协程和任务 ¶

此节提纲的高级 asyncio API 能操控协程和任务。

协程  ¶

协程采用 async/await 句法声明是编写 asyncio 应用程序的首选方式。例如，以下代码片段打印 hello，等待 1 秒，然后打印 world：

>>> import asyncio
>>> async def main():
...     print('hello')
...     await asyncio.sleep(1)
...     print('world')
>>> asyncio.run(main())
hello
world

注意，简单调用协程不会调度它的执行：

>>> main()
<coroutine object main at 0x1053bb7c8>

要实际运行协程，asyncio 提供下列机制：

The asyncio.run() 函数运行顶层入口点 main() 函数 (见以上范例)。

等待协程。以下代码片段将在等待 1 秒后打印 hello，然后打印 world 后于等待 another 2 秒：

import asyncio
import time
async def say_after(delay, what):
    await asyncio.sleep(delay)
    print(what)
async def main():
    print(f"started at {time.strftime('%X')}")
    await say_after(1, 'hello')
    await say_after(2, 'world')
    print(f"finished at {time.strftime('%X')}")
asyncio.run(main())

期望输出：

started at 17:13:52
hello
world
finished at 17:13:55

The asyncio.create_task() 函数并发运行协程如 asyncio Tasks .

让我们修改以上范例并运行 2 say_after 协程 concurrently :

async def main():
    task1 = asyncio.create_task(
        say_after(1, 'hello'))
    task2 = asyncio.create_task(
        say_after(2, 'world'))
    print(f"started at {time.strftime('%X')}")
    # Wait until both tasks are completed (should take
    # around 2 seconds.)
    await task1
    await task2
    print(f"finished at {time.strftime('%X')}")

注意，现在期望的输出展示片段运行速度比之前快了 1 秒：

started at 17:14:32
hello
world
finished at 17:14:34

The asyncio.TaskGroup 类提供了更现代替代对于 create_task() 。使用此 API，范例最后变成：

async def main():
    async with asyncio.TaskGroup() as tg:
        task1 = tg.create_task(
            say_after(1, 'hello'))
        task2 = tg.create_task(
            say_after(2, 'world'))
        print(f"started at {time.strftime('%X')}")
    # The await is implicit when the context manager exits.
    print(f"finished at {time.strftime('%X')}")

计时和输出应该如同先前版本。

Added in version 3.11: asyncio.TaskGroup .

可期待  ¶

我们说对象是 awaitable object if it can be used in an await expression. Many asyncio APIs are designed to accept awaitables.

有 3 种主要类型的 awaitable 对象：协程 , 任务，和未来 .

协程

Python 协程是 awaitables 因此可以从其它协程等待：

import asyncio
async def nested():
    return 42
async def main():
    # Nothing happens if we just call "nested()".
    # A coroutine object is created but not awaited,
    # so it *won't run at all*.
    nested()  # will raise a "RuntimeWarning".
    # Let's do it differently now and await it:
    print(await nested())  # will print "42".
asyncio.run(main())

重要

In this documentation the term “coroutine” can be used for two closely related concepts:

a 协程函数 ： async def 函数；
a 协程对象 : an object returned by calling a 协程函数 .

任务

任务被用于调度协程 concurrently .

When a coroutine is wrapped into a Task with functions like asyncio.create_task() the coroutine is automatically scheduled to run soon:

import asyncio
async def nested():
    return 42
async def main():
    # Schedule nested() to run soon concurrently
    # with "main()".
    task = asyncio.create_task(nested())
    # "task" can now be used to cancel "nested()", or
    # can simply be awaited to wait until it is complete:
    await task
asyncio.run(main())

未来

A Future is a special 低级 awaitable object that represents an eventual result of an asynchronous operation.

When a Future object is awaited it means that the coroutine will wait until the Future is resolved in some other place.

Future objects in asyncio are needed to allow callback-based code to be used with async/await.

Normally there is no need to create Future objects at the application level code.

Future objects, sometimes exposed by libraries and some asyncio APIs, can be awaited:

async def main():
    await function_that_returns_a_future_object()
    # this is also valid:
    await asyncio.gather(
        function_that_returns_a_future_object(),
        some_python_coroutine()
    )

A good example of a low-level function that returns a Future object is loop.run_in_executor() .

创建任务  ¶

源代码： Lib/asyncio/tasks.py

asyncio. create_task ( coro , * , 名称 = None , context = None ) ¶

包裹 coro 协程成 Task 并调度其执行。返回任务对象。

若 name 不是 None ，将它设为任务名称使用 Task.set_name() .

