asyncio.gather
和 asyncio.wait
似乎有相似的用途:我有一堆我想要执行/等待的异步事情(不一定要在下一个开始之前等待一个完成)。它们使用不同的语法,并且在某些细节上有所不同,但对我来说,拥有两个在功能上具有如此巨大重叠的函数似乎非常不符合 Python 标准。我错过了什么?
尽管在一般情况下类似(“运行并获取许多任务的结果”),但每个函数对于其他情况都有一些特定的功能:
asyncio.gather()
返回一个 Future 实例,允许对任务进行高级分组:
import asyncio
from pprint import pprint
import random
async def coro(tag):
print(">", tag)
await asyncio.sleep(random.uniform(1, 3))
print("<", tag)
return tag
loop = asyncio.get_event_loop()
group1 = asyncio.gather(*[coro("group 1.{}".format(i)) for i in range(1, 6)])
group2 = asyncio.gather(*[coro("group 2.{}".format(i)) for i in range(1, 4)])
group3 = asyncio.gather(*[coro("group 3.{}".format(i)) for i in range(1, 10)])
all_groups = asyncio.gather(group1, group2, group3)
results = loop.run_until_complete(all_groups)
loop.close()
pprint(results)
可以通过调用 group2.cancel()
甚至 all_groups.cancel()
来取消组中的所有任务。另见.gather(..., return_exceptions=True)
,
asyncio.wait()
支持在第一个任务完成后或指定超时后等待停止,允许较低级别的操作精度:
import asyncio
import random
async def coro(tag):
print(">", tag)
await asyncio.sleep(random.uniform(0.5, 5))
print("<", tag)
return tag
loop = asyncio.get_event_loop()
tasks = [coro(i) for i in range(1, 11)]
print("Get first result:")
finished, unfinished = loop.run_until_complete(
asyncio.wait(tasks, return_when=asyncio.FIRST_COMPLETED))
for task in finished:
print(task.result())
print("unfinished:", len(unfinished))
print("Get more results in 2 seconds:")
finished2, unfinished2 = loop.run_until_complete(
asyncio.wait(unfinished, timeout=2))
for task in finished2:
print(task.result())
print("unfinished2:", len(unfinished2))
print("Get all other results:")
finished3, unfinished3 = loop.run_until_complete(asyncio.wait(unfinished2))
for task in finished3:
print(task.result())
loop.close()
asyncio.wait
的级别比 asyncio.gather
低。
顾名思义,asyncio.gather
主要侧重于收集结果。它等待一堆期货并以给定的顺序返回它们的结果。
asyncio.wait
只是等待未来。而不是直接给你结果,它给出了已完成和待处理的任务。您必须手动收集这些值。
此外,您可以使用 wait
指定等待所有期货完成或仅等待第一个期货完成。
asyncio.wait
有一个名为 return_when
的参数,您可以使用它来控制事件循环何时应让步给您。 asyncio.gather
没有这样的参数,事件循环仅在所有任务完成/失败时才返回给您。在此处阅读官方文档:docs.python.org/3/library/asyncio-task.html#asyncio.wait
asyncio.wait
的名为 return_when
的参数已在 Python 3.5.9 中可用!见这里:docs.python.org/3.5/library/asyncio-task.html#asyncio.wait
python -m timeit "print('hello')"
每个循环提供 36.6 微秒,因此 10000000000000 print('hello')
将需要 11.6 年才能完成 print()
功能
当涉及到异常时,很容易忽略的一个非常重要的区别是这两个函数的默认行为。
我将使用这个例子来模拟一个会引发异常的协程,有时 -
import asyncio
import random
async def a_flaky_tsk(i):
await asyncio.sleep(i) # bit of fuzz to simulate a real-world example
if i % 2 == 0:
print(i, "ok")
else:
print(i, "crashed!")
raise ValueError
coros = [a_flaky_tsk(i) for i in range(10)]
await asyncio.gather(*coros)
输出 -
0 ok
1 crashed!
Traceback (most recent call last):
File "/Users/dev/PycharmProjects/trading/xxx.py", line 20, in <module>
asyncio.run(main())
File "/Users/dev/.pyenv/versions/3.8.2/lib/python3.8/asyncio/runners.py", line 43, in run
return loop.run_until_complete(main)
File "/Users/dev/.pyenv/versions/3.8.2/lib/python3.8/asyncio/base_events.py", line 616, in run_until_complete
return future.result()
File "/Users/dev/PycharmProjects/trading/xxx.py", line 17, in main
await asyncio.gather(*coros)
File "/Users/dev/PycharmProjects/trading/xxx.py", line 12, in a_flaky_tsk
raise ValueError
ValueError
如您所见,索引 1
之后的 coros 永远不会执行。
但 await asyncio.wait(coros)
继续执行任务,即使其中一些任务失败 -
0 ok
1 crashed!
