PyPy
PyPy is probably the most exciting alternative implementation of Python, as its goal is to rewrite Python in Python. PyPy in the Python interpreter is written in Python. We have a C code layer carrying out the nuts-and-bolts work for CPython. But in PyPy, this C code layer is rewritten in pure Python.
This means that you can change the interpreter's behavior during execution time, and implement code patterns that couldn't be easily done in CPython.
PyPy is currently fully compatible with Python version 2.7.13, while the latest PyPy3 is compatible with Python version 3.5.3.
In the past, PyPy was mostly interesting for theoretical reasons, and it interested those who enjoyed going deep into the details of the language. It was not generally used in production, but this has changed through the years. Nowadays, many benchmarks show that, surprisingly, PyPy is often way faster than the CPython implementation. This project has its own benchmarking site that tracks performance of each version measured using dozens of different benchmarks (refer to http://speed.pypy.org/). It clearly shows that PyPy with JIT enabled is usually at least few times faster than CPython. This and other features of PyPy makes more and more developers decide to use PyPy in their production environments.
The main differences of PyPy compared to CPython implementation are as follows:
- Garbage collection used instead of reference counting
- Integrated tracing JIT compiler that allows impressive improvements in performance
- Application-level Stackless features borrowed from Stackless Python
Like almost every other alternative Python implementation, PyPy lacks the full official support of C's Python Extension API. Still, it at least provides some sort of support for C extensions through its CPyExt subsystem, although it is poorly documented and still not feature complete. Also, there is an ongoing effort within the community in porting NumPy to PyPy because it is the most requested feature.
The official PyPy project page can be found at http://pypy.org.