Python asyncio Exception Groups in TaskGroup

TaskGroup combines related failures into ExceptionGroup or BaseExceptionGroup after cancelling and waiting for siblings.

Understanding the Concept

TaskGroup combines related failures into ExceptionGroup or BaseExceptionGroup after cancelling and waiting for siblings. Handle selected exception types with except*.

Task cancellation and grouped errors have structured semantics; avoid suppressing unrelated failures.

How the event loop makes progress

When a coroutine reaches await, it may suspend so another ready Task can execute. This scheduling is cooperative: blocking operations that do not yield hold up every coroutine sharing the event loop. Assign ownership of background tasks, put timeouts around operations that may wait, and release resources on cancellation.

An async function does not automatically use several CPU cores. Use async-compatible I/O for waiting tasks and executors or separate processes for blocking or CPU-heavy work when appropriate.

Worked Example

Read the code from the first line to the last. Identify which values are inputs, which operation is being demonstrated and what should happen before running it.

import asyncio

async def fail():
    raise ValueError("invalid task input")

async def main():
    try:
        async with asyncio.TaskGroup() as group:
            group.create_task(fail())
    except* ValueError as group_error:
        print(len(group_error.exceptions))

asyncio.run(main())

Walkthrough: What Each Part Does

• The imports make asyncio available. These are standard-library building blocks unless the example explicitly names an external tool.
• The definition fail, main creates reusable behavior. Its body runs when called, awaited or instantiated; defining it alone does not run the body of an ordinary function.
• The with or async with scope controls lifetime. Setup takes place on entry, and cleanup runs on exit even when an exception interrupts the body.
• The coordination call waits for a result or a completion condition. Predict what would happen if the worker failed or never became ready.
• The top-level entry point starts event-loop execution and prints a result after the awaited operation is complete.

This demonstration focuses on one practical behavior. Before changing several lines, vary one input or one configuration choice and check whether the result changes for the reason you expected.

Example Output

1

The output above was captured by running the example in the available Python environment. The order of concurrent events, durations and platform-dependent values can legitimately differ on your computer.

Important Behavior and Edge Cases

Using ordinary except ValueError around TaskGroup and missing its grouped exception contract.

Task cancellation and grouped errors have structured semantics; avoid suppressing unrelated failures.

Practice Exercise

Schedule a second child that fails with TypeError and use separate except* clauses.

Write down the result you expect before running your changed code. Check at least one normal case and one boundary or failure case. If your example starts workers or opens resources, verify how they are cleaned up.

Further reading: official documentation and API reference.

Common Beginner Mistakes

• Using ordinary except ValueError around TaskGroup and missing its grouped exception contract.
• Calling a blocking function inside the event loop, or creating tasks without awaiting, cancelling or owning them.

Key Takeaways

• TaskGroup combines related failures into ExceptionGroup or BaseExceptionGroup after cancelling and waiting for siblings.
• Task cancellation and grouped errors have structured semantics; avoid suppressing unrelated failures.
• Practice: Schedule a second child that fails with TypeError and use separate except* clauses.