I don't -- generally -- think of myself as an edge-of-the-envelope developer. I'm a tried-and-proven kind of engineer. I want stuff that's been around for years, with a long history of changes.
Currently, I'm revising Mastering Object-Oriented Python. Second Edition.
That means upgrading everything to Python 3.7 with full type hints throughout almost all of the 18 chapters. (SQLAlchemy presents some problems, so we're not going deep there.)
The chapter on foundational WSGI applications is *totally* broken. I can't get anything to work with mypy. (The unit tests run, but mypy complains. Loudly.) Of course, I tried every wrong thing for three solid days. Then I pulled the stub file from typeshed and realized how dumb I was.
Okay. I finally got the correct type hints. Yay!
Something in mypy is balking at the start_response() function calls. Too many arguments.
Read the issues. Hm. Stack Overflow. Hm.
Just to be sure, I updated to the new 0.630 release in September, 2018.
Problem solved. So. I've arrived at the edge of the envelope. I now require the absolutely latest and greatest mypy release. By the time I'm done with the rewrites, this release will be ancient history. But today, it was wonderful to get past the examples.