PrintAs was mentioned earlier, when you use the import statement you can import a single .py file. In addition, the import statement can point to a folder containing a set of .py files, or a library written in a different programming language, such as C++.
You may be wondering how Python finds the module or package you specified since you only specify the name. Your own modules may have been in the current directory with the program using it, but arcpy, for example, isn’t. What happens is that Python has a list of locations that it uses in order to find the necessary packages. It traverses the list in the specific order, and, once it finds all the packages it needs, it stops looking. Here is the search order1 that Python uses:
- The home directory where the currently executing .py file is located
- PYTHONPATH directories: PYTHONPATH is a variable that is optionally set in the operating system. For example, on a Windows machine you would set it in Environmental Variables in System Settings.
- Standard library directories: The location where all standard libraries are installed on the local machine – if you have ArcGIS 10.6 Desktop installed to a default location the standard libraries can be found at C:\Python27\ArcGIS10.6\Lib. Browse to the folder, and take a quick look of all the packages that are installed.
- The contents of any .pth files: These are text files that can be created to add additional library paths; this option is used by ArcGIS Desktop 10.6. In a standard installation you can find the .pth file at C:\Python27\ArcGIS10.6\Lib\site-packages\Desktop10.6.pth. ArcGIS pro has its own .pth file: C:\Users\<username>\AppData\Local\ESRI\conda\envs\arcgispro-py3-clone\Lib\site-packages\ArcGISPro.pth or C:\Program Files\ArcGIS\Pro\bin\Python\envs\arcgispro-py3\Lib\site-packages\ArcGISPro.pth depending on your version of Pro.
- The site-package home or third-party extensions: Packages placed in Libs\site-packages directory. In a standard install of ArcGIS 10.6 Desktop that would be in the C:\Python27\ArcGIS10.6\Lib\site-packages folder.
Because of the way Python finds the code it needs to import, you need to be careful how you name your modules and packages, and where you place them. For example, if you were to create an arcpy.py module and put it in the home directory, the ArcGIS arcpy package would not be loaded.
This list above may look intimidating, but the good news is that packages you are likely to need will be packaged with special Python utilities (either pip or conda) and thus setup to place themselves in the appropriate paths without any manual intervention on your part, beyond the installation step. The other good news is that both pip and conda are fairly straightforward to use when it comes to installing packages and managing Python environments. Creating your own pip or conda packages can be a bit more involved though as you will also see in this section but still provides a convenient way for deploying and sharing your own Python applications.
1Mark Lutz: Learning Python, 5th Edition