I did a small small experiment in dependent typing for Python (and catching those errors before run time). It's a decently good fit, and with HM Type inference some neat things just fall out of the design.

But the real challenge is deciding how much of Python's semantics to preserve. Once you start enforcing constraints that depend on values, a lot of idiomatic dynamic behavior stops making sense. Curious what ways there are to work around this, but I think that's for a person better at Python than I :)