Description

pablogsal

opened on May 9, 2025

Issue body actions

In PR #128768 (gh-128308), the asyncio task factory signature was changed from (loop, coro, context=None) to (loop, coro, **kwargs). This seemingly minor change breaks backward compatibility for existing task factory implementations.

The documentation was updated to reflect this new contract:

• Old: (loop, coro, context=None), where the callable must return a asyncio.Future-compatible object.
• New: (loop, coro, **kwargs), where the callable must pass on all *kwargs, and return a asyncio.Task-compatible object.

This change causes runtime errors for any code with task factories implemented according to the previous contract, as these factories now unexpectedly receive keyword arguments like name that they aren't designed to handle.

Reproducer

import asyncio

# Task factory implemented according to the previous API contract
def old_style_task_factory(loop, coro, context=None):
    print(f"Creating task with loop={loop}, coro={coro}, context={context}")
    return asyncio.Task(coro, loop=loop)

async def main():
    # Set up a custom task factory following the old documented API
    loop = asyncio.get_event_loop()
    loop.set_task_factory(old_style_task_factory)
    
    # This will fail with TypeError due to unexpected 'name' argument
    print("Attempting to create task with name...")
    await loop.create_task(asyncio.sleep(0.1), name="test_task")
    print("Task completed successfully")  # We won't reach this line

if __name__ == "__main__":
    try:
        asyncio.run(main())
    except TypeError as e:
        print(f"ERROR: {e}")
        # Output: ERROR: old_style_task_factory() got an unexpected keyword argument 'name'

This prints:

Attempting to create task with name...
Creating task with loop=<_UnixSelectorEventLoop running=False closed=False debug=False>, coro=<coroutine object BaseEventLoop.shutdown_asyncgens at 0x72b413aa5450>, context=None
Creating task with loop=<_UnixSelectorEventLoop running=False closed=False debug=False>, coro=<coroutine object BaseEventLoop.shutdown_default_executor at 0x72b413cc5910>, context=None
ERROR: old_style_task_factory() got an unexpected keyword argument 'name'
<sys>:0: RuntimeWarning: coroutine 'sleep' was never awaited
RuntimeWarning: Enable tracemalloc to get the object allocation traceback

This is a breaking change introduced in a patch release, which violates our versioning policy. The change was intended to support the name keyword argument for eager tasks, but the implementation method has unintentionally broken existing code that followed the previously documented API contract.

CPython versions tested on:

3.14

Operating systems tested on:

Linux

Linked PRs

• [3.13] gh-133745: Fix asyncio task factory name/context kwarg breaks #133948

Activity

1. pablogsal

   added

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on May 9, 2025
2. pablogsal

   assigned

   kumaraditya303

   and

   Yhg1s

   on May 9, 2025
3. picnixz

   added

   topic-asyncio

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   3.13only security fixesonly security fixes

   on May 9, 2025
4. github-project-automation

   added this to asyncioon May 9, 2025
5. github-project-automation

   moved this to Todo in asyncioon May 9, 2025
6. picnixz

   added

   3.14bugs and security fixesbugs and security fixes

   3.15bugs and security fixesbugs and security fixes

   release-blocker

   on May 9, 2025
7. ezio-melotti

   added this to Release and Deferred blockers 🚫on May 9, 2025
8. github-project-automation

   moved this to Todo in Release and Deferred blockers 🚫on May 9, 2025
9. picnixz commented on May 9, 2025

   

   picnixz

   on May 9, 2025

   Member

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   (I'm marking it as release-blocker just so that we can find the issue more easily)
10. vstinner commented on May 9, 2025

    

    vstinner

    on May 9, 2025

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    I suppose that the following issue in the toga project is related to this:

    • Use the documented prototype for task factories. beeware/toga#3395
    • beeware/toga@bdd2e7b
    • https://bugzilla.redhat.com/show_bug.cgi?id=2363033

    There is already a commit to work around the issue.
11. Yhg1s commented on May 9, 2025

    

    Yhg1s

    on May 9, 2025

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    @asvetlov @kumaraditya303 @graingert for 3.13, do you want to propose anything other than a rollback of the relevant PR (and anything that depends on it)? We'll probably want to do a new release to fix the breakage.

    For 3.14 it may also be worth considering not breaking the documented API (/cc @hugovk), but we have a bit more time to get that sorted.
12. serhiy-storchaka commented on May 9, 2025

    

    serhiy-storchaka

    on May 9, 2025

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    No hurry. 3.13.3 was released a month ago, and only now we found out about that breakage. A few more days is not critical.
13. serhiy-storchaka commented on May 9, 2025

    

    serhiy-storchaka

    on May 9, 2025

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    Other breakage of that documented contract happened in 3.11 (bpo-46994/#91150).

    I encountered a similar problem in #74696. It was needed to pass additional argument to the user registered function. I solved it by testing the optional supports_root_dir attribute of the function.

    We could do the same here. The users which need to pass the name to the constructor (eager tasks) should set a special attribute of the factory. By default, the old method will be used. We should document that this is temporary, and the factory should support arbitrary keyword arguments.
14. kumaraditya303 commented on May 10, 2025

    

    kumaraditya303

    on May 10, 2025

    Contributor

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    I created #133808 to revert the breaking change for 3.13.
15. graingert

    added a sub-issue on May 11, 2025

    • positional name and context are broken eg create_task(coro(), name, context) #133877
16. graingert

    added a commit that references this issue on May 12, 2025

    pythongh-133745: Fix asyncio task factory name/context kwarg breaks

    5c3c953
17. kumaraditya303

    marked create_task(name=...) silently fails to set name with custom task_factory after gh-128308 #132593 as a duplicate of this issue on May 16, 2025
18. gvanrossum

    added a commit that references this issue on May 18, 2025

    [3.13] gh-133745: Fix asyncio task factory name/context kwarg breaks (#…

    fd6a602
19. kumaraditya303 commented on May 21, 2025

    

    kumaraditya303

    on May 21, 2025

    Contributor

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    Fixed by #133948

    Reacted by Alice
20. kumaraditya303

    closed this as completedon May 21, 2025
21. github-project-automation

    moved this from Todo to Done in asyncioon May 21, 2025
22. github-project-automation

    moved this from Todo to Done in Release and Deferred blockers 🚫on May 21, 2025

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