.. currentmodule:: asyncio

Source code: :source:`Lib/asyncio/futures.py`, :source:`Lib/asyncio/base_futures.py`

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

.. 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);
     in this case the coroutine will be scheduled by
     ``ensure_future()``.

   * 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::

      Save a reference to the result of this function, to avoid
      a task disappearing mid-execution.

      See also the :func:`create_task` function which is the
      preferred way for creating new tasks or use :class:`asyncio.TaskGroup`
      which keeps reference to the task internally.

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

   .. deprecated:: 3.10
      Deprecation warning is emitted if *obj* is not a Future-like object
      and *loop* is not specified and there is no running event loop.

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

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

   .. deprecated:: 3.10
      Deprecation warning is emitted if *future* is not a Future-like object
      and *loop* is not specified and there is no running event loop.

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. A Future can be awaited multiple times and the result is same.

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.

.. deprecated:: 3.10
   Deprecation warning is emitted if *loop* is not specified
   and there is no running event loop.

.. 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
   an :exc:`InvalidStateError` exception.

.. method:: set_result(result)

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

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

.. method:: set_exception(exception)

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

   Raises an :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(msg=None)

   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``.

   .. versionchanged:: 3.9
      Added the *msg* parameter.

.. 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

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())