C and Java both have terrible productivity loops because they have a tedious compile step. Python you just run after editing. I was more productive in python using a test editor the very first week I learnt it than I ever was in Java or C++ even with IDE support and years of experience.
Google App Engine python dev server reloads modified python files whithout server restart. That feature blew my mind after doing Java EE stuff (although that experience is 8 years old so maybe new Java servers can do this??). Talk about productivity!
Having a REPL is a big productivity boost for Python, not avoiding an explicit compile step. A compile step is utterly trivial to avoid (e.g. "make run" which builds and runs). Likewise, live reloading is possible in C using dlopen().
