.. currentmodule:: asyncio
.. _asyncio-futures:
=======
Futures
=======
**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.
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);
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::
See also the :func:`create_task` function which is the
preferred way for creating new Tasks.
Save a reference to the result of this function, to avoid
a task disappearing mid-execution.
.. 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.
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. 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
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(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
.. _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.
- :meth:`asyncio.Future.cancel` accepts an optional ``msg`` argument,
but :meth:`concurrent.futures.Future.cancel` does not.