How does the machine know where the interpreter is?

To be able to execute Python scripts directly (that is, to make the scripts run by double-clicking on them), the computer will also need to know where the interpreter sits within its folder structure. To accomplish this requires both administrative account access, and advanced knowledge of how Windows searches for a program. If you have this, you can adjust an environment variable within the advanced system settings dialogue to register the interpreter with the system path.

On a Windows 7/10 machine, click on the Start menu, and right-click on Computer. Then select Properties from the menu. On a Windows 8 machine, open up Windows explorer, right click on This PC, and select Properties from the menu. These commands are shortcuts to get to the Control Panel's System and Security/System menu. Select Advanced system settings from the panel on the left. Click on the Environment Variables button at the bottom of the System Properties menu that appears. In the lower portion of the Environment Variables menu, scroll in the System variables window until the Path variable appears. Select it by clicking on it, and click on the Edit button. The Edit System Variable window will appear like this:

This variable has two components: Variable name (Path) and Variable value. The value is a series of folder paths separated by semicolons. This is the path that is searched when Windows looks for specific executables that have been associated with a file extension. In our case, we will add in the folder path that contains the Python interpreter. Type C:\Python27\ArcGIS10.5 (or the equivalent on your machine) into the Variable value field, making sure to separate it from the value before it with a semi-colon. Press OK to exit the Edit dialogue, OK to exit the Environment Variables menu, and OK to exit the System Properties menu. The machine will now know where the Python interpreter is, as it will search all folders contained within the Path variable to look for an executable called Python. To test that the path adjustment worked correctly, open up a command window (Start Menu/Run, and type "cmd"), and type python.

The interpreter should start directly in the command window:

If the Python header with version information and the triple chevron appears, the path adjustment has worked correctly.

If there is no admin access available, there is a workaround. In a command-line window, pass the entire path to the Python interpreter (for example, C:\Python27\ArcGIS10.5\python.exe) to start the interpreter.