2 ok
3 crashed!
4 ok
5 crashed!
6 ok
7 crashed!
8 ok
9 crashed!
Task exception was never retrieved
future: <Task finished name='Task-10' coro=<a_flaky_tsk() done, defined at /Users/dev/PycharmProjects/trading/xxx.py:6> exception=ValueError()>
Traceback (most recent call last):
File "/Users/dev/PycharmProjects/trading/xxx.py", line 12, in a_flaky_tsk
raise ValueError
ValueError
Task exception was never retrieved
future: <Task finished name='Task-8' coro=<a_flaky_tsk() done, defined at /Users/dev/PycharmProjects/trading/xxx.py:6> exception=ValueError()>
Traceback (most recent call last):
File "/Users/dev/PycharmProjects/trading/xxx.py", line 12, in a_flaky_tsk
raise ValueError
ValueError
Task exception was never retrieved
future: <Task finished name='Task-2' coro=<a_flaky_tsk() done, defined at /Users/dev/PycharmProjects/trading/xxx.py:6> exception=ValueError()>
Traceback (most recent call last):
File "/Users/dev/PycharmProjects/trading/xxx.py", line 12, in a_flaky_tsk
raise ValueError
ValueError
Task exception was never retrieved
future: <Task finished name='Task-9' coro=<a_flaky_tsk() done, defined at /Users/dev/PycharmProjects/trading/xxx.py:6> exception=ValueError()>
Traceback (most recent call last):
File "/Users/dev/PycharmProjects/trading/xxx.py", line 12, in a_flaky_tsk
raise ValueError
ValueError
Task exception was never retrieved
future: <Task finished name='Task-3' coro=<a_flaky_tsk() done, defined at /Users/dev/PycharmProjects/trading/xxx.py:6> exception=ValueError()>
Traceback (most recent call last):
File "/Users/dev/PycharmProjects/trading/xxx.py", line 12, in a_flaky_tsk
raise ValueError
ValueError
当然,可以通过使用 -
asyncio.gather(..., return_exceptions=True)
或者,
asyncio.wait([...], return_when=asyncio.FIRST_EXCEPTION)
但这并没有结束!
注意:Task exception was never retrieved
在上面的日志中。
asyncio.wait()
不会从子任务中重新引发异常,直到您单独await
它们。 (日志中的堆栈跟踪只是消息,它们无法被捕获!)
done, pending = await asyncio.wait(coros)
for tsk in done:
try:
await tsk
except Exception as e:
print("I caught:", repr(e))
输出 -
0 ok
1 crashed!
2 ok
3 crashed!
4 ok
5 crashed!
6 ok
7 crashed!
8 ok
9 crashed!
I caught: ValueError()
I caught: ValueError()
I caught: ValueError()
I caught: ValueError()
I caught: ValueError()
另一方面,要使用 asyncio.gather()
捕获异常,您必须 -
results = await asyncio.gather(*coros, return_exceptions=True)
for result_or_exc in results:
if isinstance(result_or_exc, Exception):
print("I caught:", repr(result_or_exc))
(与之前的输出相同)
Task exception was never retrieved
错误。非常感谢您的精彩解释..
