======= 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.