[英]Shutdown infinite async generator
我試圖在此處的在線REPL中重現該錯誤。 但是,它與我的實際代碼(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()
實現的方式(以及行為)並不完全相同。
我在某個地方定義了這樣的協程:
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是無限的(或可能非常持久)。 我從這樣的示例代碼中使用它:
async def print_altitude():
async for position in drone.telemetry.position():
print(f"Altitude: {position.relative_altitude_m}")
和print_altitude()
在循環上運行:
asyncio.ensure_future(print_altitude())
asyncio.get_event_loop().run_forever()
那很好。 現在,在某個時候,我想關閉來自調用方的流。 我以為我可以只運行asyncio.ensure_future(loop.shutdown_asyncgens())
並等待我上面的finally
關閉被調用,但這沒有發生。
相反,我收到有關未檢索到的異常的警告:
Task exception was never retrieved
future: <Task finished coro=<print_altitude() done, defined at [...]
為什么會這樣,又如何使所有異步生成器實際上都關閉(並運行其finally
子句)?
首先,如果您stop
循環,則協程將沒有機會正常關閉。 基本上,調用close
意味着不可逆轉地破壞循環。
如果您不關心這些正在運行的任務會發生什么,只需將其全部cancel
即可,這也會停止異步生成器:
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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