Upgrading to 10.4 -- don't forget to adjust your PYTHON environemnt variables
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03-01-2016 04:03 PM
Had an install of ArcGIS 10.4 where ArcMap would just falter and never open.
Spent the day with Process Monitor looking for missing Global cache, and then realized that I'd left the PYTHONHOME environment variable set to C:\Python2.7\ArcGIS10.3, and the PYTHONPATH composed from it.
Suffice to say that bad things happen with ArcMAP when PYTHONHOME & PYTHONPATH are not reset following an upgrade. 
Add it to your upgrade/install check list.
