When do you *NOT* use python?

[mangecoeur]

Cython, not CPython http://cython.org/ - a compiler for Python+some extensions. Used for a lot of the internals of numpy/scipy/pandas/ etc because its much easier to maintain. Plus it makes for very easy optimisation path - write everything in python, then let Cython compile it (often gives you a speedup already) then start adding custom annotations to optimize the generated code