可选仅关键词 context 自变量允许指定自定义 contextvars.Context 为 coro 以在其中运行。创建当前上下文副本当没有 context 被提供。

在循环中执行的任务的返回通过 get_running_loop() , RuntimeError 被引发若当前线程中没有正运行的循环。

注意

asyncio.TaskGroup.create_task() is a new alternative leveraging structural concurrency; it allows for waiting for a group of related tasks with strong safety guarantees.

重要

保存对此函数结果的引用，以避免任务在执行过程中消失。事件循环只保持任务的弱引用。在其它地方未引用的任务可能在任何时候被垃圾收集，甚至在完成之前。对于可靠 "激发并忘记" 的后台任务，把它们收集起来：

background_tasks = set()
for i in range(10):
    task = asyncio.create_task(some_coro(param=i))
    # Add task to the set. This creates a strong reference.
    background_tasks.add(task)
    # To prevent keeping references to finished tasks forever,
    # make each task remove its own reference from the set after
    # completion:
    task.add_done_callback(background_tasks.discard)

3.7 版添加。

3.8 版改变：添加 name 参数。

3.11 版改变：添加 context 参数。

任务消除  ¶

可以轻松且安全地取消任务。当取消任务时， asyncio.CancelledError will be raised in the task at the next opportunity.

It is recommended that coroutines use try/finally blocks to robustly perform clean-up logic. In case asyncio.CancelledError is explicitly caught, it should generally be propagated when clean-up is complete. asyncio.CancelledError directly subclasses BaseException so most code will not need to be aware of it.

The asyncio components that enable structured concurrency, like asyncio.TaskGroup and asyncio.timeout() , are implemented using cancellation internally and might misbehave if a coroutine swallows asyncio.CancelledError . Similarly, user code should not generally call uncancel . However, in cases when suppressing asyncio.CancelledError is truly desired, it is necessary to also call uncancel() to completely remove the cancellation state.

任务组  ¶

Task groups combine a task creation API with a convenient and reliable way to wait for all tasks in the group to finish.

class asyncio. TaskGroup ¶

An 异步上下文管理器 holding a group of tasks. Tasks can be added to the group using create_task() . All tasks are awaited when the context manager exits.

Added in version 3.11.

create_task ( coro , * , 名称 = None , context = None ) ¶

Create a task in this task group. The signature matches that of asyncio.create_task() . If the task group is inactive (e.g. not yet entered, already finished, or in the process of shutting down), we will close the given coro .

Changed in version 3.13: Close the given coroutine if the task group is not active.

范例：

async def main():
    async with asyncio.TaskGroup() as tg:
        task1 = tg.create_task(some_coro(...))
        task2 = tg.create_task(another_coro(...))
    print(f"Both tasks have completed now: {task1.result()}, {task2.result()}")

The async with statement will wait for all tasks in the group to finish. While waiting, new tasks may still be added to the group (for example, by passing tg into one of the coroutines and calling tg.create_task() in that coroutine). Once the last task has finished and the async with block is exited, no new tasks may be added to the group.

The first time any of the tasks belonging to the group fails with an exception other than asyncio.CancelledError , the remaining tasks in the group are cancelled. No further tasks can then be added to the group. At this point, if the body of the async with statement is still active (i.e., __aexit__() hasn’t been called yet), the task directly containing the async with statement is also cancelled. The resulting asyncio.CancelledError will interrupt an await , but it will not bubble out of the containing async with 语句。

Once all tasks have finished, if any tasks have failed with an exception other than asyncio.CancelledError , those exceptions are combined in an ExceptionGroup or BaseExceptionGroup (as appropriate; see their documentation) which is then raised.

Two base exceptions are treated specially: If any task fails with KeyboardInterrupt or SystemExit , the task group still cancels the remaining tasks and waits for them, but then the initial KeyboardInterrupt or SystemExit is re-raised instead of ExceptionGroup or BaseExceptionGroup .

If the body of the async with statement exits with an exception (so __aexit__() is called with an exception set), this is treated the same as if one of the tasks failed: the remaining tasks are cancelled and then waited for, and non-cancellation exceptions are grouped into an exception group and raised. The exception passed into __aexit__() , unless it is asyncio.CancelledError , is also included in the exception group. The same special case is made for KeyboardInterrupt and SystemExit as in the previous paragraph.

