Understanding Asyncio Event Loops in Python

Mar 25, 2024 ยท 3 min read

The event loop is the core of asyncio in Python. It handles executing asynchronous code and callbacks when futures, tasks, etc complete. Getting, creating, and managing the event loop properly is key to writing efficient asyncio programs.

What is an Event Loop?

An event loop essentially runs a program's asynchronous code and callbacks. It:

  • Tracks pending async operations and callbacks
  • Executes callbacks when futures/tasks finish
  • Decides what to run next
  • The event loop allows asyncio to execute code concurrently while it waits on long running operations like network requests. This is more efficient than traditional synchronous code.

    Here's a simple event loop example:

    import asyncio
    async def my_async_func():
        print('hello from async function')
    loop = asyncio.get_event_loop()

    This executes my_async_func asynchronously via the event loop.

    Getting the Event Loop

    There are a few ways to get the event loop:

    loop = asyncio.get_event_loop() # new event loop
    loop = asyncio.get_running_loop() # current loop
    loop = asyncio.new_event_loop() # new event loop
  • asyncio.get_event_loop() creates a new event loop to run asynchronous tasks and callbacks. This is the most common way.
  • asyncio.get_running_loop() returns the current running event loop inside a coroutine. Useful to get a reference to the current loop.
  • asyncio.new_event_loop() creates a brand new event loop instance. Rarely needed explicitly.
  • Best practice is to keep a reference to the loop instead of calling get_event_loop() everywhere. For example:

    loop = asyncio.get_event_loop()
    async def main():
       print(asyncio.get_running_loop() is loop) # True

    Reusing the loop allows proper cleanup and resource management.

    Running the Event Loop

    Once you have a loop, you need to run it to execute coroutines and callbacks:

  • loop.run_until_complete() runs until the passed coroutine finishes
  • loop.run_forever() runs perpetually until loop.stop() is called
  • For example:

    async def main():
    loop = asyncio.get_event_loop()
    loop.run_until_complete(main()) # runs main()
    loop.close() # cleanup

    This runs main() until completion before closing the loop.

    Cleaning Up After Using the Event Loop

    It's important to properly clean up after the event loop when done:

  • loop.stop() stops the perpetual execution of loop.run_forever()
  • loop.close() cleans up resources and handlers used by loop.
  • Failing to close event loops can lead to issues like resource warnings or pending callbacks/tasks not running.

    So in summary, best practice is:

    1. Get an event loop
    2. Run coroutines/futures with run_until_complete() or run_forever()
    3. Call loop.stop() to halt loop execution
    4. Clean up with loop.close()

    Event Loop Gotchas

    There are some common event loop pitfalls:

  • Forgetting to close the loop, leading to resource issues
  • Creating multiple event loops instead of reusing one
  • Calling run_forever() without loop.stop(), causing the program to hang
  • So be mindful that the event loop runs perpetually once started via run_forever(). Also remember to clean up properly after use.


    The event loop is the secret sauce making asyncio concurrent and efficient. Getting, running, and closing it properly ensures programs work smoothly without issues. Following best practices around creating and reusing the event loop prevents tricky bugs. With a solid grasp of event loops, leveraging asyncio for writing asynchronous Python becomes much simpler.

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