Handling Errors Gracefully with Asyncio Exceptions

Asyncio provides an asynchronous programming framework in Python that allows you to write non-blocking and efficient I/O code. A key part of any application is properly handling errors and exceptions. When working with asyncio, there are some unique exception handling challenges to be aware of.

The Basics

Most exceptions raised in asyncio asynchronous coroutines and tasks can be handled normally using try/except blocks:

import asyncio

async def main():
    try:
        result = await some_api_call()
    except ValueError:
        print("Oops, invalid input")

asyncio.run(main())

This will catch a ValueError in the coroutine and handle it.

CancelledError

One special exception to watch out for is asyncio.CancelledError. This gets raised if a task or coroutine is cancelled:

import asyncio

async def main():
    task = asyncio.create_task(other_func())
    
    await asyncio.sleep(1)
    
    task.cancel()
    try:
        await task
    except asyncio.CancelledError:
        print("Task was cancelled")

asyncio.run(main()) 

So make sure to catch CancelledError if you anticipate tasks being cancelled.

Propagating Exceptions

Another common pitfall is exceptions not propagating back properly from tasks. Use asyncio.create_task(coro()) instead of asyncio.ensure_future(coro()) and await any spawned tasks to avoid this issue.

Overall, handling exceptions with asyncio does take some special care. Set up try/except blocks for expected errors, watch out for CancelledError, and properly await spawned tasks. With these tips, you can gracefully handle errors in your asynchronous code!