18.5.3. 任务和协程 ¶

18.5.3.1. 协程 ¶

Coroutines used with asyncio may be implemented using the async def statement, or by using generators . async def type of coroutine was added in Python 3.5, and is recommended if there is no need to support older Python versions.

Generator-based coroutines should be decorated with @asyncio.coroutine , although this is not strictly enforced. The decorator enables compatibility with async def coroutines, and also serves as documentation. Generator-based coroutines use the yield from syntax introduced in PEP 380 , instead of the original yield 句法。

The word “coroutine”, like the word “generator”, is used for two different (though related) concepts:

• The function that defines a coroutine (a function definition using async def or decorated with @asyncio.coroutine ). If disambiguation is needed we will call this a 协程函数 ( iscoroutinefunction() 返回 True ).
• The object obtained by calling a coroutine function. This object represents a computation or an I/O operation (usually a combination) that will complete eventually. If disambiguation is needed we will call it a coroutine object ( iscoroutine() 返回 True ).

Things a coroutine can do:

• result = await future or result = yield from future – suspends the coroutine until the future is done, then returns the future’s result, or raises an exception, which will be propagated. (If the future is cancelled, it will raise a CancelledError exception.) Note that tasks are futures, and everything said about futures also applies to tasks.
• result = await 协程 or result = yield from 协程 – wait for another coroutine to produce a result (or raise an exception, which will be propagated). 协程 expression must be a call to another coroutine.
• return expression – produce a result to the coroutine that is waiting for this one using await or yield from .
• raise exception – raise an exception in the coroutine that is waiting for this one using await or yield from .

Calling a coroutine does not start its code running – the coroutine object returned by the call doesn’t do anything until you schedule its execution. There are two basic ways to start it running: call await 协程 or yield from 协程 from another coroutine (assuming the other coroutine is already running!), or schedule its execution using the ensure_future() function or the AbstractEventLoop.create_task() 方法。

Coroutines (and tasks) can only run when the event loop is running.

@ asyncio. 协程 ¶

Decorator to mark generator-based coroutines. This enables the generator use yield from to call async def coroutines, and also enables the generator to be called by async def coroutines, for instance using an await 表达式。

There is no need to decorate async def coroutines themselves.

If the generator is not yielded from before it is destroyed, an error message is logged. See Detect coroutines never scheduled .

import asyncio
async def hello_world():
    print("Hello World!")
loop = asyncio.get_event_loop()
# Blocking call which returns when the hello_world() coroutine is done
loop.run_until_complete(hello_world())
loop.close()
									

import asyncio
import datetime
async def display_date(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)
loop = asyncio.get_event_loop()
# Blocking call which returns when the display_date() coroutine is done
loop.run_until_complete(display_date(loop))
loop.close()
									

18.5.3.1.3. Example: Chain coroutines ¶

Example chaining coroutines:

import asyncio
async def compute(x, y):
    print("Compute %s + %s ..." % (x, y))
    await asyncio.sleep(1.0)
    return x + y
async def print_sum(x, y):
    result = await compute(x, y)
    print("%s + %s = %s" % (x, y, result))
loop = asyncio.get_event_loop()
loop.run_until_complete(print_sum(1, 2))
loop.close()
									

compute() is chained to print_sum() : print_sum() coroutine waits until compute() is completed before returning its result.

Sequence diagram of the example:

The “Task” is created by the AbstractEventLoop.run_until_complete() method when it gets a coroutine object instead of a task.

The diagram shows the control flow, it does not describe exactly how things work internally. For example, the sleep coroutine creates an internal future which uses AbstractEventLoop.call_later() to wake up the task in 1 second.

18.5.3.2. InvalidStateError ¶

exception asyncio. InvalidStateError ¶

The operation is not allowed in this state.

18.5.3.3. TimeoutError ¶

exception asyncio. TimeoutError ¶

The operation exceeded the given deadline.

18.5.3.4. Future ¶

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

此类是 almost compatible with concurrent.futures.Future .

Differences:

• result() and exception() do not take a timeout argument and raise an exception when the future isn’t done yet.
• Callbacks registered with add_done_callback() are always called via the event loop’s call_soon() .
• This class is not compatible with the wait() and as_completed() functions in the concurrent.futures package.

此类是 not thread safe .

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 .

cancelled ( ) ¶

返回 True if the future was cancelled.

done ( ) ¶

返回 True if the future is done.

Done means either that a result / exception are available, or that the future was cancelled.

result ( ) ¶

Return the result this future represents.

If the future has been cancelled, raises CancelledError . If the future’s result isn’t yet available, raises InvalidStateError . If the future is done and has an exception set, this exception is raised.

