PyPy is an interpreter for the Python programming language, which is itself written in Python. Since the programmer can experiment with a Python implementation in Python (and not in C, as is the case with the reference implementation ), PyPy makes it easier to find areas where the Python implementation can be improved. In addition, PyPy allows developers due to its flexibility to experiment with multiple implementations of a special features. One of the goals of the project is to write an optimized Python implementation that is faster than the current C implementation, which was first provided in March 2008. Meanwhile PyPy is faster than CPython in a vast number of benchmarks.

PyPy consists of two main components:

Among others, there since version 1.2 a JIT compiler that performs mainly composed of integer arithmetic code in some cases significantly faster than Psyco.

PyPy was funded by the European Union Research Framework Programme. The promotion ran out in the first quarter of 2007 after 28 months.

While version 1.0 mainly pooled the results of the EU research project to version 1.1 focuses on compatibility with CPython 2.5, and the stability and accuracy. Version 1.5 is again compatible with Python 2.7. Version 1.5 should be 25% faster than the previous version.