[ty] Treat custom enum __new__ values as dynamic#25136
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Typing conformance resultsNo changes detected ✅Current numbersThe percentage of diagnostics emitted that were expected errors held steady at 89.36%. The percentage of expected errors that received a diagnostic held steady at 85.49%. The number of fully passing files held steady at 88/134. |
Memory usage reportSummary
Significant changesClick to expand detailed breakdownprefect
trio
sphinx
flake8
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| Lint rule | Added | Removed | Changed |
|---|---|---|---|
invalid-assignment |
3 | 0 | 0 |
| Total | 3 | 0 | 0 |
Raw diff:
mypy (https://github.com/python/mypy)
+ mypy/typeshed/stdlib/ssl.pyi:268:5 error[invalid-assignment] Enum member `SERVER_AUTH` is incompatible with `__new__`
+ mypy/typeshed/stdlib/ssl.pyi:269:5 error[invalid-assignment] Enum member `CLIENT_AUTH` is incompatible with `__new__`
typeshed-stats (https://github.com/AlexWaygood/typeshed-stats)
+ tests/test_gather.py:114:13 error[invalid-assignment] Enum member `NOT_A_STRING` is incompatible with `__new__`
charliermarsh
requested review from
AlexWaygood,
carljm,
dcreager,
ibraheemdev and
sharkdp
as code owners
| reveal_type(Planet2.MERCURY._value_) # revealed: Any | ||
| ``` | ||
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| ### `__new__` without `_value_` annotation |
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Should we test for __new__ coming from a parent class? The code makes explicit allowance for inherited __new__.
| class Base(Enum): | ||
| def __new__(cls, value: str, connector_id: int = 0) -> "Base": | ||
| obj = object.__new__(cls) | ||
| obj._value_ = value |
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Do we give any errors if this is set to a different type instead? Probably not essential but would be nice to note.
I tried this sample in multiplay, just changing the annotation on the __new__ to value: int, and only pyright detects the issue.
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It appears not today, but I can fix that in a follow-up.
JelleZijlstra
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May 13, 2026
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Summary
For enums with a custom
__new__, the member RHS is input to the constructor, but__new__can assign_value_independently. We now treat such values as dynamically typed, unless_value_is explicitly annotated.To quote @JelleZijlstra: "Looks right and fairly simple, behavior matches pyright and mypy."