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Python is a open source, general purpose multi platform computer language.
It is available for a variety of platforms including Windows, OS/X, and a variety of Unix variants including Linux.
It is preinstalled on Macs and often preinstalled on Linux.  It is not preinstalled with Windows but can be easily installed by downloading any version freely available at the Python.org website.
One of the characteristics of Python is that it can be embedded in another application for executing macros.  Blender, Maya, ArcGIS are among applications that use an embedded version of Python.
Python was conceived as a cross between a shell scripting language and a full blown general purpose computer language.  Like most shell scripting languages, it's interpreted.  However, unlike shell scripting languages such as BASH, it has many features and extensions that give it nearly as much power as C and Java.
Syntactically, Python is similar to C.  One of the unique and most beloved characteristics of Python is that code blocks are grouped by indentation rather than by surrounding the block with markers such as curly braces.
Python was created by Dutch progammer Guido Van Rossum in .  It has been steadily enhanced and improved over the years.  Due to its open source nature, enhancements have been provided by numerous contributors.
The two major revisions in common use today are the 2.X series and the 3.X series.