cpython/Doc/library/asyncio-task.rst

.. currentmodule:: asyncio

Tasks and coroutines
====================

.. _coroutine:

Coroutines
----------

A coroutine is a generator that follows certain conventions.  For
documentation purposes, all coroutines should be decorated with
``@asyncio.coroutine``, but this cannot be strictly enforced.

Coroutines use the ``yield from`` syntax introduced in :pep:`380`,
instead of the original ``yield`` syntax.

The word "coroutine", like the word "generator", is used for two
different (though related) concepts:

- The function that defines a coroutine (a function definition
  decorated with ``@asyncio.coroutine``).  If disambiguation is needed
  we will call this a *coroutine function*.

- The object obtained by calling a coroutine function.  This object
  represents a computation or an I/O operation (usually a combination)
  that will complete eventually.  If disambiguation is needed we will
  call it a *coroutine object*.

Things a coroutine can do:

- ``result = yield from future`` -- suspends the coroutine until the
  future is done, then returns the future's result, or raises an
  exception, which will be propagated.  (If the future is cancelled,
  it will raise a ``CancelledError`` exception.)  Note that tasks are
  futures, and everything said about futures also applies to tasks.

- ``result = yield from coroutine`` -- wait for another coroutine to
  produce a result (or raise an exception, which will be propagated).
  The ``coroutine`` expression must be a *call* to another coroutine.

- ``return expression`` -- produce a result to the coroutine that is
  waiting for this one using ``yield from``.

- ``raise exception`` -- raise an exception in the coroutine that is
  waiting for this one using ``yield from``.

Calling a coroutine does not start its code running -- it is just a
generator, and the coroutine object returned by the call is really a
generator object, which doesn't do anything until you iterate over it.
In the case of a coroutine object, there are two basic ways to start
it running: call ``yield from coroutine`` from another coroutine
(assuming the other coroutine is already running!), or convert it to a
:class:`Task`.

Coroutines (and tasks) can only run when the event loop is running.

.. decorator:: coroutine

    Decorator to mark coroutines.

    If the coroutine is not yielded from before it is destroyed, an error
    message is logged. See :ref:`Detect coroutines never scheduled
    <asyncio-coroutine-not-scheduled>`.


.. _asyncio-hello-world-coroutine:

Example: "Hello World" coroutine
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Print ``"Hello World"`` every two seconds using a coroutine::

    import asyncio

    @asyncio.coroutine
    def greet_every_two_seconds():
        while True:
            print('Hello World')
            yield from asyncio.sleep(2)

    loop = asyncio.get_event_loop()
    loop.run_until_complete(greet_every_two_seconds())

.. seealso::

   :ref:`Hello World example using a callback <asyncio-hello-world-callback>`.


Example: Chain coroutines
^^^^^^^^^^^^^^^^^^^^^^^^^

Example chaining coroutines::

    import asyncio

    @asyncio.coroutine
    def compute(x, y):
        print("Compute %s + %s ..." % (x, y))
        yield from asyncio.sleep(1.0)
        return x + y

    @asyncio.coroutine
    def print_sum(x, y):
        result = yield from compute(x, y)
        print("%s + %s = %s" % (x, y, result))

    loop = asyncio.get_event_loop()
    loop.run_until_complete(print_sum(1, 2))
    loop.close()

``compute()`` is chained to ``print_sum()``: ``print_sum()`` coroutine waits
until ``compute()`` is completed before returing its result.

Sequence diagram of the example:

.. image:: tulip_coro.png
   :align: center

The "Task" is created by the :meth:`BaseEventLoop.run_until_complete` method
when it gets a coroutine object instead of a task.

The diagram shows the control flow, it does not describe exactly how things
work internally. For example, the sleep coroutine creates an internal future
which uses :meth:`BaseEventLoop.call_later` to wake up the task in 1 second.


InvalidStateError
-----------------

.. exception:: InvalidStateError

   The operation is not allowed in this state.


Future
------

.. class:: Future(\*, loop=None)

   This class is *almost* compatible with :class:`concurrent.futures.Future`.

   Differences:

   - :meth:`result` and :meth:`exception` do not take a timeout argument and
     raise an exception when the future isn't done yet.

