PEP696 + PEP749: Defer evaluation of defaults when parametrizing
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stdlib
topic-typing
type-bug
- Dominant language
- Python
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Description
Bug report
Bug description:
With PEP 696 + PEP 749, the following works just fine:
class Scalar[CompatT, TruthT = BoolScalar]: ...
class BoolScalar(Scalar): ...
class IntScalar(Scalar[int]): ... # OK ✅️
However, if we switch around the order of definition, the code breaks:
class Scalar[CompatT, TruthT = BoolScalar]: ...
class IntScalar(Scalar[int]): ... # ❌️ NameError: name 'BoolScalar' is not defined
class BoolScalar(Scalar): ...
I guess since the default gets evaluated here. Not sure whether this is a bug or feature request, but it would be nice if the evaluation of the default could be deferred in is case as well.
CPython versions tested on:
3.14
Operating systems tested on:
Linux
Linked PRs
- gh-152208
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Run the two class-definition examples from the issue on CPython 3.14 and compare their behavior. Read the linked PEP 696 and PEP 749 discussions, then review linked PR gh-152208; done means the forward-definition example no longer raises NameError while the existing example continues to work.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- python
- Domain
- backend
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 35/100