我还注意到,您可以通过简单地指定列表在 wait() 中提供一组协程:
result=loop.run_until_complete(asyncio.wait([
say('first hello', 2),
say('second hello', 1),
say('third hello', 4)
]))
而在 gather() 中的分组仅通过指定多个协程来完成:
result=loop.run_until_complete(asyncio.gather(
say('first hello', 2),
say('second hello', 1),
say('third hello', 4)
))
gather()
一起使用,例如:asyncio.gather(*task_list)
await
! group = asyncio.gather(*aws)
直接返回组的可等待/未来,它代表所有组合任务。任务可以在 asyncio.gather
调用后很快运行,例如,当有一个 await
表示其他东西(如 asyncio.sleep
)或访问未来时(如 group.done()
)。当您想确保任务已完成或取消并收集所有结果时,您只需要使用 await group
。
除了前面的所有答案之外,我还想介绍一下 gather()
和 wait()
的不同行为,以防它们被取消。
Gather() 取消
如果 gather()
被取消,所有提交的等待项(尚未完成)也将被取消。
等待()取消
如果取消 wait()
任务,它只会抛出一个 CancelledError
并且等待的任务保持不变。
简单的例子:
import asyncio
async def task(arg):
await asyncio.sleep(5)
return arg
async def cancel_waiting_task(work_task, waiting_task):
await asyncio.sleep(2)
waiting_task.cancel()
try:
await waiting_task
print("Waiting done")
except asyncio.CancelledError:
print("Waiting task cancelled")
try:
res = await work_task
print(f"Work result: {res}")
except asyncio.CancelledError:
print("Work task cancelled")
async def main():
work_task = asyncio.create_task(task("done"))
waiting = asyncio.create_task(asyncio.wait({work_task}))
await cancel_waiting_task(work_task, waiting)
work_task = asyncio.create_task(task("done"))
waiting = asyncio.gather(work_task)
await cancel_waiting_task(work_task, waiting)
asyncio.run(main())
输出:
asyncio.wait()
Waiting task cancelled
Work result: done
----------------
asyncio.gather()
Waiting task cancelled
Work task cancelled
申请
有时需要结合 wait()
和 gather()
功能。例如,我们希望等待至少一个任务完成,然后取消其余待处理的任务,如果waiting
本身被取消,然后也取消所有待处理的任务。
作为真实的例子,假设我们有一个断开连接事件和一个工作任务。而我们想要等待工作任务的结果,但是如果连接丢失了,那么就取消它。或者我们将发出多个并行请求,但在完成至少一个响应后,取消所有其他请求。
可以这样做:
import asyncio
from typing import Optional, Tuple, Set
async def wait_any(
tasks: Set[asyncio.Future], *, timeout: Optional[int] = None,
) -> Tuple[Set[asyncio.Future], Set[asyncio.Future]]:
tasks_to_cancel: Set[asyncio.Future] = set()
try:
done, tasks_to_cancel = await asyncio.wait(
tasks, timeout=timeout, return_when=asyncio.FIRST_COMPLETED
)
return done, tasks_to_cancel
except asyncio.CancelledError:
tasks_to_cancel = tasks
raise
finally:
for task in tasks_to_cancel:
task.cancel()
async def task():
await asyncio.sleep(5)
async def cancel_waiting_task(work_task, waiting_task):
await asyncio.sleep(2)
waiting_task.cancel()
try:
await waiting_task
print("Waiting done")
except asyncio.CancelledError:
print("Waiting task cancelled")
try:
res = await work_task
print(f"Work result: {res}")
except asyncio.CancelledError:
print("Work task cancelled")
async def check_tasks(waiting_task, working_task, waiting_conn_lost_task):
try:
await waiting_task
print("waiting is done")
except asyncio.CancelledError:
print("waiting is cancelled")
try:
await waiting_conn_lost_task
print("connection is lost")
except asyncio.CancelledError:
print("waiting connection lost is cancelled")
try:
await working_task
print("work is done")
except asyncio.CancelledError:
print("work is cancelled")
async def work_done_case():
working_task = asyncio.create_task(task())
connection_lost_event = asyncio.Event()
waiting_conn_lost_task = asyncio.create_task(connection_lost_event.wait())
waiting_task = asyncio.create_task(wait_any({working_task, waiting_conn_lost_task}))
await check_tasks(waiting_task, working_task, waiting_conn_lost_task)
async def conn_lost_case():
working_task = asyncio.create_task(task())
connection_lost_event = asyncio.Event()
waiting_conn_lost_task = asyncio.create_task(connection_lost_event.wait())
waiting_task = asyncio.create_task(wait_any({working_task, waiting_conn_lost_task}))
await asyncio.sleep(2)
connection_lost_event.set() # <---
await check_tasks(waiting_task, working_task, waiting_conn_lost_task)
async def cancel_waiting_case():
working_task = asyncio.create_task(task())
connection_lost_event = asyncio.Event()
waiting_conn_lost_task = asyncio.create_task(connection_lost_event.wait())
waiting_task = asyncio.create_task(wait_any({working_task, waiting_conn_lost_task}))
await asyncio.sleep(2)
waiting_task.cancel() # <---
await check_tasks(waiting_task, working_task, waiting_conn_lost_task)
async def main():
print("Work done")
print("-------------------")
await work_done_case()
print("\nConnection lost")
print("-------------------")
await conn_lost_case()
print("\nCancel waiting")
print("-------------------")
await cancel_waiting_case()
asyncio.run(main())
输出:
Work done
-------------------
waiting is done
waiting connection lost is cancelled
work is done
Connection lost
-------------------
waiting is done
connection is lost
work is cancelled
Cancel waiting
-------------------
waiting is cancelled
waiting connection lost is cancelled
work is cancelled
asyncio.gather()
代码中,如果创建这三个组的代码包含在函数体中,您可以去掉loop = asyncio.get_event_loop()
并重构代码,将await
添加到asyncio.gather(group1, group2, group3)
使其稍微简单一些,并且不再需要与循环变量相关的所有行