I use SublimeText with a Cython highlighter; @redcrowbar uses Atom. I started coding uvloop & asyncpg directly in Cython, basically from scratch.

Debugging & refactoring are pretty much as easy as for pure Python code (which is a huge win over C). Sometimes when you're writing low-level code you can get a segfault, in which case lldb (or gdb) help you understand what's going on relatively quickly.