异常“线程 'MainThread' 中没有当前事件循环”，而新循环 运行

Question

这是简单的测试代码和结果。

import asyncio

async def test():
    await asyncio.sleep(1)

if __name__ == '__main__':

    asyncio.set_event_loop(None)      # Clear the main loop.
    loop = asyncio.new_event_loop()   # Create a new loop.
    loop.run_until_complete(test())   # Run coroutine over the new loop

---

Traceback (most recent call last):
  File "test_test.py", line 11, in <module>
    loop.run_until_complete(test())
  File "/usr/lib/python3.5/asyncio/base_events.py", line 387, in run_until_complete
    return future.result()
  File "/usr/lib/python3.5/asyncio/futures.py", line 274, in result
    raise self._exception
  File "/usr/lib/python3.5/asyncio/tasks.py", line 239, in _step
    result = coro.send(None)
  File "test_test.py", line 5, in test
    await asyncio.sleep(1)
  File "/usr/lib/python3.5/asyncio/tasks.py", line 510, in sleep
    loop = events.get_event_loop()
  File "/usr/lib/python3.5/asyncio/events.py", line 632, in get_event_loop
    return get_event_loop_policy().get_event_loop()
  File "/usr/lib/python3.5/asyncio/events.py", line 578, in get_event_loop
    % threading.current_thread().name)
RuntimeError: There is no current event loop in thread 'MainThread'.

我 运行 async def test() 在 new loop 之上，并期望由 test() 嵌套的 asyncio.sleep(1) 也使用 new loop。

与此相反，sleep() 似乎仍然可以访问 main loop 我设置为 None。

我知道我可以在调用 run_until_complete() 之前用 asyncio.set_event_loop(loop) 将 main loop 重新设置为 new loop 并且它将毫无例外地工作。

但是，我想知道 asyncio 必须设置 main loop 并且用于协程是正常的，而不管协程是 运行 的循环。

Answer 1

I want to know it is normal for asyncio that main loop Must be set and is used for coroutines regardless of a loop over which coroutine is run.

这在 Python 3.6 之前是必需的。原因是像asyncio.sleep()这样的函数需要一个事件循环才能使用loop.call_later()来安排唤醒调用来完成未来。

从 Python 3.6（或 3.5.3，其中包含对 the issue 的修复）开始，当从事件循环驱动的协程调用 get_event_loop() 时，它总是returns 驱动它的事件循环。因此，您的代码可以正常工作。

联机文档中未提及新行为，但在 the docstring:

When called from a coroutine or a callback (e.g. scheduled with call_soon or similar API), this function will always return the running event loop.