typing.TypeIs used

Detects usage of typing.TypeIs in return type annotations, which can lead to runtime errors if the type narrowing function is implemented incorrectly.

typing.TypeIs is used for type narrowing functions that tell type checkers to narrow the type of a variable. Type checkers trust TypeIs functions completely, so incorrect implementations bypass all type safety guarantees.

What gets flagged

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from typing_extensions import TypeIs

def is_int(x: object) -> TypeIs[int]:
    return True

value = "hello"
if is_int(value):
    # Type checker thinks value is int, but it's actually str
    result = value + 1

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warning[typing-type-is-used]: Using `typing.TypeIs` can lead to runtime type errors.
 --> main.py:3:26
  |
1 | from typing_extensions import TypeIs
2 |
3 | def is_int(x: object) -> TypeIs[int]:
  |                          ^^^^^^^^^^^
4 |     return True
  |
info: rule `typing-type-is-used` was selected in the configuration file

Better alternative

Instead of using TypeIs, use isinstance() checks directly in your code:

This provides actual runtime safety instead of just telling the type checker to trust your narrowing logic.

What is okay

If your TypeIs function correctly validates the type at runtime, it should be safe. However, since there's no way for type checkers to verify the implementation matches the signature, it's easy to introduce bugs.

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from typing_extensions import TypeIs

def is_int(x: object) -> TypeIs[int]:
    return isinstance(x, int)

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warning[typing-type-is-used]: Using `typing.TypeIs` can lead to runtime type errors.
 --> main.py:3:26
  |
1 | from typing_extensions import TypeIs
2 |
3 | def is_int(x: object) -> TypeIs[int]:
  |                          ^^^^^^^^^^^
4 |     return isinstance(x, int)
  |
info: rule `typing-type-is-used` was selected in the configuration file