::: currentmodule asyncio ::: # Futures {#asyncio-futures} **Source code:** `Lib/asyncio/futures.py`{.interpreted-text role="source"}, `Lib/asyncio/base_futures.py`{.interpreted-text role="source"} ------------------------------------------------------------------------ *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 `asyncio.Future`{.interpreted-text role="class"}, - an instance of `asyncio.Task`{.interpreted-text role="class"}, - 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 `Future`{.interpreted-text role="class"}, a `Task`{.interpreted-text role="class"}, or a Future-like object (`isfuture`{.interpreted-text role="func"} is used for the test.) - a `Task`{.interpreted-text role="class"} object wrapping *obj*, if *obj* is a coroutine (`iscoroutine`{.interpreted-text role="func"} is used for the test); in this case the coroutine will be scheduled by `ensure_future()`. - a `Task`{.interpreted-text role="class"} object that would await on *obj*, if *obj* is an awaitable (`inspect.isawaitable`{.interpreted-text role="func"} is used for the test.) If *obj* is neither of the above a `TypeError`{.interpreted-text role="exc"} is raised. :::: important ::: title Important ::: Save a reference to the result of this function, to avoid a task disappearing mid-execution. See also the `create_task`{.interpreted-text role="func"} function which is the preferred way for creating new tasks or use `asyncio.TaskGroup`{.interpreted-text role="class"} which keeps reference to the task internally. :::: ::: versionchanged 3.5.1 The function accepts any `awaitable`{.interpreted-text role="term"} 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 `concurrent.futures.Future`{.interpreted-text role="class"} object in a `asyncio.Future`{.interpreted-text role="class"} 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 {#asyncio-future-obj} :::::::::::::::::: {.Future(*, .loop=None)} A Future represents an eventual result of an asynchronous operation. Not thread-safe. Future is an `awaitable`{.interpreted-text role="term"} 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 `transports `{.interpreted-text role="ref"}) 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`{.interpreted-text role="meth"}. This way alternative event loop implementations can inject their own optimized implementations of a Future object. ::: versionchanged 3.7 Added support for the `contextvars`{.interpreted-text role="mod"} 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 `set_result`{.interpreted-text role="meth"} method, the result value is returned. If the Future is *done* and has an exception set by the `set_exception`{.interpreted-text role="meth"} method, this method raises the exception. If the Future has been *cancelled*, this method raises a `CancelledError`{.interpreted-text role="exc"} exception. If the Future\'s result isn\'t yet available, this method raises an `InvalidStateError`{.interpreted-text role="exc"} exception. ::: ::: method set_result(result) Mark the Future as *done* and set its result. Raises an `InvalidStateError`{.interpreted-text role="exc"} error if the Future is already *done*. ::: ::: method set_exception(exception) Mark the Future as *done* and set an exception. Raises an `InvalidStateError`{.interpreted-text role="exc"} 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 `set_result`{.interpreted-text role="meth"} or `set_exception`{.interpreted-text role="meth"} 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 `loop.call_soon`{.interpreted-text role="meth"}. An optional keyword-only *context* argument allows specifying a custom `contextvars.Context`{.interpreted-text role="class"} for the *callback* to run in. The current context is used when no *context* is provided. `functools.partial`{.interpreted-text role="func"} 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 `567`{.interpreted-text role="pep"} 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 `CancelledError`{.interpreted-text role="exc"} exception. If the Future isn\'t *done* yet, this method raises an `InvalidStateError`{.interpreted-text role="exc"} 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 ::: title Important ::: The Future object was designed to mimic `concurrent.futures.Future`{.interpreted-text role="class"}. Key differences include: - unlike asyncio Futures, `concurrent.futures.Future`{.interpreted-text role="class"} instances cannot be awaited. - `asyncio.Future.result`{.interpreted-text role="meth"} and `asyncio.Future.exception`{.interpreted-text role="meth"} do not accept the *timeout* argument. - `asyncio.Future.result`{.interpreted-text role="meth"} and `asyncio.Future.exception`{.interpreted-text role="meth"} raise an `InvalidStateError`{.interpreted-text role="exc"} exception when the Future is not *done*. - Callbacks registered with `asyncio.Future.add_done_callback`{.interpreted-text role="meth"} are not called immediately. They are scheduled with `loop.call_soon`{.interpreted-text role="meth"} instead. - asyncio Future is not compatible with the `concurrent.futures.wait`{.interpreted-text role="func"} and `concurrent.futures.as_completed`{.interpreted-text role="func"} functions. - `asyncio.Future.cancel`{.interpreted-text role="meth"} accepts an optional `msg` argument, but `concurrent.futures.Future.cancel`{.interpreted-text role="meth"} does not. ::::