asyncio-future.rst

.. currentmodule:: asyncio


.. _asyncio-futures:

=======
Futures
=======

*Future* objects are used to bridge **low-level callback-based code**
with high-level async/await code.


Future Functions
================

.. function:: isfuture(obj)

   Return ``True`` if *obj* is either of:

   * an instance of :class:`asyncio.Future`,
   * an instance of :class:`asyncio.Task`,
   * a Future-like object with a ``_asyncio_future_blocking``
     attribute.

   .. versionadded:: 3.5


.. function:: ensure_future(obj, \*, loop=None)

   Return:

   * *obj* argument as is, if *obj* is a :class:`Future`,
     a :class:`Task`, or a Future-like object (:func:`isfuture`
     is used for the test.)

   * a :class:`Task` object wrapping *obj*, if *obj* is a
     coroutine (:func:`iscoroutine` is used for the test.)

   * a :class:`Task` object that would await on *obj*, if *obj* is an
     awaitable (:func:`inspect.isawaitable` is used for the test.)

   If *obj* is neither of the above a :exc:`TypeError` is raised.

   .. important::

      See also the :func:`create_task` function which is the
      preferred way for creating new Tasks.

   .. versionchanged:: 3.5.1
      The function accepts any :term:`awaitable` object.


.. function:: wrap_future(future, \*, loop=None)

   Wrap a :class:`concurrent.futures.Future` object in a
   :class:`asyncio.Future` object.


Future Object
=============

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

   A Future represents an eventual result of an asynchronous
   operation.  Not thread-safe.

   Future is an :term:`awaitable` object.  Coroutines can await on
   Future objects until they either have a result or an exception
   set, or until they are cancelled.

   Typically Futures are used to enable low-level
   callback-based code (e.g. in protocols implemented using asyncio
   :ref:`transports <asyncio-transports-protocols>`)
   to interoperate with high-level async/await code.

   The rule of thumb is to never expose Future objects in user-facing
   APIs, and the recommended way to create a Future object is to call
   :meth:`loop.create_future`.  This way alternative event loop
   implementations can inject their own optimized implementations
   of a Future object.

   .. versionchanged:: 3.7
      Added support for the :mod:`contextvars` module.

   .. method:: result()

      Return the result of the Future.

      If the Future is *done* and has a result set by the
      :meth:`set_result` method, the result value is returned.

      If the Future is *done* and has an exception set by the
      :meth:`set_exception` method, this method raises the exception.

      If the Future has been *cancelled*, this method raises
      a :exc:`CancelledError` exception.

      If the Future's result isn't yet available, this method raises
      a :exc:`InvalidStateError` exception.

   .. method:: set_result(result)

      Mark the Future as *done* and set its result.

      Raises a :exc:`InvalidStateError` error if the Future is
      already *done*.

   .. method:: set_exception(exception)

      Mark the Future as *done* and set an exception.

      Raises a :exc:`InvalidStateError` error if the Future is
      already *done*.

   .. method:: done()

      Return ``True`` if the Future is *done*.

      A Future is *done* if it was *cancelled* or if it has a result
      or an exception set with :meth:`set_result` or
      :meth:`set_exception` calls.

   .. method:: cancelled()

      Return ``True`` if the Future was *cancelled*.

      The method is usually used to check if a Future is not
      *cancelled* before setting a result or an exception for it::

          if not fut.cancelled():
              fut.set_result(42)

   .. method:: add_done_callback(callback, *, context=None)

      Add a callback to be run when the Future is *done*.

      The *callback* is called with the Future object as its only
      argument.

      If the Future is already *done* when this method is called,
      the callback is scheduled with :meth:`loop.call_soon`.

      An optional keyword-only *context* argument allows specifying a
      custom :class:`contextvars.Context` for the *callback* to run in.
      The current context is used when no *context* is provided.

      :func:`functools.partial` can be used to pass parameters
      to the callback, e.g.::

          # Call 'print("Future:", fut)' when "fut" is done.
          fut.add_done_callback(
              functools.partial(print, "Future:"))

      .. versionchanged:: 3.7
         The *context* keyword-only parameter was added.
         See :pep:`567` for more details.

   .. method:: remove_done_callback(callback)

      Remove *callback* from the callbacks list.

      Returns the number of callbacks removed, which is typically 1,
      unless a callback was added more than once.

   .. 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:: 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*, this method raises a
      :exc:`CancelledError` exception.

      If the Future isn't *done* yet, this method raises an
      :exc:`InvalidStateError` exception.

   .. method:: get_loop()

      Return the event loop the Future object is bound to.

      .. versionadded:: 3.7


.. _asyncio_example_future:

This example creates a Future object, creates and schedules an
asynchronous Task to set result for the Future, and waits until
the Future has a result::

    async def set_after(fut, delay, value):
        # Sleep for *delay* seconds.
        await asyncio.sleep(delay)

        # Set *value* as a result of *fut* Future.
        fut.set_result(value)

    async def main():
        # Get the current event loop.
        loop = asyncio.get_running_loop()

        # Create a new Future object.
        fut = loop.create_future()

        # Run "set_after()" coroutine in a parallel Task.
        # We are using the low-level "loop.create_task()" API here because
        # we already have a reference to the event loop at hand.
        # Otherwise we could have just used "asyncio.create_task()".
        loop.create_task(
            set_after(fut, 1, '... world'))

        print('hello ...')

        # Wait until *fut* has a result (1 second) and print it.
        print(await fut)

    asyncio.run(main())


.. important::

   The Future object was designed to mimic
   :class:`concurrent.futures.Future`.  Key differences include:

   - unlike asyncio Futures, :class:`concurrent.futures.Future`
     instances cannot be awaited.

   - :meth:`asyncio.Future.result` and :meth:`asyncio.Future.exception`
     do not accept the *timeout* argument.

   - :meth:`asyncio.Future.result` and :meth:`asyncio.Future.exception`
     raise an :exc:`InvalidStateError` exception when the Future is not
     *done*.

   - Callbacks registered with :meth:`asyncio.Future.add_done_callback`
     are not called immediately.  They are scheduled with
     :meth:`loop.call_soon` instead.

   - asyncio Future is not compatible with the
     :func:`concurrent.futures.wait` and
     :func:`concurrent.futures.as_completed` functions.