Task groups are careful not to mix up the internal cancellation used to “wake up” their __aexit__() with cancellation requests for the task in which they are running made by other parties. In particular, when one task group is syntactically nested in another, and both experience an exception in one of their child tasks simultaneously, the inner task group will process its exceptions, and then the outer task group will receive another cancellation and process its own exceptions.

In the case where a task group is cancelled externally and also must raise an ExceptionGroup , it will call the parent task’s cancel() method. This ensures that a asyncio.CancelledError will be raised at the next await , so the cancellation is not lost.

Task groups preserve the cancellation count reported by asyncio.Task.cancelling() .

Changed in version 3.13: Improved handling of simultaneous internal and external cancellations and correct preservation of cancellation counts.

Terminating a Task Group ¶

While terminating a task group is not natively supported by the standard library, termination can be achieved by adding an exception-raising task to the task group and ignoring the raised exception:

import asyncio
from asyncio import TaskGroup
class TerminateTaskGroup(Exception):
    """Exception raised to terminate a task group."""
async def force_terminate_task_group():
    """Used to force termination of a task group."""
    raise TerminateTaskGroup()
async def job(task_id, sleep_time):
    print(f'Task {task_id}: start')
    await asyncio.sleep(sleep_time)
    print(f'Task {task_id}: done')
async def main():
    try:
        async with TaskGroup() as group:
            # spawn some tasks
            group.create_task(job(1, 0.5))
            group.create_task(job(2, 1.5))
            # sleep for 1 second
            await asyncio.sleep(1)
            # add an exception-raising task to force the group to terminate
            group.create_task(force_terminate_task_group())
    except* TerminateTaskGroup:
        pass
asyncio.run(main())

期望输出：

Task 1: start
Task 2: start
Task 1: done

休眠  ¶

协程 asyncio. sleep ( delay , result = None ) ¶

阻塞 delay 秒。

若 result 有提供，将它返回给调用者当协程完成时。

sleep() 始终挂起当前任务，允许其它任务运行。

将延迟设为 0，提供了允许运行其它任务的优化路径。长时间运行的函数可以通过使用这，以避免在函数的完整调用期间阻塞事件循环。

在 5 秒内每秒显示当前日期的协程范例：

import asyncio
import datetime
async def display_date():
    loop = asyncio.get_running_loop()
    end_time = loop.time() + 5.0
    while True:
        print(datetime.datetime.now())
        if (loop.time() + 1.0) >= end_time:
            break
        await asyncio.sleep(1)
asyncio.run(display_date())

3.10 版改变：移除 loop 参数。

Changed in version 3.13: 引发 ValueError if delay is nan .

并发运行任务  ¶

awaitable asyncio. gather ( * aws , return_exceptions = False ) ¶

运行可期待对象在 aws sequence concurrently .

若任何可期待在 aws 是协程，自动作为 Task (任务) 被调度。

If all awaitables are completed successfully, the result is an aggregate list of returned values. The order of result values corresponds to the order of awaitables in aws .

若 return_exceptions is False (default), the first raised exception is immediately propagated to the task that awaits on gather() . Other awaitables in the aws sequence won’t be cancelled and will continue to run.

若 return_exceptions is True , exceptions are treated the same as successful results, and aggregated in the result list.

若 gather() is cancelled , all submitted awaitables (that have not completed yet) are also cancelled .

If any Task or Future from the aws sequence is cancelled , it is treated as if it raised CancelledError – the gather() call is not cancelled in this case. This is to prevent the cancellation of one submitted Task/Future to cause other Tasks/Futures to be cancelled.

注意

A new alternative to create and run tasks concurrently and wait for their completion is asyncio.TaskGroup . TaskGroup provides stronger safety guarantees than gather for scheduling a nesting of subtasks: if a task (or a subtask, a task scheduled by a task) raises an exception, TaskGroup will, while gather will not, cancel the remaining scheduled tasks).

