Event Loop and asyncio.run

What is the event loop?

The event loop is the “engine” that:

• runs coroutines
• pauses them at await points
• resumes them when I/O is ready

asyncio.run

asyncio.run(coro):

• creates an event loop
• runs the coroutine until complete
• closes the loop

import asyncio
 
 
async def main():
    await asyncio.sleep(0.1)
    return 123
 
 
print(asyncio.run(main()))

Running multiple coroutines

To run multiple coroutines concurrently, you’ll use:

• tasks
• gather

(covered in next pages)

Common mistake in notebooks

Some notebook environments already have an event loop running.

If asyncio.run fails there, use notebook-safe patterns.

In normal Python scripts, prefer asyncio.run.

Visualize it

asyncio.run starts the event loop. It runs one coroutine until that coroutine hits an await, at which point the coroutine suspends and hands control back to the loop, which picks the next ready task. Nothing runs in parallel — the loop just never sits idle waiting:

  flowchart LR
  A["asyncio.run(main)"] --> B["Event loop"]
  B --> C["Run a coroutine"]
  C -->|hits await| D["Suspend & save state"]
  D --> B
  C -->|returns| E["Task done"]

Here the loop juggles three coroutines: one runs, hits await, yields, and the loop moves on to the next:

🧪 Try It Yourself

Exercise 1 – asyncio.run() Entry Point

Press Run to see what your code prints.

Exercise 2 – Get the Running Loop

Press Run to see what your code prints.

Exercise 3 – Schedule and Await

Press Run to see what your code prints.