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Type narrowing discards tuple from Union[Tuple, Sequence] in conditional #13749
Description
Bug Report
When refining an element of type Union[Tuple, Sequence] with the isinstance(x, Sequence) in a conditional, the Tuple part of the type is wrongly discarded in the branch.
To Reproduce
from typing import Sequence, Tuple, Union
def test_func(x: Union[Tuple[int], Sequence[str]]) -> str:
if isinstance(x, Sequence):
reveal_type(x) # Revealed type is "typing.Sequence[builtins.str]"
return x[0] # raises no error
return "a" # Statement is unreachableExpected Behavior
The revealed type of x should be Union[Tuple[int], Sequence[str]] or something equivalent. The line return x[0] should be marked as an issue, because currently test_func((1,)) returns 1 so the functions shouldn't typecheck.
Actual Behavior
test.py:6:21: note: Revealed type is "typing.Sequence[builtins.str]"
test.py:8:5: error: Statement is unreachable [unreachable]
Found 1 error in 1 file (checked 1 source file)Your Environment
Used a fresh virtual environment
- Mypy version used: 0.971
- Python version used: 3.8
Activity
brianschubert commentedon Dec 7, 2024
This was fixed in #16470, released with v1.8.0. Mypy now correctly infers a union type inside the if branch:
Thanks for the original report!