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Doc/library/asyncio-future.rst
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Miss Islington (bot)
[3.9] Save a reference for ensure_future and create_task (GH-29163) (GH-29573)
16 ноя 2021, 14:47
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16 ноя 2021, 14:47
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.. 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. .. 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(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.