未来 ¶

源代码： Lib/asyncio/futures.py , Lib/asyncio/base_futures.py

Future 对象用于桥接 基于回调的低级代码 采用高级 async/await 代码。

Future 函数 ¶

asyncio. isfuture ( obj ) ¶

返回 True if obj 是：

实例化的 asyncio.Future ,
实例化的 asyncio.Task ,
像 Future 对象具有 _asyncio_future_blocking 属性。

Added in version 3.5.

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

obj argument as is, if obj 是 Future ， Task , or a Future-like object ( isfuture() is used for the test.)
a Task object wrapping obj ，若 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 ，若 obj is an awaitable ( inspect.isawaitable() is used for the test.)

若 obj is neither of the above a TypeError 被引发。

重要

另请参阅 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.

3.5.1 版改变： The function accepts any awaitable 对象。

从 3.10 版起弃用： Deprecation warning is emitted if obj is not a Future-like object and loop is not specified and there is no running event loop.

asyncio. wrap_future ( future , * , loop = None ) ¶: 包裹 concurrent.futures.Future 对象在 asyncio.Future 对象。

从 3.10 版起弃用： 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 对象 ¶

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

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

Future 是 awaitable 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 传输 ) 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.

3.7 版改变：添加支持 contextvars 模块。

从 3.10 版起弃用： Deprecation warning is emitted if loop is not specified and there is no running event loop.

result ( ) ¶

返回 Future 的结果。

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

若 Future 是 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 异常。

If the Future’s result isn’t yet available, this method raises an InvalidStateError 异常。

set_result ( result ) ¶

Mark the Future as done and set its result.

引发 InvalidStateError error if the Future is already done .

set_exception ( exception ) ¶

Mark the Future as done and set an exception.

引发 InvalidStateError error if the Future is already done .

done ( ) ¶

返回 True 若 Future 是 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() 调用。

cancelled ( ) ¶

返回 True 若 Future 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() .

可选仅关键词 context 自变量允许指定自定义 contextvars.Context 为 callback to run in. The current context is used when no context 被提供。

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:"))

3.7 版改变： The context keyword-only parameter was added. See PEP 567 了解更多细节。

remove_done_callback ( callback ) ¶

移除 callback 从回调列表。

Returns the number of callbacks removed, which is typically 1, unless a callback was added more than once.

cancel ( msg = None ) ¶

取消 Future 并调度回调。

If the Future is already done or cancelled ，返回 False . Otherwise, change the Future’s state to cancelled , schedule the callbacks, and return True .

3.9 版改变：添加 msg 参数。

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 异常。

If the Future isn’t done yet, this method raises an InvalidStateError 异常。

get_loop ( ) ¶

Return the event loop the Future object is bound to.

3.7 版添加。

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())

重要

The Future object was designed to mimic concurrent.futures.Future . Key differences include:

不像 asyncio 未来， concurrent.futures.Future 实例无法等待。
asyncio.Future.result() and asyncio.Future.exception() do not accept the timeout 自变量。
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() 代替。
asyncio Future is not compatible with the concurrent.futures.wait() and concurrent.futures.as_completed() 函数。
asyncio.Future.cancel() accepts an optional msg argument, but concurrent.futures.Future.cancel() 不会。