Python asyncio.TaskGroup: Structured Concurrency

TaskGroup is structured concurrency: related tasks finish before the async with block exits, and a child failure cancels remaining tasks and groups errors.

Understanding the Concept

TaskGroup is structured concurrency: related tasks finish before the async with block exits, and a child failure cancels remaining tasks and groups errors.

TaskGroup cancellation behavior is more structured than default gather behavior.

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 double(value):
    await asyncio.sleep(0)
    return value * 2

async def main():
    async with asyncio.TaskGroup() as group:
        first = group.create_task(double(3))
        second = group.create_task(double(5))
    print(first.result(), second.result())

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 double, 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.
• Each await is a suspension point. When the awaited operation is not ready, the loop can schedule other tasks instead of blocking the entire thread.
• The coordination call waits for a result or a completion condition. Predict what would happen if the worker failed or never became ready.

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

6 10

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 TaskGroup but leaving unrelated background tasks outside its ownership without managing them.

TaskGroup cancellation behavior is more structured than default gather behavior.

Practice Exercise

Introduce a failing child and catch the resulting ExceptionGroup with except*.

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 TaskGroup but leaving unrelated background tasks outside its ownership without managing them.
• Calling a blocking function inside the event loop, or creating tasks without awaiting, cancelling or owning them.

Key Takeaways

• TaskGroup is structured concurrency: related tasks finish before the async with block exits, and a child failure cancels remaining tasks and groups errors.
• TaskGroup cancellation behavior is more structured than default gather behavior.
• Practice: Introduce a failing child and catch the resulting ExceptionGroup with except*.