mirror of
https://github.com/python/cpython.git
synced 2025-10-29 17:38:56 +00:00
09de8d7aaf
Reason: we were too hasty in deprecating this. We shouldn't deprecate it before we have a replacement.
279 lines
8.3 KiB
ReStructuredText
279 lines
8.3 KiB
ReStructuredText
.. 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 :func:`concurrent.futures.cancel` does not.
|