Cla*_*ude 107 python python-asyncio
asyncio.gather并且asyncio.wait似乎有类似的用途:我有一堆异步的东西,我想要执行/等待(不一定等待一个在下一个开始之前完成).它们使用不同的语法,并且在一些细节上有所不同,但是对于我来说,拥有2个功能上具有如此巨大重叠的功能似乎非常不灵活.我错过了什么？
Udi*_*Udi 132
虽然在一般情况下类似("运行并获得许多任务的结果"),但每个函数对于其他情况都有一些特定的功能:
asyncio.gather()返回Future实例,允许高级别的任务分组:
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()甚至取消组中的所有任务all_groups.cancel().另见.gather(..., return_exceptions=True),
asyncio.wait()支持在第一个任务完成后或在指定的超时后等待停止,从而允许更低级别的操作精度:
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()
osp*_*der 33
asyncio.wait比...更低asyncio.gather.
顾名思义,asyncio.gather主要集中在收集结果上.它等待一堆期货并按给定的顺序返回结果.
asyncio.wait等待期货.而不是直接给你结果,它提供完成和待处理的任务.你必须手工收集价值观.
此外,您可以指定等待所有期货完成或仅等待第一个期货wait.
Dev*_*wal 24
一个很容易被忽略的非常重要的区别是这两个函数的默认行为，当涉及到异常时。
我将使用这个例子来模拟一个有时会引发异常的协程 -
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，索引之后的 coros1永远不会执行。
但await asyncio.wait(coros)继续执行任务，即使其中一些失败 -
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，这两种行为都可以通过使用 -
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await单独使用它们。（日志中的堆栈跟踪只是消息，无法捕获！）
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，要捕获异常asyncio.gather()，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))
（与之前相同的输出）
小智 11
我还注意到你可以通过简单地指定列表在wait()中提供一组协同程序:
result=loop.run_until_complete(asyncio.wait([
say('first hello', 2),
say('second hello', 1),
say('third hello', 4)
]))
而只需指定多个协程,就可以在gather()中进行分组:
result=loop.run_until_complete(asyncio.gather(
say('first hello', 2),
say('second hello', 1),
say('third hello', 4)
))
除了之前的所有答案之外，我还想谈谈 的不同行为gather()以及wait()它们被取消的情况。
如果gather()被取消，所有提交的等待（尚未完成）也被取消。
如果wait()任务被取消，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
有时需要结合wait()和gather()功能。例如，我们要等待至少一个任务完成并在此之后取消其余的挂起任务，如果waiting本身被取消，那么也取消所有挂起的任务。
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。并且我们要等待工作任务的结果，但是如果连接丢失，则取消它。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