(🐞) `isinstance` with generic type narrows the generic parameters to `Any` when it should narrow to a safe type
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bug
topic-type-narrowing
- Dominant language
- Python
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Description
a: object
if isinstance(a, list):
reveal_type(a) # list[Any]
b = a[0] # Any
Why is a narrowed to list[Any]? It should be narrowed to list[T_co] where T_co = TypeVar("T_co", covariant=True)
related: #12015
also:
i: Iterable[int]
assert isinstance(i, list):
reveal_type(i) # list[Any]
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
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- Open a pull request that references the issue number.
Research direction
Reproduce the two examples in the issue and inspect how isinstance narrows generic types in mypy's type-checking path. Compare the behavior with related issue #12015. Done means list values retain a safe generic element type rather than becoming list[Any], including when narrowing Iterable[int].
Written by the indexing model from the issue text.
Assessment
- Tech stack
- python
- Domain
- devtools
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 38/100