Python asyncio.gather: a sibling can continue after the first failure

Operation contract

The failing receipt job raises immediately. Its sibling is already waiting on a gate. After gather raises, the caller checks that the sibling task is still pending, then opens the gate and awaits it. This makes the lifetime visible rather than relying on a scheduling sleep. The caller owns cleanup of a task it intentionally allowed to continue.

Failure and ownership boundary

A cancelled gather future and a child cancellation follow different rules, and another child may also fail later. TaskGroup applies a different sibling-cancellation contract. Choose one explicitly before starting work with side effects. Python asyncio TaskGroup: cancel sibling work and retain failure evidence, Python asyncio timeout: cancellation and cleanup ownership and Python cancellation review: release a permit before propagating task cancellation provide the comparison.

Working program

import asyncio

async def reject_receipt():
    raise ValueError("invalid receipt")

async def delayed_receipt(gate):
    await gate.wait()
    return "R42 stored"

async def main():
    gate = asyncio.Event()
    sibling = asyncio.create_task(delayed_receipt(gate))
    try:
        await asyncio.gather(reject_receipt(), sibling)
    except ValueError:
        print("sibling pending:", not sibling.done())
    gate.set()
    print("sibling result:", await sibling)

asyncio.run(main())

Output

sibling pending: True
sibling result: R42 stored

Costs and limits

Task creation and scheduling add runtime work; the fixture runs two tasks. A real batch needs a bound on in-flight tasks and explicit handling of every task result or failure. A continuing sibling may already have side effects when the caller sees the first error.

Common Mistakes

• Do not assume gather cancels every sibling on the first ordinary exception.
• Retain and observe tasks that continue after an early failure.

Connected lessons

Python asyncio TaskGroup: cancel sibling work and retain failure evidence, Python asyncio timeout: cancellation and cleanup ownership, Python cancellation review: release a permit before propagating task cancellation.

Continue with Python eager asyncio tasks: a coroutine can finish before create_task returns.