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Python Implementations
An "implementation" of Python should be taken to mean a program or environment which provides support for the execution of programs written in the Python language, or some broadly similar dialect of the language.
There have been and are several distinct software packages providing of what we all recognize as Python, although some of those are more like distributions or variants of some existing implementation than a completely new implementation of the language.
CPython Variants
CPython - the reference Python implementation
CrossTwine Linker - a combination of CPython and an add-on library offering improved performance (currently proprietary)
Stackless Python - CPython with an emphasis on concurrency using tasklets and channels (used by dspython for the Nintendo DS)
Tiny Python - not to be confused with tinypy, below
unladen-swallow - "an optimization branch of CPython, intended to be fully compatible and significantly faster", potentially merging with CPython subject to PEP 3146
wpython - a re-implementation of CPython using "wordcode" instead of bytecode
Other Implementations
Berp - an implementation of Python 3 in Haskell, providing an interactive environment as well as a compiler
CLPython - Python in Common Lisp
GCC Python Front-End - an in-progress effort to compile Python code within the GCC infrastructure
HotPy - a virtual machine for Python supporting bytecode optimisation and translation (to native code) using type information gathered at run-time
IronPython - Python in C# for the Common Language Runtime (CLR/.NET) and the FePy project's IronPython Community Edition (IPCE)
Jython - Python in Java for the Java platform
PyMite - Python for embedded devices
PyPy - Python in Python, targeting several environments
pyjamas - a Python to JavaScript compiler plus Web/GUI framework
Pystacho is, like Skulpt, Python in JavaScript
pyvm - a Python-related virtual machine and software suite providing a nearly self-contained "userspace" system
Skulpt - Python in JavaScript
SNAPpy - "a subset of the Python language that has been optimized for use in low-power embedded devices" (apparently proprietary)
tinypy - a minimalist implementation of python in 64k of code
Extensions
These are typically part of CPython (or some other implementation) but change the implementation's behaviour:
Psyco - a just-in-time specialising compiler for CPython
Compilers
These compilers usually implement something close to Python, although some compilers may impose restrictions that alter the nature of the language:
Compyler - an attempt to "transliterate the bytecode into x86 assembly" (now abandoned)
Cython - an optimising Python-to-C compiler, CPython extension module generator, and wrapper language for binding external libraries. Interacts with CPython runtime and supports embedding CPython in stand-alone binaries.
Nuitka - a Python-to-C++ compiler using libpython at run-time, attempting some compile-time and run-time optimisations. Interacts with CPython runtime.
Pyc - performs static analysis in order to compile Python programs, uses similar techniques to Shed Skin
Shed Skin - a Python-to-C++ compiler
unPython - a Python to C compiler using type annotations
Similar but Distinct Languages
These languages don't attempt to be directly compatible even with a subset of Python, choosing to provide their own set of features:
Converge - inspired by Python, Haskell, Icon and Smalltalk, provides macros which can be evaluated at compile-time
Delight - based on the D programming language
Genie - based on the same foundations (Gtk+, GNOME) as the Vala programming language, supposedly inspired by Boo
Mython - an extensible variant of the Python programming language, apparently a front-end for CPython
Nimrod - statically typed, compiles to C, features parameterised types, macros, and so on
Serpent - inspired by Python, supporting real-time garbage collection and multiple virtual machines in separate threads (more information)
Trylon - inspired heavily by Smalltalk
Wirbel - a compilable language with similar restrictions to Shed Skin (statically typed names, lists cannot mix elements of different types)
Comparisons:
Comparisons of Genie and Wirbel and Genie and Python by the lead developer of the Puppy Linux distribution
Some Genie code samples indicating the differences between that language and other Python variants
Topic Guides
(Fun) Python Preprocessors
There are even some tongue-in-cheek dialects of Python which you might find fun.
This page aims to replace one formerly maintained as "Cameron Laird's personal notes on varieties of Python implementation". Also of interest will be IntegratingPythonWithOtherLanguages, which, among other variants, mentions embeddings of Python in other languages.