asyncio's call_later raises 'generator' object is not callable with coroutine object

Question

I have some simple code code made with Python 3.4's asyncio using call_later. The code should print, waits 10 seconds, and then print again (but instead raises TypeError when end() should be excecuted, see below):

import asyncio

@asyncio.coroutine
def begin():
    print("Starting to wait.")
    asyncio.get_event_loop().call_later(10, end())

@asyncio.coroutine
def end():
    print("completed")

if __name__ == "__main__":
    try:
        loop = asyncio.get_event_loop()
        loop.create_task(begin())
        loop.run_forever()
    except KeyboardInterrupt:
        print("Goodbye!")

Gives the error:

Exception in callback <generator object coro at 0x7fc88eeaddc8>()
handle: <TimerHandle when=31677.188005054 <generator object coro at 0x7fc88eeaddc8>()>
Traceback (most recent call last):
  File "/usr/lib64/python3.4/asyncio/events.py", line 119, in _run
    self._callback(*self._args)
TypeError: 'generator' object is not callable

From what I can tell from the docs (https://docs.python.org/3/library/asyncio-task.html#coroutine), call_later takes a coroutine object, which is obtained by calling a coroutine function. This appears to be what I've done, but asyncio does not call end() properly.

How is this supposed to be done?

Answer 1

call_later is designed to take a callback (meaning a regular function object), not a coroutine. Newer versions of Python will actually say this explicitly:

Starting to wait.
Task exception was never retrieved
future: <Task finished coro=<coro() done, defined at /usr/lib/python3.4/asyncio/coroutines.py:139> exception=TypeError('coroutines cannot be used with call_at()',)>
Traceback (most recent call last):
  File "/usr/lib/python3.4/asyncio/tasks.py", line 238, in _step
    result = next(coro)
  File "/usr/lib/python3.4/asyncio/coroutines.py", line 141, in coro
    res = func(*args, **kw)
  File "aio.py", line 6, in begin
    asyncio.get_event_loop().call_later(10, end())
  File "/usr/lib/python3.4/asyncio/base_events.py", line 392, in call_later
    timer = self.call_at(self.time() + delay, callback, *args)
  File "/usr/lib/python3.4/asyncio/base_events.py", line 404, in call_at
    raise TypeError("coroutines cannot be used with call_at()")
TypeError: coroutines cannot be used with call_at()

To make your code work, end needs to be a regular function, which you then pass to call_later:

import asyncio

@asyncio.coroutine
def begin():
    print("Starting to wait.")
    asyncio.get_event_loop().call_later(10, end)

def end():
    print("completed")

if __name__ == "__main__":
    try:
        loop = asyncio.get_event_loop()
        loop.create_task(begin())
        loop.run_forever()
    except KeyboardInterrupt:
        print("Goodbye!")

Output:

Starting to wait.
completed
Goodbye!

If end needs to be a coroutine, a more natural way to call it after a delay would be to use asyncio.sleep:

import asyncio

@asyncio.coroutine
def begin():
    print("Starting to wait.")
    yield from asyncio.sleep(10)
    yield from end()

@asyncio.coroutine
def end():
    print("completed")

if __name__ == "__main__":
    try:
        loop = asyncio.get_event_loop()
        loop.create_task(begin())
        loop.run_forever()
    except KeyboardInterrupt:
        print("Goodbye!")

Though technically, this does work:

asyncio.get_event_loop().call_later(10, lambda: asyncio.async(end()))