范例：

import asyncio
async def factorial(name, number):
    f = 1
    for i in range(2, number + 1):
        print(f"Task {name}: Compute factorial({number}), currently i={i}...")
        await asyncio.sleep(1)
        f *= i
    print(f"Task {name}: factorial({number}) = {f}")
    return f
async def main():
    # Schedule three calls *concurrently*:
    L = await asyncio.gather(
        factorial("A", 2),
        factorial("B", 3),
        factorial("C", 4),
    )
    print(L)
asyncio.run(main())
# Expected output:
#
#     Task A: Compute factorial(2), currently i=2...
#     Task B: Compute factorial(3), currently i=2...
#     Task C: Compute factorial(4), currently i=2...
#     Task A: factorial(2) = 2
#     Task B: Compute factorial(3), currently i=3...
#     Task C: Compute factorial(4), currently i=3...
#     Task B: factorial(3) = 6
#     Task C: Compute factorial(4), currently i=4...
#     Task C: factorial(4) = 24
#     [2, 6, 24]

注意

若 return_exceptions is false, cancelling gather() after it has been marked done won’t cancel any submitted awaitables. For instance, gather can be marked done after propagating an exception to the caller, therefore, calling gather.cancel() after catching an exception (raised by one of the awaitables) from gather won’t cancel any other awaitables.

3.7 版改变：若 gather itself is cancelled, the cancellation is propagated regardless of return_exceptions .

3.10 版改变：移除 loop 参数。

从 3.10 版起弃用： Deprecation warning is emitted if no positional arguments are provided or not all positional arguments are Future-like objects and there is no running event loop.

Eager Task Factory ¶

asyncio. eager_task_factory ( loop , coro , * , 名称 = None , context = None ) ¶

A task factory for eager task execution.

When using this factory (via loop.set_task_factory(asyncio.eager_task_factory) ), coroutines begin execution synchronously during Task construction. Tasks are only scheduled on the event loop if they block. This can be a performance improvement as the overhead of loop scheduling is avoided for coroutines that complete synchronously.

A common example where this is beneficial is coroutines which employ caching or memoization to avoid actual I/O when possible.

注意

Immediate execution of the coroutine is a semantic change. If the coroutine returns or raises, the task is never scheduled to the event loop. If the coroutine execution blocks, the task is scheduled to the event loop. This change may introduce behavior changes to existing applications. For example, the application’s task execution order is likely to change.

3.12 版添加。

asyncio. create_eager_task_factory ( custom_task_constructor ) ¶

Create an eager task factory, similar to eager_task_factory() , using the provided custom_task_constructor when creating a new task instead of the default Task .

custom_task_constructor 必须为 callable with the signature matching the signature of Task.__init__ . The callable must return a asyncio.Task -compatible object.

This function returns a callable intended to be used as a task factory of an event loop via loop.set_task_factory(factory) ).

3.12 版添加。

从消除屏蔽  ¶

awaitable asyncio. shield ( aw ) ¶

Protect an awaitable object from being cancelled .

若 aw 是协程，自动作为 Task (任务) 被调度。

语句：

task = asyncio.create_task(something())
res = await shield(task)

相当于：

res = await something()

except that if the coroutine containing it is cancelled, the Task running in something() is not cancelled. From the point of view of something() , the cancellation did not happen. Although its caller is still cancelled, so the “await” expression still raises a CancelledError .

若 something() is cancelled by other means (i.e. from within itself) that would also cancel shield() .

If it is desired to completely ignore cancellation (not recommended) the shield() function should be combined with a try/except clause, as follows:

task = asyncio.create_task(something())
try:
    res = await shield(task)
except CancelledError:
    res = None

重要

Save a reference to tasks passed to this function, to avoid a task disappearing mid-execution. The event loop only keeps weak references to tasks. A task that isn’t referenced elsewhere may get garbage collected at any time, even before it’s done.

3.10 版改变：移除 loop 参数。

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

超时  ¶

asyncio. timeout ( delay ) ¶

返回异步上下文管理器 that can be used to limit the amount of time spent waiting on something.

delay 可以是 None , or a float/int number of seconds to wait. If delay is None , no time limit will be applied; this can be useful if the delay is unknown when the context manager is created.

In either case, the context manager can be rescheduled after creation using Timeout.reschedule() .

范例：

async def main():
    async with asyncio.timeout(10):
        await long_running_task()

若 long_running_task takes more than 10 seconds to complete, the context manager will cancel the current task and handle the resulting asyncio.CancelledError internally, transforming it into a TimeoutError which can be caught and handled.

注意

The asyncio.timeout() context manager is what transforms the asyncio.CancelledError 成 TimeoutError ，这意味着 TimeoutError can only be caught outside of the context manager.

Example of catching TimeoutError :

async def main():
    try:
        async with asyncio.timeout(10):
            await long_running_task()
    except TimeoutError:
        print("The long operation timed out, but we've handled it.")
    print("This statement will run regardless.")

