This is intended behavior, actually! By default, mypy doesn't type check the interior of functions that don't have any type annotations. This lets you slowly

This is intended behavior, actually! By default, mypy doesn't type check the interior of functions that don't have any type annotations. This lets you slowly introduce types to a large codebase without having to convert everything at once (or even file by file). Adding a type annotation to "bar" will cause mypy to check it: def foo(n: int): pass def bar() -> None: foo("123") >> main: note: In function "bar": >> main:4: error: Argument 1 to "foo" has incompatible type "str"; expected "int"