   - Callbacks registered with :meth:`add_done_callback` are always called
     via the event loop's :meth:`~BaseEventLoop.call_soon_threadsafe`.

   - This class is not compatible with the :func:`~concurrent.futures.wait` and
     :func:`~concurrent.futures.as_completed` functions in the
     :mod:`concurrent.futures` package.

   .. method:: cancel()

      Cancel the future and schedule callbacks.

      If the future is already done or cancelled, return ``False``. Otherwise,
      change the future's state to cancelled, schedule the callbacks and return
      ``True``.

   .. method:: cancelled()

      Return ``True`` if the future was cancelled.

   .. method:: done()

      Return True if the future is done.

      Done means either that a result / exception are available, or that the
      future was cancelled.

   .. method:: result()

      Return the result this future represents.

      If the future has been cancelled, raises :exc:`CancelledError`. If the
      future's result isn't yet available, raises :exc:`InvalidStateError`. If
      the future is done and has an exception set, this exception is raised.

   .. method:: exception()

      Return the exception that was set on this future.

      The exception (or ``None`` if no exception was set) is returned only if
      the future is done. If the future has been cancelled, raises
      :exc:`CancelledError`. If the future isn't done yet, raises
      :exc:`InvalidStateError`.

   .. method:: add_done_callback(fn)

      Add a callback to be run when the future becomes done.

      The callback is called with a single argument - the future object. If the
      future is already done when this is called, the callback is scheduled
      with :meth:`~BaseEventLoop.call_soon`.

   .. method:: remove_done_callback(fn)

      Remove all instances of a callback from the "call when done" list.

      Returns the number of callbacks removed.

   .. method:: set_result(result)

      Mark the future done and set its result.

      If the future is already done when this method is called, raises
      :exc:`InvalidStateError`.

   .. method:: set_exception(exception)

      Mark the future done and set an exception.

      If the future is already done when this method is called, raises
      :exc:`InvalidStateError`.


Example: Future with run_until_complete()
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Example combining a :class:`Future` and a :ref:`coroutine function
<coroutine>`::

    import asyncio

    @asyncio.coroutine
    def slow_operation(future):
        yield from asyncio.sleep(1)
        future.set_result('Future in done!')

    loop = asyncio.get_event_loop()
    future = asyncio.Future()
    asyncio.Task(slow_operation(future))
    loop.run_until_complete(future)
    print(future.result())
    loop.close()

The coroutine function is responsible of the computation (which takes 1 second)
and it stores the result into the future. The
:meth:`~BaseEventLoop.run_until_complete` method waits for the completion of
the future.

.. note::
   The :meth:`~BaseEventLoop.run_until_complete` method uses internally the
   :meth:`~Future.add_done_callback` method to be notified when the future is
   done.


Example: Future with run_forever()
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The previous example can be written differently using the
:meth:`Future.add_done_callback` method to describe explicitly the control
flow::

    import asyncio

    @asyncio.coroutine
    def slow_operation(future):
        yield from asyncio.sleep(1)
        future.set_result('Future in done!')

    def got_result(future):
        print(future.result())
        loop.stop()

    loop = asyncio.get_event_loop()
    future = asyncio.Future()
    asyncio.Task(slow_operation(future))
    future.add_done_callback(got_result)
    loop.run_forever()

In this example, the future is responsible to display the result and to stop
the loop.

.. note::
   The "slow_operation" coroutine object is only executed when the event loop
   starts running, so it is possible to add a "done callback" to the future
   after creating the task scheduling the coroutine object.



Task
----

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

   A coroutine object wrapped in a :class:`Future`. Subclass of :class:`Future`.

   .. 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.

   .. classmethod:: current_task(loop=None)

      Return the currently running task in an event loop or ``None``.

      By default the current task for the current event loop is returned.

      ``None`` is returned when called not in the context of a :class:`Task`.

   .. method:: get_stack(self, \*, limit=None)

      Return the list of stack frames for this task's coroutine.

      If the coroutine is active, 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.

      The optional limit gives the maximum nummber of frames to return; by
      default all available frames are returned.  Its meaning 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.)

      For reasons beyond our control, only one stack frame is returned for a
      suspended coroutine.

   .. method:: print_stack(\*, limit=None, file=None)

      Print the stack or traceback for this task's coroutine.

