asyncio.run

Learn the event loop with asyncio.run.

asyncio.run

asyncio.run runs a coroutine from the top level. It opens a new event loop, awaits the coroutine and closes the loop.

import asyncio

async def main():
    return 42

asyncio.run(main())  # 42

asyncio.run is called once in a program; typically it runs the main() coroutine. Inside the same loop, other coroutines are run with await.

Python 3.7+ standardized this; it replaces the old loop.run_until_complete pattern.

Event loop lifecycle

asyncio.run creates an event loop behind the scenes. The loop runs pending coroutines in turn; when one hits an await point, the loop moves to others.

When the program ends, the loop is closed; remaining resources are cleaned up. Calling asyncio.run multiple times is impractical; better to organize work inside a single main coroutine.

You cannot nest asyncio.run; trying to open a new loop while one is already running raises an error.

Exercise Run main with asyncio.run

Start with import asyncio. The function async def main() returns 42. Add the call print(asyncio.run(main())).

Show example solution

import asyncio

async def main():
    return 42

print(asyncio.run(main()))

Test yourself

asyncio.run call

import asyncio

async def main():
    return 42

print(____(main()))

Show answer

import asyncio

async def main():
    return 42

print(asyncio.run(main()))

Match the asyncio.run pieces

asyncio.run(main())

Top-level call
main()

Coroutine object
event loop

Running loop
await

Waiting point
How often is asyncio.run called in a program?
- Usually once, at the top
- Once per coroutine
- Unlimited, nested
- Never, only await is used
Show answer

Usually once, at the top — asyncio.run is usually called once at the top for main().

Next lesson: async def

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