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| ::: 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 <asyncio-transports-protocols>`{.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. | |
| :::: | |