      This produces output similar to that of the traceback module, for the
      frames retrieved by get_stack().  The limit argument is passed to
      get_stack().  The file argument is an I/O stream to which the output
      goes; by default it goes to sys.stderr.


Example: Parallel execution of tasks
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Example executing 3 tasks (A, B, C) in parallel::

    import asyncio

    @asyncio.coroutine
    def factorial(name, number):
        f = 1
        for i in range(2, number+1):
            print("Task %s: Compute factorial(%s)..." % (name, i))
            yield from asyncio.sleep(1)
            f *= i
        print("Task %s: factorial(%s) = %s" % (name, number, f))

    tasks = [
        asyncio.Task(factorial("A", 2)),
        asyncio.Task(factorial("B", 3)),
        asyncio.Task(factorial("C", 4))]

    loop = asyncio.get_event_loop()
    loop.run_until_complete(asyncio.wait(tasks))
    loop.close()

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

A task is automatically scheduled for execution when it is created. The event
loop stops when all tasks are done.


Task functions
--------------

.. note::

   In the functions below, the optional *loop* argument allows to explicitly set
   the event loop object used by the underlying task or coroutine.  If it's
   not provided, the default event loop is used.

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

   Return an iterator whose values, when waited for, are :class:`Future`
   instances.

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

   Example::

       for f in as_completed(fs):
           result = yield from f  # The 'yield from' may raise
           # Use result

   .. note::

      The futures ``f`` are not necessarily members of fs.

.. function:: async(coro_or_future, \*, loop=None)

   Wrap a :ref:`coroutine object <coroutine>` in a future.

   If the argument is a :class:`Future`, it is returned directly.

.. function:: gather(\*coros_or_futures, loop=None, return_exceptions=False)

   Return a future aggregating results from the given coroutine objects or
   futures.

   All futures must share the same event loop.  If all the tasks are done
   successfully, the returned future's result is the list of results (in the
   order of the original sequence, not necessarily the order of results
   arrival).  If *result_exception* is True, exceptions in the tasks are
   treated the same as successful results, and gathered in the result list;
   otherwise, the first raised exception will be immediately propagated to the
   returned future.

   Cancellation: if the outer Future is cancelled, all children (that have not
   completed yet) are also cancelled.  If any child is cancelled, this is
   treated as if it raised :exc:`~concurrent.futures.CancelledError` -- the
   outer Future is *not* cancelled in this case.  (This is to prevent the
   cancellation of one child to cause other children to be cancelled.)

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

   Create a :ref:`coroutine object <coroutine>` that completes after a given
   time (in seconds).  If *result* is provided, it is produced to the caller
   when the coroutine completes.

.. function:: shield(arg, \*, loop=None)

   Wait for a future, shielding it from cancellation.

   The statement::

       res = yield from shield(something())

   is exactly equivalent to the statement::

       res = yield from 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.  But its caller is still
   cancelled, so the yield-from expression still raises
   :exc:`~concurrent.futures.CancelledError`.  Note: If ``something()`` is
   cancelled by other means this will still cancel ``shield()``.

   If you want to completely ignore cancellation (not recommended) you can
   combine ``shield()`` with a try/except clause, as follows::

       try:
           res = yield from shield(something())
       except CancelledError:
           res = None

.. function:: wait(futures, \*, loop=None, timeout=None, return_when=ALL_COMPLETED)

   Wait for the Futures and coroutine objects given by the sequence *futures*
   to complete.  Coroutines will be wrapped in Tasks. Returns two sets of
   :class:`Future`: (done, pending).

   *timeout* can be used to control the maximum number of seconds to wait before
   returning.  *timeout* can be an int or float.  If *timeout* is not specified
   or ``None``, there is no limit to the wait time.

   *return_when* indicates when this function should return.  It must be one of
   the following constants of the :mod:`concurrent.futures` module:

   .. 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.       |
   +-----------------------------+----------------------------------------+

   This function returns a :ref:`coroutine object <coroutine>`.

   Usage::

        done, pending = yield from asyncio.wait(fs)

   .. note::

      This does not raise :exc:`TimeoutError`! Futures that aren't done when
      the timeout occurs are returned in the second set.