Asyncio Cancel Scope

A utility for cancelling asyncio task groups in the absence of one from the standard library.

Installation

pip install asyncio_cancel_scope

Usage

The cancel_scope function allows you to establish a task group that can be cancelled with the cancel_group function. This is useful for managing multiple tasks that should be stopped together.

import asyncio
from asyncio_cancel_scope import cancel_scope, cancel_group

async def main():
    async with cancel_scope(asyncio.TaskGroup()) as tg:
        tg.create_task(asyncio.sleep(1))
        tg.create_task(asyncio.sleep(2))
        tg.create_task(asyncio.sleep(3))
        tg.create_task(asyncio.sleep(4))
        cancel_group(tg)  # cancels all tasks in the group and exits without an exception

asyncio.run(main())

You can link cancel scopes so that parent task groups are cancelled when inner task groups are.

import asyncio
from asyncio_cancel_scope import cancel_scope, cancel_group

async def outer():
    forever = asyncio.Event()
    async with cancel_scope(asyncio.TaskGroup()) as outer_tg:
        outer_tg.create_task(forever.wait())
        outer_tg.create_task(inner(outer_tg))

async def inner(parent_tg):
    async with cancel_scope(asyncio.TaskGroup(), parent_tg) as inner_tg:
        cancel_group(inner_tg)

asyncio.run(outer())

Alternatives

Without this you'd need to manually cancel each task in the group, which can be cumbersome and error-prone:

import asyncio


async def main():
    tasks = []
    async with asyncio.TaskGroup() as tg:
        tasks.append(tg.create_task(asyncio.sleep(1)))
        tasks.append(tg.create_task(asyncio.sleep(2)))
        tasks.append(tg.create_task(asyncio.sleep(3)))
        tasks.append(tg.create_task(asyncio.sleep(4)))
        for task in tasks:
            task.cancel()  # manually cancel each task

Under the Hood

The cancel_group function adds a task to the task group that immediately raises a special exception. When this happens the behavior of asyncio.TaskGroup is to cancel all tasks in the group and propagate the exception. The purpose of cancel_scope is to supress that special exception so that it does not propagate to the caller of cancel_scope.