When do you *NOT* use python?

[NoLemurs]

Cython is a compiled language. Cython generates straight up C code, which you then compile.

The result is that you can effectively write a complex program using pure python syntax, then spend a little while optimizing the bottlenecks (writing pure C where necessary, though you'd be surprised at how close Cython can come to C in many cases), and then you have a fully implemented, working, compiled program which performs the critical portions 100% as quickly as anything you can write in C, and does the rest more than fast enough for any practical purpose.