Python 3.10 introduces pattern matching: I've not had the chance to really play with it yet so can't say how fully featured it is, but you can do something like:

    def foo(login_data):
       match login_data:
          case {"username": "foo", "password": "blah"}:
              return True
          case _:
              return False

In addition you can use type hinting:

    def foo(login_data: dict[str, str]) -> bool:
        match login_data:
          case {"username": "foo", "password": "blah"}:
              return True
          case _:
              return False

Unfortunately I don't think you can yet do the neat argument unpacking as per your Elixir example, but it's some of the way there.