The context manager produced by asyncio.timeout() can be rescheduled to a different deadline and inspected.

class asyncio. Timeout ( 当 ) ¶

An 异步上下文管理器 for cancelling overdue coroutines.

when should be an absolute time at which the context should time out, as measured by the event loop’s clock:

若 when is None , the timeout will never trigger.
若 when < loop.time() , the timeout will trigger on the next iteration of the event loop.

当 ( ) → float | None ¶

Return the current deadline, or None if the current deadline is not set.

reschedule ( 当 : float | None ) ¶

Reschedule the timeout.

expired ( ) → bool ¶

Return whether the context manager has exceeded its deadline (expired).

范例：

async def main():
    try:
        # We do not know the timeout when starting, so we pass ``None``.
        async with asyncio.timeout(None) as cm:
            # We know the timeout now, so we reschedule it.
            new_deadline = get_running_loop().time() + 10
            cm.reschedule(new_deadline)
            await long_running_task()
    except TimeoutError:
        pass
    if cm.expired():
        print("Looks like we haven't finished on time.")

Timeout context managers can be safely nested.

Added in version 3.11.

asyncio. timeout_at ( 当 ) ¶

类似于 asyncio.timeout() ，除了当 is the absolute time to stop waiting, or None .

范例：

async def main():
    loop = get_running_loop()
    deadline = loop.time() + 20
    try:
        async with asyncio.timeout_at(deadline):
            await long_running_task()
    except TimeoutError:
        print("The long operation timed out, but we've handled it.")
    print("This statement will run regardless.")

Added in version 3.11.

协程 asyncio. wait_for ( aw , timeout ) ¶

等待 aw awaitable 完成采用超时。

若 aw 是协程，自动作为 Task (任务) 被调度。

timeout 可以是 None or a float or int number of seconds to wait for. If timeout is None , block until the future completes.

If a timeout occurs, it cancels the task and raises TimeoutError .

要避免任务 cancellation ，将它包裹在 shield() .

The function will wait until the future is actually cancelled, so the total wait time may exceed the timeout . If an exception happens during cancellation, it is propagated.

If the wait is cancelled, the future aw is also cancelled.

范例：

async def eternity():
    # Sleep for one hour
    await asyncio.sleep(3600)
    print('yay!')
async def main():
    # Wait for at most 1 second
    try:
        await asyncio.wait_for(eternity(), timeout=1.0)
    except TimeoutError:
        print('timeout!')
asyncio.run(main())
# Expected output:
#
#     timeout!

3.7 版改变：当 aw is cancelled due to a timeout, wait_for waits for aw to be cancelled. Previously, it raised TimeoutError 立即。

3.10 版改变：移除 loop 参数。

3.11 版改变：引发 TimeoutError 而不是 asyncio.TimeoutError .

等待原语  ¶

协程 asyncio. wait ( aws , * , timeout = None , return_when = ALL_COMPLETED ) ¶

运行 Future and Task instances in the aws iterable concurrently and block until the condition specified by return_when .

The aws iterable must not be empty.

Returns two sets of Tasks/Futures: (done, pending) .

用法：

done, pending = await asyncio.wait(aws)

timeout (a float or int), if specified, can be used to control the maximum number of seconds to wait before returning.

Note that this function does not raise TimeoutError . Futures or Tasks that aren’t done when the timeout occurs are simply returned in the second set.

return_when indicates when this function should return. It must be one of the following constants:

常量	描述
asyncio. FIRST_COMPLETED ¶	The function will return when any future finishes or is cancelled.
asyncio. FIRST_EXCEPTION ¶	The function will return when any future finishes by raising an exception. If no future raises an exception then it is equivalent to `ALL_COMPLETED` .
asyncio. ALL_COMPLETED ¶	The function will return when all futures finish or are cancelled.

不像 wait_for() , wait() does not cancel the futures when a timeout occurs.

3.10 版改变：移除 loop 参数。

3.11 版改变：将协程对象传递给 wait() 直接是禁止的。

3.12 版改变： Added support for generators yielding tasks.

asyncio. as_completed ( aws , * , timeout = None ) ¶

运行可期待对象在 aws iterable concurrently. The returned object can be iterated to obtain the results of the awaitables as they finish.

