Naughty coroutines in Python

naughty.md

Naughty coroutines

Unexpected behavior when working with asyncio in Python. The behavior is likely intended, but intuitive.

import asyncio


async def fine_job(i: int):
    # A fine job that is well behaved without exceptions!
    await asyncio.sleep(3)
    print(f"Fine job {i} completed.")


async def run_and_try():
    await asyncio.sleep(0.33)

    print("Starting loop. Numbers 1, 2, 3, 4 and 5:")
    for i in range(1, 6):
        try:
            await fine_job(i)
        except:
            print(f"Exception caught! {i}")


async def naughty_job(sleep: float):
    # A naughty job with errors...
    await asyncio.sleep(sleep)
    raise ValueError("Bad!")


async def main():
    # If naughty wait is zero (less than 0.33), the program crashes immediately
    # NAUGHTY_WAIT: float = 0

    # If it's 1 (higher than 0.33), run_and_try enters its try-except, and will
    # catch the raised error! One fine_job won't be completed. But the rest will.
    NAUGHTY_WAIT: float = 1

    await asyncio.gather(
        naughty_job(NAUGHTY_WAIT),
        run_and_try(),
    )

    # The "Done is never reached..."
    print("Done.")


asyncio.run(main())