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Breaking API Change in Python 3.13.3: Task Factory Signature Change #133745
Description
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
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picnixz commented on May 9, 2025
(I'm marking it as release-blocker just so that we can find the issue more easily)
vstinner commented on May 9, 2025
I suppose that the following issue in the toga project is related to this:
There is already a commit to work around the issue.
Yhg1s commented on May 9, 2025
@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.
serhiy-storchaka commented on May 9, 2025
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.
serhiy-storchaka commented on May 9, 2025
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.
kumaraditya303 commented on May 10, 2025
I created #133808 to revert the breaking change for 3.13.
pythongh-133745: Fix asyncio task factory name/context kwarg breaks
[3.13] gh-133745: Fix asyncio task factory name/context kwarg breaks (#…
kumaraditya303 commented on May 21, 2025
Fixed by #133948