| .. currentmodule:: asyncio |
| |
| |
| .. _asyncio-futures: |
| |
| ======= |
| 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. |