We (Zulip) added type hints to most of our Python Django codebase a while back. The blog post[1] below has a lot of details on this. Also included a link to the hacker news discussion of the blog post[2]. Feel free to come and ask in https://chat.zulip.org community server if you have any further queries :)
1. https://blog.zulip.org/2016/10/13/static-types-in-python-oh-...