The object returned by as_completed() can be iterated as an 异步迭代器 or a plain iterator . When asynchronous iteration is used, the originally-supplied awaitables are yielded if they are tasks or futures. This makes it easy to correlate previously-scheduled tasks with their results. Example:

ipv4_connect = create_task(open_connection("127.0.0.1", 80))
ipv6_connect = create_task(open_connection("::1", 80))
tasks = [ipv4_connect, ipv6_connect]
async for earliest_connect in as_completed(tasks):
    # earliest_connect is done. The result can be obtained by
    # awaiting it or calling earliest_connect.result()
    reader, writer = await earliest_connect
    if earliest_connect is ipv6_connect:
        print("IPv6 connection established.")
    else:
        print("IPv4 connection established.")

During asynchronous iteration, implicitly-created tasks will be yielded for supplied awaitables that aren’t tasks or futures.

When used as a plain iterator, each iteration yields a new coroutine that returns the result or raises the exception of the next completed awaitable. This pattern is compatible with Python versions older than 3.13:

ipv4_connect = create_task(open_connection("127.0.0.1", 80))
ipv6_connect = create_task(open_connection("::1", 80))
tasks = [ipv4_connect, ipv6_connect]
for next_connect in as_completed(tasks):
    # next_connect is not one of the original task objects. It must be
    # awaited to obtain the result value or raise the exception of the
    # awaitable that finishes next.
    reader, writer = await next_connect

A TimeoutError is raised if the timeout occurs before all awaitables are done. This is raised by the async for loop during asynchronous iteration or by the coroutines yielded during plain iteration.

3.10 版改变：移除 loop 参数。

从 3.10 版起弃用： Deprecation warning is emitted if not all awaitable objects in the aws iterable are Future-like objects and there is no running event loop.

3.12 版改变： Added support for generators yielding tasks.

Changed in version 3.13: The result can now be used as either an 异步迭代器 or as a plain iterator (previously it was only a plain iterator).

在线程中运行  ¶

协程 asyncio. to_thread ( func , / , * args , ** kwargs ) ¶

异步运行函数 func 在单独线程中。

Any *args and **kwargs supplied for this function are directly passed to func . Also, the current contextvars.Context is propagated, allowing context variables from the event loop thread to be accessed in the separate thread.

Return a coroutine that can be awaited to get the eventual result of func .

This coroutine function is primarily intended to be used for executing IO-bound functions/methods that would otherwise block the event loop if they were run in the main thread. For example:

def blocking_io():
    print(f"start blocking_io at {time.strftime('%X')}")
    # Note that time.sleep() can be replaced with any blocking
    # IO-bound operation, such as file operations.
    time.sleep(1)
    print(f"blocking_io complete at {time.strftime('%X')}")
async def main():
    print(f"started main at {time.strftime('%X')}")
    await asyncio.gather(
        asyncio.to_thread(blocking_io),
        asyncio.sleep(1))
    print(f"finished main at {time.strftime('%X')}")
asyncio.run(main())
# Expected output:
#
# started main at 19:50:53
# start blocking_io at 19:50:53
# blocking_io complete at 19:50:54
# finished main at 19:50:54

直接调用 blocking_io() in any coroutine would block the event loop for its duration, resulting in an additional 1 second of run time. Instead, by using asyncio.to_thread() , we can run it in a separate thread without blocking the event loop.

注意

由于 GIL , asyncio.to_thread() can typically only be used to make IO-bound functions non-blocking. However, for extension modules that release the GIL or alternative Python implementations that don’t have one, asyncio.to_thread() can also be used for CPU-bound functions.

Added in version 3.9.

从其它线程调度  ¶

asyncio. run_coroutine_threadsafe ( coro , loop ) ¶

将协程提交给给定事件循环。是线程安全的。

返回 concurrent.futures.Future to wait for the result from another OS thread.

This function is meant to be called from a different OS thread than the one where the event loop is running. Example:

# Create a coroutine
coro = asyncio.sleep(1, result=3)
# Submit the coroutine to a given loop
future = asyncio.run_coroutine_threadsafe(coro, loop)
# Wait for the result with an optional timeout argument
assert future.result(timeout) == 3

If an exception is raised in the coroutine, the returned Future will be notified. It can also be used to cancel the task in the event loop:

try:
    result = future.result(timeout)
except TimeoutError:
    print('The coroutine took too long, cancelling the task...')
    future.cancel()
except Exception as exc:
    print(f'The coroutine raised an exception: {exc!r}')
else:
    print(f'The coroutine returned: {result!r}')

见并发和多线程章节的文档编制。

Unlike other asyncio functions this function requires the loop argument to be passed explicitly.

