.. currentmodule:: asyncio

Coroutines and Tasks

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

Coroutines
Awaitables
Running an asyncio Program
Creating Tasks
Sleeping
Running Tasks Concurrently
Shielding From Cancellation
Timeouts
Waiting Primitives
Scheduling From Other Threads
Introspection
Task Object
Generator-based Coroutines

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(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

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.

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.

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`.

Running an asyncio Program

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

    Execute the :term:`coroutine` *coro* and return the result.

    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.

    Example::

        async def main():
            await asyncio.sleep(1)
            print('hello')

        asyncio.run(main())

    .. 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)

   Wrap the *coro* :ref:`coroutine <coroutine>` into a :class:`Task`
   and schedule its execution.  Return the Task object.

   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

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.

   ``sleep()`` always suspends the current task, allowing other tasks
   to run.

   The *loop* argument is deprecated and scheduled for removal
   in Python 3.10.

   .. _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 3.10.

   .. _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 3.10.

   *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(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

Task Object

Generator-based Coroutines

Note

Support for generator-based coroutines is deprecated and is scheduled for removal in Python 3.10.

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 3.10.

    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.

.. 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>`.

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