Wrong type narrowing when using type without an argument in isinstance call

To Reproduce

from typing import TypeVar

T = TypeVar("T")


def fun(x: T, klass: type) -> T:
    if not isinstance(x, klass):
        raise TypeError
    return x

Expected Behavior

No error.

Actual Behavior

error: Incompatible return value type (got "object", expected "T") [return-value]

Your Environment

Mypy version used: 0.971

Adding a constraint to the TypeVar seems to help

T = TypeVar("T", str, str)

def fun(val: T) -> T:
    if not isinstance(val, str):
        raise TypeError
    return val

Narrow type variable bounds in binder (python#19183)

@ilevkivskyi

@ilevkivskyi How exactly #19183 closed this one?
I still see the same error in the pretty brain dead scenarios.

    def rename_deep(self, data: T, col_map: dict[str, str]) -> T:
        if isinstance(data, str):
            return self.rename_all(data, col_map)
        if isinstance(data, dict):
            return {key: self.rename_deep(value, col_map) for key, value in data.items()}
        if isinstance(data, list):
            return [self.rename_deep(item, col_map) for item in data]
        return data

error: Incompatible return value type (got "str", expected "T")  [return-value]
error: Incompatible return value type (got "dict[Any, Any]", expected "T")  [return-value]
error: Incompatible return value type (got "list[Any]", expected "T")  [return-value]

Using mypy 1.17.1 which should include #19183

Navigation Menu

Repository navigation

Wrong type narrowing when using type without an argument in isinstance call #13462

Metadata

Assignees

Labels

Projects

Milestone

Relationships

Development

Wrong type narrowing when using type without an argument in isinstance call #13462

Description

Activity

kellerza commented on Jul 25, 2024

pkit commented on Aug 16, 2025

Metadata

Metadata

Assignees

Labels

Projects

Milestone

Relationships

Development

Issue actions