PyPy is a Python implementation written in Python. Of late people have started to implement a language in itself to demonstrate the versatility of the language and to show its applicability for large projects. Writing operating systems and compiler/interpreters for languages are the most complex things in software development. Since PyPy is implemented in Python, it provides the high level of abstraction and compactness of the language. This makes the implementation easy to understand than in C. Currently it is not well optimised, but in future they may be optimisations to allow prorgams to run faster than CPython.