This was also my initial reaction. However, if you couple type annotations with an LSP server[1] that takes the annotations into consideration[2], you'll get on-the-fly type checking in your editor or IDE. That way you can catch type errors as you write your typed Python code.

Besides VS Code, these days lightweight editors like Kate, Sublime Text, Vim, etc can all take advantage of type checking LSP servers for Python.

[1] https://github.com/palantir/python-language-server

[2] https://github.com/tomv564/pyls-mypy