Python is an interpreted, interactive, object-oriented programming language. It incorporates modules, exceptions, dynamic typing, very high level dynamic data types, and classes.
Python can be installed as a standalone (not associated with ArcGIS) program.
When installing ArcGIS on a computer with a standalone installation of Python, use the Custom install option in ArcGIS and deselect Python from the list. Some functions in ArcGIS require Numpy, Scipy, and Matplotlib, be sure these are installed prior to installing ArcGIS.
Python 2.7.10 is also installed as part of ArcGIS 10.1 and above if a complete installation is selected.
Free, open Source
Python installation instructions in progress.
IMPORTANT: These instructions are for a custom install of Python. Also used if not installing Python that is installed with ArcGIS on the same computer (doing a custom install of ArcGIS and deselecting Python).
Install Python 2.7.12 or newer. Python 2.7.11 has a flaw that prevents ArcGIS 10.4.1 from running.
Newer versions of Python can be download from Python.
Site packages for Python can be downloaded from http://www.lfd.uci.edu/~gohlke/pythonlibs/