Futures — Python 3.8.6 documentation

# Futures

Source code: Lib/asyncio/futures.py, Lib/asyncio/base_futures.py

Future objects are used to bridge low-level callback-based code with high-level async/await code.

## Future Functions

`asyncio.` `isfuture`(obj)

  Return `True` if obj is either of:

  - an instance of `asyncio.Future`,

  - an instance of `asyncio.Task`,

  - a Future-like object with a `_asyncio_future_blocking` attribute.

  New in version 3.5.

`asyncio.` `ensure_future`(obj, *, loop=None)

  Return:

  - obj argument as is, if obj is a `Future`, a `Task`, or a Future-like object (`isfuture()` is used for the test.)

  - a `Task` object wrapping obj, if obj is a coroutine (`iscoroutine()` is used for the test); in this case the coroutine will be scheduled by `ensure_future()`.

  - a `Task` object that would await on obj, if obj is an awaitable (`inspect.isawaitable()` is used for the test.)

  If obj is neither of the above a `TypeError` is raised.

  Important

  See also the `create_task()` function which is the preferred way for creating new Tasks.

  Changed in version 3.5.1: The function accepts any awaitable object.

`asyncio.` `wrap_future`(future, *, loop=None)

  Wrap a `concurrent.futures.Future` object in a `asyncio.Future` object.

## Future Object

class `asyncio.` `Future`(*, loop=None)

  A Future represents an eventual result of an asynchronous operation. Not thread-safe.

  Future is an 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 transports) 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 `loop.create_future()`. This way alternative event loop implementations can inject their own optimized implementations of a Future object.

  Changed in version 3.7: Added support for the `contextvars` module.

  `result`()

    Return the result of the Future.

    If the Future is done and has a result set by the `set_result()` method, the result value is returned.

    If the Future is done and has an exception set by the `set_exception()` method, this method raises the exception.

    If the Future has been cancelled, this method raises a `CancelledError` exception.

    If the Future’s result isn’t yet available, this method raises a `InvalidStateError` exception.

  `set_result`(result)

    Mark the Future as done and set its result.

    Raises a `InvalidStateError` error if the Future is already done.

  `set_exception`(exception)

    Mark the Future as done and set an exception.

    Raises a `InvalidStateError` error if the Future is already done.

  `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 `set_result()` or `set_exception()` calls.

  `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)

  `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 `loop.call_soon()`.

    An optional keyword-only context argument allows specifying a custom `contextvars.Context` for the callback to run in. The current context is used when no context is provided.

    `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:"))

    Changed in version 3.7: The context keyword-only parameter was added. See PEP 567 for more details.

  `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.

  `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`.

  `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 `CancelledError` exception.

    If the Future isn’t done yet, this method raises an `InvalidStateError` exception.

  `get_loop`()

    Return the event loop the Future object is bound to.

    New in version 3.7.

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 `concurrent.futures.Future`. Key differences include:

- unlike asyncio Futures, `concurrent.futures.Future` instances cannot be awaited.

- `asyncio.Future.result()` and `asyncio.Future.exception()` do not accept the timeout argument.

- `asyncio.Future.result()` and `asyncio.Future.exception()` raise an `InvalidStateError` exception when the Future is not done.

- Callbacks registered with `asyncio.Future.add_done_callback()` are not called immediately. They are scheduled with `loop.call_soon()` instead.

- asyncio Future is not compatible with the `concurrent.futures.wait()` and `concurrent.futures.as_completed()` functions.