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, raises CancelledError . If the future isn’t done yet, raises InvalidStateError .

add_done_callback ( fn ) ¶

Add a callback to be run when the future becomes done.

The callback is called with a single argument - the future object. If the future is already done when this is called, the callback is scheduled with call_soon() .

Use functools.partial to pass parameters to the callback 。例如， fut.add_done_callback(functools.partial(print, "Future:", flush=True)) will call print("Future:", fut, flush=True) .

remove_done_callback ( fn ) ¶

Remove all instances of a callback from the “call when done” list.

Returns the number of callbacks removed.

set_result ( result ) ¶

Mark the future done and set its result.

If the future is already done when this method is called, raises InvalidStateError .

set_exception ( exception ) ¶

Mark the future done and set an exception.

If the future is already done when this method is called, raises InvalidStateError .

import asyncio
async def slow_operation(future):
    await asyncio.sleep(1)
    future.set_result('Future is done!')
loop = asyncio.get_event_loop()
future = asyncio.Future()
asyncio.ensure_future(slow_operation(future))
loop.run_until_complete(future)
print(future.result())
loop.close()
									

import asyncio
async def slow_operation(future):
    await asyncio.sleep(1)
    future.set_result('Future is done!')
def got_result(future):
    print(future.result())
    loop.stop()
loop = asyncio.get_event_loop()
future = asyncio.Future()
asyncio.ensure_future(slow_operation(future))
future.add_done_callback(got_result)
try:
    loop.run_forever()
finally:
    loop.close()
									

18.5.3.5. 任务 ¶

class asyncio. Task ( coro , * , loop=None ) ¶

Schedule the execution of a 协程 : wrap it in a future. A task is a subclass of Future .

A task is responsible for executing a coroutine object in an event loop. If the wrapped coroutine yields from a future, the task suspends the execution of the wrapped coroutine and waits for the completion of the future. When the future is done, the execution of the wrapped coroutine restarts with the result or the exception of the future.

Event loops use cooperative scheduling: an event loop only runs one task at a time. Other tasks may run in parallel if other event loops are running in different threads. While a task waits for the completion of a future, the event loop executes a new task.

The cancellation of a task is different from the cancelation of a future. Calling cancel() will throw a CancelledError to the wrapped coroutine. cancelled() only returns True if the wrapped coroutine did not catch the CancelledError exception, or raised a CancelledError 异常。

If a pending task is destroyed, the execution of its wrapped 协程 did not complete. It is probably a bug and a warning is logged: see Pending task destroyed .

Don’t directly create Task instances: use the ensure_future() function or the AbstractEventLoop.create_task() 方法。

此类是 not thread safe .

classmethod all_tasks ( loop=None ) ¶

Return a set of all tasks for an event loop.

By default all tasks for the current event loop are returned.

classmethod current_task ( loop=None ) ¶

Return the currently running task in an event loop or None .

By default the current task for the current event loop is returned.

None is returned when called not in the context of a Task .

cancel ( ) ¶

Request that this task cancel itself.

This arranges for a CancelledError to be thrown into the wrapped coroutine on the next cycle through the event loop. The coroutine then has a chance to clean up or even deny the request using try/except/finally.

不像 Future.cancel() , this does not guarantee that the task will be cancelled: the exception might be caught and acted upon, delaying cancellation of the task or preventing cancellation completely. The task may also return a value or raise a different exception.

Immediately after this method is called, cancelled() will not return True (unless the task was already cancelled). A task will be marked as cancelled when the wrapped coroutine terminates with a CancelledError exception (even if cancel() was not called).

get_stack ( * , limit=None ) ¶

Return the list of stack frames for this task’s coroutine.

If the 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.

The optional limit gives the maximum number of frames to return; by default all available frames are returned. Its meaning 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.)

For reasons beyond our control, only one stack frame is returned for a suspended coroutine.

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

Print the stack or traceback for this task’s coroutine.

This produces output similar to that of the traceback module, for the frames retrieved by get_stack(). The limit argument is passed to get_stack(). The file argument is an I/O stream to which the output is written; by default output is written to sys.stderr.

import asyncio
async def factorial(name, number):
    f = 1
    for i in range(2, number+1):
        print("Task %s: Compute factorial(%s)..." % (name, i))
        await asyncio.sleep(1)
        f *= i
    print("Task %s: factorial(%s) = %s" % (name, number, f))
loop = asyncio.get_event_loop()
loop.run_until_complete(asyncio.gather(
    factorial("A", 2),
    factorial("B", 3),
    factorial("C", 4),
))
loop.close()
									

Task A: Compute factorial(2)...
Task B: Compute factorial(2)...
Task C: Compute factorial(2)...
Task A: factorial(2) = 2
Task B: Compute factorial(3)...
Task C: Compute factorial(3)...
Task B: factorial(3) = 6
Task C: Compute factorial(4)...
Task C: factorial(4) = 24
									

18.5.3.6. 任务函数 ¶

asyncio. as_completed ( fs , * , loop=None , timeout=None ) ¶

Return an iterator whose values, when waited for, are Future 实例。

引发 asyncio.TimeoutError if the timeout occurs before all Futures are done.

