Funny that you mentioned it, because there's such tool and one of it orginates from Dropbox - PyAnnotate[1] There's also MonekyType[2] from instagram, but it only works on Python 3. I will also drop PyType[3] from Google, although this one tries to statically determine the types and when I used it felt a bit buggy.

[1] https://github.com/dropbox/pyannotate [2] https://github.com/instagram/MonkeyType [3] https://github.com/google/pytype