Using Python's sys module to add a module

Python's sys module allows the user to take advantage of the system tools built into the Python interpreter. One of the most useful properties of the sys module is sys.path. It is a list of file paths which the user can modify to adjust where Python will look for a module to import, without needing administrative access.

When Python 2.7 is installed by the ArcGIS 10.5 installer, the installer takes advantage of sys.path functions to add C:\Program Files (x86)\ArcGIS\Desktop10.5\arcpy to the system path. To test this, start up the Python interpreter or an IDE, and type the following:

>>> import sys
>>> print sys.path
['', 'C:\\WINDOWS\\SYSTEM32\\python27.zip', 'C:\\Python27\\ArcGIS10.5\\Dlls', 'C:\\Python27\\ArcGIS10.5\\lib', 'C:\\Python27\\ArcGIS10.5\\lib\\plat-win', 'C:\\Python27\\ArcGIS10.5', 'C:\\Program Files (x86)\\ArcGIS\\Desktop10.5\\arcpy', 'C:\\Program Files (x86)\\ArcGIS\\Desktop10.5\\ArcToolbox\\Scripts']

The system path (stored in the sys property sys.path) includes all of the folders that ArcPy requires to automate ArcGIS. The system path incorporates all directories listed in the PYTHONPATH environment variable (if one has been created); this is separate from the Windows Path environment variable discussed earlier. The two separate path variables work together to help Python locate modules.