范例：

for f in as_completed(fs):
    result = yield from f  # The 'yield from' may raise
    # Use result
										

注意

The futures f are not necessarily members of fs.

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

Schedule the execution of a coroutine object : wrap it in a future. Return a Task 对象。

If the argument is a Future , it is returned directly.

3.4.4 版新增。

3.5.1 版改变： The function accepts any awaitable 对象。

另请参阅

AbstractEventLoop.create_task() 方法。

asyncio. async ( coro_or_future , * , loop=None ) ¶

A deprecated alias to ensure_future() .

Deprecated since version 3.4.4.

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

Wrap a concurrent.futures.Future object in a Future 对象。

asyncio. gather ( *coros_or_futures , loop=None , return_exceptions=False ) ¶

Return a future aggregating results from the given coroutine objects or futures.

All futures must share the same event loop. If all the tasks are done successfully, the returned future’s result is the list of results (in the order of the original sequence, not necessarily the order of results arrival). If return_exceptions is true, exceptions in the tasks are treated the same as successful results, and gathered in the result list; otherwise, the first raised exception will be immediately propagated to the returned future.

Cancellation: if the outer Future is cancelled, all children (that have not completed yet) are also cancelled. If any child is cancelled, this is treated as if it raised CancelledError – the outer Future is not cancelled in this case. (This is to prevent the cancellation of one child to cause other children to be cancelled.)

Changed in version 3.6.6: 若 gather itself is cancelled, the cancellation is propagated regardless of return_exceptions .

asyncio. iscoroutine ( obj ) ¶

返回 True if obj 是 coroutine object , which may be based on a generator or an async def coroutine.

asyncio. iscoroutinefunction ( func ) ¶

返回 True if func is determined to be a 协程函数 , which may be a decorated generator function or an async def 函数。

asyncio. run_coroutine_threadsafe ( coro , loop ) ¶

Submit a coroutine object to a given event loop.

返回 concurrent.futures.Future to access the result.

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

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

见 并发和多线程 section of the documentation.

注意

Unlike other functions from the module, run_coroutine_threadsafe() requires the loop argument to be passed explicitly.

New in version 3.5.1.

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

创建 协程 that completes after a given time (in seconds). If result is provided, it is produced to the caller when the coroutine completes.

The resolution of the sleep depends on the granularity of the event loop .

This function is a 协程 .

协程 asyncio. shield ( arg , * , loop=None ) ¶

Wait for a future, shielding it from cancellation.

The statement:

res = yield from shield(something())
										

is exactly equivalent to the statement:

res = yield from 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. But its caller is still cancelled, so the yield-from expression still raises CancelledError . Note: If something() is cancelled by other means this will still cancel shield() .

If you want to completely ignore cancellation (not recommended) you can combine shield() with a try/except clause, as follows:

try:
    res = yield from shield(something())
except CancelledError:
    res = None
										

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

Wait for the Futures and coroutine objects given by the sequence futures to complete. Coroutines will be wrapped in Tasks. Returns two sets of Future : (done, pending).

The sequence futures must not be empty.

timeout can be used to control the maximum number of seconds to wait before returning. timeout can be an int or float. If timeout 未指定或 None , there is no limit to the wait time.

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

常量	Description
FIRST_COMPLETED	The function will return when any future finishes or is cancelled.
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 .
ALL_COMPLETED	The function will return when all futures finish or are cancelled.

This function is a 协程 .

用法：

done, pending = yield from asyncio.wait(fs)
										

注意

This does not raise asyncio.TimeoutError ! Futures that aren’t done when the timeout occurs are returned in the second set.

协程 asyncio. wait_for ( fut , timeout , * , loop=None ) ¶

Wait for the single Future or coroutine object to complete with timeout. If timeout is None , block until the future completes.

Coroutine will be wrapped in Task .

Returns result of the Future or coroutine. When a timeout occurs, it cancels the task and raises asyncio.TimeoutError . To avoid the task cancellation, wrap it in shield() .

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

This function is a 协程 , usage:

result = yield from asyncio.wait_for(fut, 60.0)
										

3.4.3 版改变： If the wait is cancelled, the future fut is now also cancelled.