Added in version 3.5.1.

自省  ¶

asyncio. current_task ( loop = None ) ¶

返回目前正运行的 Task 实例，或 None 若没有任务在运行。

若 loop is None get_running_loop() 用于获取当前循环。

3.7 版添加。

asyncio. all_tasks ( loop = None ) ¶

Return a set of not yet finished Task objects run by the loop.

若 loop is None , get_running_loop() is used for getting current loop.

3.7 版添加。

asyncio. iscoroutine ( obj ) ¶

返回 True if obj is a coroutine object.

Added in version 3.4.

任务对象  ¶

class asyncio. Task ( coro , * , loop = None , 名称 = None , context = None , eager_start = False ) ¶

A Future-like 对象运行 Python 协程。不是线程安全的。

Tasks are used to run coroutines in event loops. If a coroutine awaits on a Future, the Task suspends the execution of the coroutine and waits for the completion of the Future. When the Future is done , the execution of the wrapped coroutine resumes.

Event loops use cooperative scheduling: an event loop runs one Task at a time. While a Task awaits for the completion of a Future, the event loop runs other Tasks, callbacks, or performs IO operations.

Use the high-level asyncio.create_task() function to create Tasks, or the low-level loop.create_task() or ensure_future() functions. Manual instantiation of Tasks is discouraged.

To cancel a running Task use the cancel() method. Calling it will cause the Task to throw a CancelledError exception into the wrapped coroutine. If a coroutine is awaiting on a Future object during cancellation, the Future object will be cancelled.

cancelled() can be used to check if the Task was cancelled. The method returns True if the wrapped coroutine did not suppress the CancelledError exception and was actually cancelled.

asyncio.Task 继承自 Future all of its APIs except Future.set_result() and Future.set_exception() .

可选仅关键词 context 自变量允许指定自定义 contextvars.Context 为 coro to run in. If no context is provided, the Task copies the current context and later runs its coroutine in the copied context.

可选仅关键词 eager_start argument allows eagerly starting the execution of the asyncio.Task at task creation time. If set to True and the event loop is running, the task will start executing the coroutine immediately, until the first time the coroutine blocks. If the coroutine returns or raises without blocking, the task will be finished eagerly and will skip scheduling to the event loop.

3.7 版改变：添加支持 contextvars 模块。

3.8 版改变：添加 name 参数。

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

3.11 版改变：添加 context 参数。

3.12 版改变：添加 eager_start 参数。

done ( ) ¶

返回 True 若 Task done .

A Task is done when the wrapped coroutine either returned a value, raised an exception, or the Task was cancelled.

result ( ) ¶

Return the result of the Task.

If the Task is done , the result of the wrapped coroutine is returned (or if the coroutine raised an exception, that exception is re-raised.)

If the Task has been cancelled , this method raises a CancelledError 异常。

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

exception ( ) ¶

Return the exception of the Task.

If the wrapped coroutine raised an exception that exception is returned. If the wrapped coroutine returned normally this method returns None .

If the Task has been cancelled , this method raises a CancelledError 异常。

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

add_done_callback ( callback , * , context = None ) ¶

Add a callback to be run when the Task is done .

This method should only be used in low-level callback-based code.

见文档编制为 Future.add_done_callback() 了解更多细节。

remove_done_callback ( callback ) ¶

移除 callback 从回调列表。

This method should only be used in low-level callback-based code.

见文档编制为 Future.remove_done_callback() 了解更多细节。

get_stack ( * , limit = None ) ¶

Return the list of stack frames for this Task.

If the wrapped coroutine is not done, this returns the stack where it is suspended. If the coroutine has completed successfully or was cancelled, this returns an empty list. If the coroutine was terminated by an exception, this returns the list of traceback frames.

The frames are always ordered from oldest to newest.

Only one stack frame is returned for a suspended coroutine.

可选 limit argument sets the maximum number of frames to return; by default all available frames are returned. The ordering of the returned list differs depending on whether a stack or a traceback is returned: the newest frames of a stack are returned, but the oldest frames of a traceback are returned. (This matches the behavior of the traceback module.)

print_stack ( * , limit = None , file = None ) ¶

Print the stack or traceback for this Task.

This produces output similar to that of the traceback module for the frames retrieved by get_stack() .

