How to use async/await in Python 3.5?

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

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


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

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

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

    import asyncio

    # Python 3.7+


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

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:

    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "/.../lib/python3.7/asyncio/", line 43, in run
        return loop.run_until_complete(main)
      File "/.../lib/python3.7/asyncio/", 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.