How to use async/await in Python 3.5?

Question

#!/usr/bin/env python3
# -*- coding: utf-8 -*-
import time

async def foo():
  await time.sleep(1)

foo()

I couldn't make this dead simple example to run:

RuntimeWarning: coroutine 'foo' was never awaited foo()

Answer 1

Running coroutines requires an event loop. Use the asyncio() library to create one:

import asyncio

# Python 3.7+
asyncio.run(foo())

or

# Python 3.6 and older
loop = asyncio.get_event_loop()
loop.run_until_complete(foo())

Also see the Tasks and Coroutines chapter of the asyncio documentation. If you already have a loop running, you'd want to run additional coroutines concurrently by creating a task (asyncio.create_task(...) in Python 3.7+, asyncio.ensure_future(...) in older versions).

Note however that time.sleep() is not an awaitable object. It returns None so you get an exception after 1 second:

>>> asyncio.run(foo())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/.../lib/python3.7/asyncio/runners.py", line 43, in run
    return loop.run_until_complete(main)
  File "/.../lib/python3.7/asyncio/base_events.py", line 573, in run_until_complete
    return future.result()
  File "<stdin>", line 2, in foo
TypeError: object NoneType can't be used in 'await' expression

In this case you should use the asyncio.sleep() coroutine instead:

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

which is cooperates with the loop to enable other tasks to run. For blocking code from third-party libraries that do not have asyncio equivalents, you could run that code in an executor pool. See Running Blocking Code in the asyncio development guide.

Answer 2

If you already have a loop running (with some other tasks), you can add new tasks with:

asyncio.ensure_future(foo())

otherwise you might get

The event loop is already running

error.