Module Paths

How does python find modules? It has a search order:

• current directory

• PYTHONPATH environment variable (this follows the same format as the shell PATH environment variable)

• System-wide python installation default path (usually has a site-packages directory)

We can look at the path via sys.path. On my machine I get:

['/home/zingale/.local/bin',
 '/home/zingale/classes/python-science/content/09-packages',
 '/home/zingale/classes/numerical_exercises',
 '/home/zingale/classes/astro_animations',
 '/usr/lib64/python312.zip',
 '/usr/lib64/python3.12',
 '/usr/lib64/python3.12/lib-dynload',
 '',
 '/home/zingale/.local/lib/python3.12/site-packages',
 '/usr/lib64/python3.12/site-packages',
 '/usr/lib/python3.12/site-packages']

Note

You can explicitly add paths to the sys.path by setting the PYTHONPATH environment variable.

Notice that the general places that it looks are in ~/.local and in /usr. The first is the user-specific path—you can install things here without admin privileges. The second is a system-wide path.

You can find your user-specific path via:

python3 -m site --user-site

on my machine, this gives:

/home/zingale/.local/lib/python3.11/site-packages

Tip

Using PYTHONPATH to quickly add a module to your search path is an easy hack, but if you are developing a library that will be used by others, it is better to make the modules installable to the system search paths. This is where packaging comes into play.