Uninstalling Python
When Python, NumPy, and their components are installed by the ArcGIS for Desktop setup, they are also uninstalled when you remove ArcGIS for Desktop. You can manually install Python 2.7.8, NumPy 1.7.1, and matplotlib and the other resources prior to installing ArcGIS for Desktop. If Python and NumPy are both found by the ArcGIS for Desktop setup program, Python components will not be installed and, as a result, will not be uninstalled.