B", 3),
              factorial("C", 4),
          )

      asyncio.run(main())

      # Expected output:
      #
      #     Task A: Compute factorial(2)...
      #     Task B: Compute factorial(2)...
      #     Task C: Compute factorial(2)...
      #     Task A: factorial(2) = 2
      #     Task B: Compute factorial(3)...
      #     Task C: Compute factorial(3)...
      #     Task B: factorial(3) = 6
      #     Task C: Compute factorial(4)...
      #     Task C: factorial(4) = 24

   .. versionchanged:: 3.7
      If the *gather* itself is cancelled, the cancellation is
      propagated regardless of *return_exceptions*.


Shielding From Cancellation
===========================

.. awaitablefunction:: shield(aw, \*, loop=None)

   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::

       res = await shield(something())

   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::

       try:
           res = await shield(something())
       except CancelledError:
           res = None


Timeouts
========

.. coroutinefunction:: wait_for(aw, timeout, \*, loop=None)

   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:`asyncio.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 the wait is cancelled, the future *aw* is also cancelled.

   The *loop* argument is deprecated and scheduled for removal
   in Python 4.0.

   .. _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 asyncio.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:`asyncio.TimeoutError` immediately.


Waiting Primitives
==================

.. coroutinefunction:: wait(aws, \*, loop=None, timeout=None,\
                            return_when=ALL_COMPLETED)

   Run :ref:`awaitable objects <asyncio-awaitables>` in the *aws*
   set concurrently and block until the condition specified
   by *return_when*.

   If any awaitable in *aws* is a coroutine, it is automatically
   scheduled as a Task.  Passing coroutines objects to
   ``wait()`` directly is deprecated as it leads to
   :ref:`confusing behavior <asyncio_example_wait_coroutine>`.

   Returns two sets of Tasks/Futures: ``(done, pending)``.

   Usage::

        done, pending = await asyncio.wait(aws)

   The *loop* argument is deprecated and scheduled for removal
   in Python 4.0.

   *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:`asyncio.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.

   .. _asyncio_example_wait_coroutine:
   .. note::

      ``wait()`` schedules coroutines as Tasks automatically and later
      returns those implicitly created Task objects in ``(done, pending)``
      sets.  Therefore the following code won't work as expected::

          async def foo():
              return 42

          coro = foo()
          done, pending = await asyncio.wait({coro})

          if coro in done:
              # This branch will never be run!

      Here is how the above snippet can be fixed::

          async def foo():
              return 42

          task = asyncio.create_task(foo())
          done, pending = await asyncio.wait({task})

          if task in done:
              # Everything will work as expected now.

      Passing coroutine objects to ``wait()`` directly is
      deprecated.


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

   Run :ref:`awaitable objects <asyncio-awaitables>` in the *aws*
   set concurrently.  Return an iterator of :class:`Future` objects.
   Each Future object returned represents the earliest result
   from the set of the remaining awaitables.

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

   Example::

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


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 asyncio.TimeoutError:
         print('The coroutine took too long, cancelling the task...')
         future.cancel()
     except Exception as exc:
         print('The coroutine raised an exception: {!r}'.format(exc))
     else:
         print('The coroutine returned: {!r}'.format(result))

   See the :ref:`concurrency and multithreading <asyncio-multithreading>`
   section of the documentation.

   Unlike other asyncio functions this functions 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


Task Object
===========

.. class:: Task(coro, \*, loop=None, name=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`.

   Tasks support the :mod:`contextvars` module.  When a Task
   is created it 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.

   .. method:: cancel()

      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.

      .. _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:: 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.stderr`.

   .. 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

   .. classmethod:: all_tasks(loop=None)

      Return a set of all tasks for an event loop.

      By default all tasks for the current event loop are returned.
      If *loop* is ``None``, the :func:`get_event_loop` function
      is used to get the current loop.

      This method is **deprecated** and will be removed in
      Python 3.9.  Use the :func:`asyncio.all_tasks` function instead.

   .. classmethod:: current_task(loop=None)

      Return the currently running task or ``None``.

      If *loop* is ``None``, the :func:`get_event_loop` function
      is used to get the current loop.

      This method is **deprecated** and will be removed in
      Python 3.9.  Use the :func:`asyncio.current_task` function
      instead.


.. _asyncio_generator_based_coro:

Generator-based Coroutines
==========================

.. note::

   Support for generator-based coroutines is **deprecated** and
   is scheduled for removal in Python 4.0.

Generator-based coroutines predate async/await syntax.  They are
Python generators that use ``yield from`` expressions to await
on Futures and other coroutines.

Generator-based coroutines should be decorated with
:func:`@asyncio.coroutine <asyncio.coroutine>`, although this is not
enforced.


.. decorator:: coroutine

    Decorator to mark generator-based coroutines.

    This decorator enables legacy generator-based coroutines to be
    compatible with async/await code::

        @asyncio.coroutine
        def old_style_coroutine():
            yield from asyncio.sleep(1)

        async def main():
            await old_style_coroutine()

    This decorator is **deprecated** and is scheduled for removal in
    Python 4.0.

    This decorator should not be used for :keyword:`async def`
    coroutines.

.. function:: iscoroutine(obj)

   Return ``True`` if *obj* is a :ref:`coroutine object <coroutine>`.

   This method is different from :func:`inspect.iscoroutine` because
   it returns ``True`` for generator-based coroutines decorated with
   :func:`@coroutine <coroutine>`.

.. function:: iscoroutinefunction(func)

   Return ``True`` if *func* is a :ref:`coroutine function
   <coroutine>`.

   This method is different from :func:`inspect.iscoroutinefunction`
   because it returns ``True`` for generator-based coroutine functions
   decorated with :func:`@coroutine <coroutine>`.