Difference between $PATH, sys.path and os.environ
sys.path
Is a list of strings that specifies the search path for modules. Initialized from the environment variable PYTHONPATH, plus an installation-dependent default.
os.environ
Is a mapping object representing the string environment. For example, environ['HOME'] is the pathname of your home directory (on some platforms), and is equivalent to getenv("HOME") in C.
Environment variable PATH
Specifies a set of directories where executable programs are located. In general, each executing process or user session has its own PATH setting.
This is actually more complicated than it would seem. It's unclear by the question if you understand the Linux/MacOS $PATH environment variable. Lets start there. The $PATH variable (in Python you're able to access the system environement variables from os.environ) denotes the current users $PATH variable as defined in various shell profile and environment files. It typically contains things like "/usr/bin" and other places where programs are installed. For example when you type "ls" into the system shell, the underlying system searches the $PATH for programs named "ls". So what actually gets executed is probably something like "/usr/bin/ls" I've included additional reading below.
sys.path on the other hand is constructed by Python when the interpreter is started, based on a number of things. The first sentence in the help page is as follows. "A list of strings that specifies the search path for modules. Initialized from the environment variable $PYTHONPATH, plus an installation-dependent default." The installation-dependent portion typically defines the installation location of Python site packages. $PYTHONPATH is another environment variable (like $PATH) which can be added to facilitate the module search location and can be set the same way the system $PATH can
Typically if you have non-installed sources (ie you have Python files that you want to run outside the site-packages directory) you typically need to manipulate sys.path either directly in your scripts or add the location to the $PYTHONPATH environment variable so the interpreter knows where to find your modules. Alternatively, you could use .pth files to manipulate the module search path as well
This is just a basic overview, I hope you read the docs for better understanding
Sources
- Linux $PATH variable information
- Python sys.path
- Python site.py