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[英]Asyncio.create_subprocess_exec NotImplementedError - Fastapi Background Task
[英]Why await doesn't wait asyncio.create_subprocess_exec()
我正在根據教程編寫協程以在python中執行shell命令。 基本內容:
import asyncio
async def async_procedure():
process = await asyncio.create_subprocess_exec('ping', '-c', '2', 'google.com')
await process.wait()
print('async procedure done.')
loop = asyncio.get_event_loop()
loop.run_until_complete(async_procedure())
loop.close()
上面的這段代碼完美地工作了。 結果如下:
PING google.com (...) 56(84) bytes of data.
64 bytes from ...: icmp_seq=1 ttl=46 time=34.8 ms
64 bytes from ...: icmp_seq=2 ttl=46 time=34.5 ms
--- google.com ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 3004ms
rtt min/avg/max/mdev = 33.771/34.437/34.881/0.407 ms
Process done!
當我嘗試刪除process.wait()時:
async def async_procedure():
await asyncio.create_subprocess_exec('ping', '-c', '2', 'google.com')
print('async procedure done.')
該腳本無法正常工作:
Process done! # This line should be lastest line
PING google.com (...) 56(84) bytes of data.
64 bytes from ...: icmp_seq=1 ttl=46 time=21.1 ms
64 bytes from ...: icmp_seq=2 ttl=46 time=21.8 ms
--- google.com ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1001ms
rtt min/avg/max/mdev = 21.135/21.469/21.803/0.334 ms
但是在一個非常相似的示例中沒有問題:
async def async_procedure():
await asyncio.sleep(2)
print('async procedure done')
docs( https://docs.python.org/3/library/asyncio-task.html#coroutine )說:
結果=等待未來或結果=來自未來的收益– 暫停協程直到完成未來,然后返回未來的結果,或引發異常,該異常將被傳播。 (如果取消了future,則會引發CancelledError異常。)請注意,任務就是future,有關future的所有說明也適用於任務。
結果=等待協程或結果=從協程產生– 等待另一個協程產生結果(或引發異常,該異常將被傳播)。 協程表達式必須是對另一個協程的調用。
return expression –使用await或yield from將結果生成到等待此協程的協程。
引發異常-在協程中使用await或yield from在協程中引發異常。
這是asyncio.create_subprocess_exec()和asyncio.sleep()的源代碼協程。 它們都是協程:
@coroutine
def create_subprocess_exec(program, *args, stdin=None, stdout=None,
stderr=None, loop=None,
limit=streams._DEFAULT_LIMIT, **kwds):
if loop is None:
loop = events.get_event_loop()
protocol_factory = lambda: SubprocessStreamProtocol(limit=limit,
loop=loop)
transport, protocol = yield from loop.subprocess_exec(
protocol_factory,
program, *args,
stdin=stdin, stdout=stdout,
stderr=stderr, **kwds)
return Process(transport, protocol, loop)
@coroutine
def sleep(delay, result=None, *, loop=None):
"""Coroutine that completes after a given time (in seconds)."""
if delay == 0:
yield
return result
if loop is None:
loop = events.get_event_loop()
future = loop.create_future()
h = future._loop.call_later(delay,
futures._set_result_unless_cancelled,
future, result)
try:
return (yield from future)
finally:
h.cancel()
您等待該過程開始。 您沒有等待它完成。 await process.wait()
等待它完成。
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