I don’t understand why people try to unit test handlers. Use integration tests for this and the real services you depend on, use unit tests to test functions and business logic, not glue code like handlers. If you build a vast complex web of mocks attempting to reproduce behaviour you’ll only test the happy path and failures you can think of. It’s lots of work for no benefit.
This is interesting, but seems a little unnecessarily complex in places. Not sure they need to stray so far from Go. You can pretty much do exactly the same thing in golang just with a check(err) function which panics instead (tried to paste this into this comment as code but the formatting wasn't so good (no newlines)).
Not sure how profitable comparisons of languages like this are, but it is a choice we all face. Go mostly leaves things out (except perhaps a better concurrency story than python), so in a feature by feature comparison, it's going to look less impressive at first.
It's certainly less easy to get up to speed in if you're learning programming for the first time, and in different domains it has completely different strengths (for example python has far more mature science and data analysis libraries, because lots of people are using it for that).