Future/generator expansion inside asyncio.coroutine

return_await.py

import asyncio
import contextlib


@asyncio.coroutine
def return_coroutine_object():
    @asyncio.coroutine
    def g():
        yield from asyncio.sleep(0.01)
        return "return_coroutine_object"

    return g()


@asyncio.coroutine
def return_future():
    fut = asyncio.Future()
    fut.set_result("return_future")

    return fut


@asyncio.coroutine
def generator_return_future():
    yield from asyncio.sleep(0.01)

    fut = asyncio.Future()
    fut.set_result("generator_return_future")

    return fut


if __name__ == "__main__":
    loop = asyncio.get_event_loop()

    with contextlib.closing(loop):
        print("return_future:",
              loop.run_until_complete(return_future()))

        print("generator_return_future:",
              loop.run_until_complete(generator_return_future()))

        print("return_coroutine_object:",
              loop.run_until_complete(return_coroutine_object()))

run_with_debug.log

$ PYTHONASYNCIODEBUG=X python3 return_await.py
<CoroWrapper return_coroutine_object.<locals>.g() running at return_await.py:7, created at return_await.py:12> was never yielded from
Coroutine object created at (most recent call last):
  File "return_await.py", line 44, in <module>
    loop.run_until_complete(return_coroutine_object()))
  File "/usr/lib/python3.4/asyncio/base_events.py", line 304, in run_until_complete
    self.run_forever()
  File "/usr/lib/python3.4/asyncio/base_events.py", line 276, in run_forever
    self._run_once()
  File "/usr/lib/python3.4/asyncio/base_events.py", line 1164, in _run_once
    handle._run()
  File "/usr/lib/python3.4/asyncio/events.py", line 120, in _run
    self._callback(*self._args)
  File "/usr/lib/python3.4/asyncio/tasks.py", line 238, in _step
    result = next(coro)
  File "/usr/lib/python3.4/asyncio/coroutines.py", line 79, in __next__
    return next(self.gen)
  File "/usr/lib/python3.4/asyncio/coroutines.py", line 141, in coro
    res = func(*args, **kw)
  File "return_await.py", line 12, in return_coroutine_object
    return g()
return_future: return_future
generator_return_future: <Future finished result='generator_return_future' created at return_await.py:27>
return_coroutine_object: <CoroWrapper return_coroutine_object.<locals>.g() running at return_await.py:7, created at return_await.py:12>
$

run_without_debug.log

$ python3 return_await.py
return_future: return_future
generator_return_future: <Future finished result='generator_return_future'>
return_coroutine_object: return_coroutine_object
$