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.