Python 3.7 – asyncio.sleep() and time.sleep()

When I go to the asyncio page, the first example is a hello world program. When I run it on python 3.73, I can’t see any different from the normal one, can anyone tell me the difference and give a non-trivial example?

In [1]: import asyncio
   ...:
   ...: async def main():
   ...:     print('Hello ...')
   ...:     await asyncio.sleep(5)
   ...:     print('... World!')
   ...:
   ...: # Python 3.7+
   ...: asyncio.run(main())
Hello ...
... World!

In [2]:

In [2]: import time
   ...:
   ...: def main():
   ...:     print('Hello ...')
   ...:     time.sleep(5)
   ...:     print('... World!')
   ...:
   ...: # Python 3.7+
   ...: main()
Hello ...
... World!

I intentionally increase the time from 1s to 5s, hope to see something special but I didn’t.

Answers:

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Method 1

You aren’t seeing anything special because there’s nothing much asynchronous work in your code. However, the main difference is that time.sleep(5) is blocking, and asyncio.sleep(5) is non-blocking.

When time.sleep(5) is called, it will block the entire execution of the script and it will be put on hold, just frozen, doing nothing. But when you call await asyncio.sleep(5), it will ask the event loop to run something else while your await statement finishes its execution.

Here’s an improved example.

import asyncio

async def hello():
    print('Hello ...')
    await asyncio.sleep(5)
    print('... World!')

async def main():
    await asyncio.gather(hello(), hello())

asyncio.run(main())

Will output:

~$ python3.7 async.py
Hello ...
Hello ...
... World!
... World!

You can see that await asyncio.sleep(5) is not blocking the execution of the script.

Hope it helps 🙂

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