Python print doesn't show in async function

Question

My code doesn't output any text, and then terminates after 5 seconds. Doing so in a non-async method does show output, but blocks other processes from running. Any thoughts and/or suggestions?

import asyncio, os, sys, time

def update(text):
    # Area of interest
    global print
    print("", flush=True)
    os.system("clear")
    print(text, flush=True)
    print("What is your selection? ", flush=True)

async def main():
    a = "a"
    while True:
        update(a)
        a += "c"
        time.sleep(0.05)
loop = asyncio.get_event_loop()
loop.create_task(main())
time.sleep(5)

Take a look how async coroutines should be defined (offic. docs) — RomanPerekhrest, Mar 14 '23 at 18:33
An `async` function needs to be awaited, otherwise it doesn’t do anything. — deceze, Mar 14 '23 at 18:33
@JohnGordon, `global print` was my attempt at fixing this problem, and @deceze, how do I await an async function in the global scope? — py660, Mar 14 '23 at 18:35
Async isn’t multi-threading. The async function does not execute in parallel “in the background”, as you seem to think. — deceze, Mar 14 '23 at 18:35
`main` is async, which means *other* asynchronous could be "interleaved" with `main`, but main itself *blocks* any other function from being schedule until it is complete. You would want to use `asyncio.wait` instead of `time.sleep` at the very least. — chepner, Mar 14 '23 at 18:47
Replace `loop.create_task(main())` with `asyncio.run(main())`. — chepner, Mar 14 '23 at 18:51

score 1 · Accepted Answer · answered Mar 14 '23 at 19:00

your code in main itself never runs - you don´t call the event loop:

while asyncio.create_task will create a task and get it running "in the background" when you are already running asynchronous code, that never happens in this example:

Your modulelevel code gets a reference to an event loop, with get_event_loop, but uses create_task(main()) - that will create a task to run the main function, but you then never switch control to the asyncio event loop itself.

A more proper way isto have your synchronous code define things up to the point everything is ready, and then pass the control to the event loop so that it executes a "parent" task that may or not spawn others.

In recent Python versions, this is simply done with asyncio.run() - with no need to get an explicit reference to the loop prior to that. In older Python one would have to call get_event_loop() as you do, and would usually call loop.run_until_complete(main()).

There are other strange things in your code - including the use of os.system just to clear the screen - just print the ANSI sequence "\x1b[2J" on Unixes, or print("\n" * 70)` in any system.

The other thing is that you should use await asyncio.sleep(...) when using asyncio code, and never time.sleep

import asyncio
...

async def main():
    a = "a"
    while True:
        update(a)
        a += "c"
        await asyncio.sleep(0.05)

asyncio.run(main())

Python print doesn't show in async function

1 Answers1