[ddfisher]

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"