The limit 自变量会被传递给 get_stack() 直接。

The file argument is an I/O stream to which the output is written; by default output is written to sys.stdout .

get_coro ( ) ¶: Return the coroutine object wrapped by the Task .

注意

This will return None for Tasks which have already completed eagerly. See the Eager Task Factory .

Added in version 3.8.

3.12 版改变： Newly added eager task execution means result may be None .

get_context ( ) ¶

返回 contextvars.Context object associated with the task.

3.12 版添加。

get_name ( ) ¶

Return the name of the Task.

If no name has been explicitly assigned to the Task, the default asyncio Task implementation generates a default name during instantiation.

Added in version 3.8.

set_name ( 值 ) ¶

Set the name of the Task.

The value argument can be any object, which is then converted to a string.

In the default Task implementation, the name will be visible in the repr() output of a task object.

Added in version 3.8.

cancel ( msg = None ) ¶

Request the Task to be cancelled.

This arranges for a CancelledError exception to be thrown into the wrapped coroutine on the next cycle of the event loop.

The coroutine then has a chance to clean up or even deny the request by suppressing the exception with a try … … except CancelledError … finally block. Therefore, unlike Future.cancel() , Task.cancel() does not guarantee that the Task will be cancelled, although suppressing cancellation completely is not common and is actively discouraged. Should the coroutine nevertheless decide to suppress the cancellation, it needs to call Task.uncancel() in addition to catching the exception.

3.9 版改变：添加 msg 参数。

3.11 版改变： The msg parameter is propagated from cancelled task to its awaiter.

The following example illustrates how coroutines can intercept the cancellation request:

async def cancel_me():
    print('cancel_me(): before sleep')
    try:
        # Wait for 1 hour
        await asyncio.sleep(3600)
    except asyncio.CancelledError:
        print('cancel_me(): cancel sleep')
        raise
    finally:
        print('cancel_me(): after sleep')
async def main():
    # Create a "cancel_me" Task
    task = asyncio.create_task(cancel_me())
    # Wait for 1 second
    await asyncio.sleep(1)
    task.cancel()
    try:
        await task
    except asyncio.CancelledError:
        print("main(): cancel_me is cancelled now")
asyncio.run(main())
# Expected output:
#
#     cancel_me(): before sleep
#     cancel_me(): cancel sleep
#     cancel_me(): after sleep
#     main(): cancel_me is cancelled now

cancelled ( ) ¶

返回 True 若 Task cancelled .

The Task is cancelled when the cancellation was requested with cancel() and the wrapped coroutine propagated the CancelledError exception thrown into it.

uncancel ( ) ¶

Decrement the count of cancellation requests to this Task.

Returns the remaining number of cancellation requests.

Note that once execution of a cancelled task completed, further calls to uncancel() are ineffective.

Added in version 3.11.

This method is used by asyncio’s internals and isn’t expected to be used by end-user code. In particular, if a Task gets successfully uncancelled, this allows for elements of structured concurrency like 任务组 and asyncio.timeout() to continue running, isolating cancellation to the respective structured block. For example:

async def make_request_with_timeout():
    try:
        async with asyncio.timeout(1):
            # Structured block affected by the timeout:
            await make_request()
            await make_another_request()
    except TimeoutError:
        log("There was a timeout")
    # Outer code not affected by the timeout:
    await unrelated_code()

While the block with make_request() and make_another_request() might get cancelled due to the timeout, unrelated_code() should continue running even in case of the timeout. This is implemented with uncancel() . TaskGroup context managers use uncancel() in a similar fashion.

If end-user code is, for some reason, suppressing cancellation by catching CancelledError , it needs to call this method to remove the cancellation state.

When this method decrements the cancellation count to zero, the method checks if a previous cancel() call had arranged for CancelledError to be thrown into the task. If it hasn’t been thrown yet, that arrangement will be rescinded (by resetting the internal _must_cancel flag).

Changed in version 3.13: Changed to rescind pending cancellation requests upon reaching zero.

cancelling ( ) ¶

Return the number of pending cancellation requests to this Task, i.e., the number of calls to cancel() less the number of uncancel() 调用。

Note that if this number is greater than zero but the Task is still executing, cancelled() will still return False . This is because this number can be lowered by calling uncancel() , which can lead to the task not being cancelled after all if the cancellation requests go down to zero.

This method is used by asyncio’s internals and isn’t expected to be used by end-user code. See uncancel() 了解更多细节。

Added in version 3.11.

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协程和任务 ¶

Terminating a Task Group ¶

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