Alternative approach via stubfiles

[Stewori]

This is not an issue. I just wanted to crosslink an approach with the same goal that might be of help for some use cases (and eventually save us all from doing work twice).

TypeLogger from pytypes can observe types at runtime and then write a PEP-484 stubfile from that information.
All versions of typing are supported and it runs on Python 2.7, 3.3, 3.4, 3.5, 3.6, PyPy and Jython.
See https://github.com/Stewori/pytypes#usage-example-with-profiler.

The approach differs from pyannotate in that it does not write into existing code, but always creates an external PEP-484 conform stubfile that should be suitable for use with mypy (pytypes itself can use it for runtime typechecking).

A nice goodie is that it can take existing type annotations into account and extend them by information acquired from runtime observations.
The tool supports OOP -- classes, inner classes, static methods, class methods and properties -- and automatically writes a proper import section for the types in use.

Disclaimer: It is not perfect yet and still in beta. Please file issues as they come up. Help is welcome!

[gvanrossum]

Thanks for the link! We should combine the best of our tools.