Why python3.6 could yield from a coroutine object in a non-coroutine generator?

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zz.py

import asyncio

# @asyncio.coroutine
def main():
    yield from asyncio.sleep(1)

next(main())

  • Execute in Python 3.6.7 --> OK, no error.

  • Execute in Python 3.7.3 --> NOT OK, reported next:

      zz.py:5: RuntimeWarning: coroutine 'sleep' was never awaited
    yield from asyncio.sleep(1)
  RuntimeWarning: Enable tracemalloc to get the object allocation traceback
  Traceback (most recent call last):
    File "zz.py", line 7, in <module>
      next(main())
    File "zz.py", line 5, in main
      yield from asyncio.sleep(1)
  TypeError: cannot 'yield from' a coroutine object in a non-coroutine generator

I know I could enable @asyncio.coroutine to make both work, or async/await.

But, here my question is: if I still use generator, what's the difference here for python3.6 & python3.7? I really feel chaos here.

python-asyncio python-3.x python
1 Answers

I found the root cause after dig into the python source code:

  • In python3.6, although async/await already there, coroutine.sleep still use generator based coroutine:

    @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()

  • In python3.7, coroutine.sleep starts to switch to native coroutine:

    async def sleep(delay, result=None, *, loop=None):
    """Coroutine that completes after a given time (in seconds)."""
    if delay <= 0:
        await __sleep0()
        return result

    if loop is None:
        loop = events.get_event_loop()
    future = loop.create_future()
    h = loop.call_later(delay,
                        futures._set_result_unless_cancelled,
                        future, result)
    try:
        return await future
    finally:
        h.cancel()

So, when python3.6 execute yield from asyncio.sleep(1), it in fact did not yield from a native coroutine, surely not have error.

user_63947220
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