Just like CPython is the standard implementation of the Python specifications and is written in C (of course), PyPy is an alternative implementation of Python, both for version 2.x and 3.x. It tries to mimic the behavior of the language that is written in RPython, a limited version of Python with static types.

The PyPy project (http://pypy.org/) is a continuation of another, older project called Psycho, which was a JIT compiler for Python, written in C. It worked great on 32-bit Intel processors, but it was never updated. Its latest stable release was in 2007, so it is now deprecated. PyPy took over in 2007 with its 1.0 release. Although it was initially considered a research project, it grew over the years. Finally, in 2010, version 1.4 was released. With this version, there was an increase in confidence that systems written in PyPy were production ready and compatible with Python 2.5.

The latest stable version of PyPy, released in June 2014, is version 2.5, which, in turn, is compatible...