So you can do things like “from typing import Optional” to bring Optional into scope, and then annotate a function with -> Optional[int] to indicate it returns None or an int.

Unlike a system using special comments for type hints, the interpreter will complain if you make a typo in the word Optional or don’t bring it into scope.

But the interpreter doesn’t do anything else; if you actually return a string from that annotated function it won’t complain.

You need an external third party tool like MyPy or Pyre to consume the hint information and produce warnings.

In practice it’s quite usable, so long as you have CI enforcing the type system. You can gradually add types to an existing code base, and IDEs can use the hint information to support code navigation and error highlighting.