Merge pull request realpython#31 from kisielk/which

Added a section on different Python implementations

Author: Kenneth Reitz

docs/starting/which-python.rst

@@ -30,4 +30,68 @@ Which Python to Support?
 ~~~~~~~~~~~~~~~~~~~~~~~~
 
 If you're starting work on a new Python module, I recommend you write it for
-Python 2.5 or 2.6, and add support for Python3 in a later iteration.
+Python 2.5 or 2.6, and add support for Python3 in a later iteration.
+
+Implementations
+~~~~~~~~~~~~~~~
+
+There are several popular implementations of the Python programming language on
+different back-ends.
+
+CPython
+-------
+
+`CPython <http://www.python.org>`_ is the reference implementation of Python,
+written in C. It compiles Python code to intermediate bytecode which is then
+interpreted by a virtual machine. When people speak of *Python* they often mean
+not just the language but also this implementation. It provides the highest
+level of compatibility with Python packages and C extension modules.
+
+If you are writing open-source Python code and want to reach the widest possible
+audience, targeting CPython is your best bet. If you need to use any packages
+that are rely on C extensions for their functionality (eg: numpy) then CPython
+is your only choice.
+
+Being the reference implementation, all versions of the Python language are
+available as CPython. Python 3 is only available in a CPython implementation.
+
+PyPy
+----
+
+`PyPy <http://pypy.org/>`_ is a Python interpreter implemented in a restricted
+statically-typed subset of the Python language called RPython. The interpreter
+features a just-in-time compiler and supports multiple back-ends (C, CLI, JVM).
+
+PyPy aims for maximum compatibility with the reference CPython implementation
+while improving performance.
+
+If you are looking to squeeze more performance out of your Python code, it's
+worth giving PyPy a try.
+
+Currently PyPy supports Python 2.7.
+
+Jython
+------
+
+`Jython <http://www.jython.org/>`_ is a Python implementation that compiles
+Python code to Java byte code that then executes on a JVM. It has the additional
+advantage of being able to import and use any Java class the same as a Python
+module.
+
+If you need to interface with an existing Java codebase or have other reasons to
+need to write Python code for the JVM, Jython is the best choice.
+
+Currently Jython supports up to Python 2.5.
+
+IronPython
+----------
+
+`IronPython <http://ironpython.net/>`_  is an implementation of Python for .NET
+framework. It can use both Python a .NET framework libraries, and can also
+expose Python code to other .NET languages.
+
+`Python Tools for Visual Studio <http://ironpython.net/tools/>`_ integrate
+IronPython directly in to the Visual Studio development environment, making it
+an ideal choice for Windows developers.
+
+IronPython supports Python 2.7.