[英]Shutdown infinite async generator
我试图在此处的在线REPL中重现该错误。 但是,它与我的实际代码(在async for response in position_stream()
,我async for response in position_stream()
而不是在REPL for position in count()
中的async for response in position_stream()
实现的方式(以及行为)并不完全相同。
我在某个地方定义了这样的协程:
async def position(self):
request = telemetry_pb2.SubscribePositionRequest()
position_stream = self._stub.SubscribePosition(request)
try:
async for response in position_stream:
yield Position.translate_from_rpc(response)
finally:
position_stream.cancel()
其中position_stream是无限的(或可能非常持久)。 我从这样的示例代码中使用它:
async def print_altitude():
async for position in drone.telemetry.position():
print(f"Altitude: {position.relative_altitude_m}")
和print_altitude()
在循环上运行:
asyncio.ensure_future(print_altitude())
asyncio.get_event_loop().run_forever()
那很好。 现在,在某个时候,我想关闭来自调用方的流。 我以为我可以只运行asyncio.ensure_future(loop.shutdown_asyncgens())
并等待我上面的finally
关闭被调用,但这没有发生。
相反,我收到有关未检索到的异常的警告:
Task exception was never retrieved
future: <Task finished coro=<print_altitude() done, defined at [...]
为什么会这样,又如何使所有异步生成器实际上都关闭(并运行其finally
子句)?
首先,如果您stop
循环,则协程将没有机会正常关闭。 基本上,调用close
意味着不可逆转地破坏循环。
如果您不关心这些正在运行的任务会发生什么,只需将其全部cancel
即可,这也会停止异步生成器:
import asyncio
from contextlib import suppress
async def position_stream():
while True:
await asyncio.sleep(1)
yield 0
async def print_position():
async for position in position_stream():
print(f'position: {position}')
async def cleanup_awaiter():
await asyncio.sleep(3)
print('cleanup!')
if __name__ == '__main__':
loop = asyncio.get_event_loop()
try:
asyncio.ensure_future(print_position())
asyncio.ensure_future(print_position())
loop.run_until_complete(cleanup_awaiter())
# get all running tasks:
tasks = asyncio.gather(*asyncio.Task.all_tasks())
# schedule throwing CancelledError into the them:
tasks.cancel()
# allow them to process the exception and be cancelled:
with suppress(asyncio.CancelledError):
loop.run_until_complete(tasks)
finally:
print('closing loop')
loop.close()
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