Difference between $PATH, sys.path and os.environ - python-3.x

Question

What is the difference between $PATH variable, sys.path and os.environ? I understand that they both serve as paths where python searches packages. But it'd be nice to have more elaborate response.
Just a working case from my practice is when I used the script with only os.environ before import on Ubuntu 16.04 I got ImportError: No module named XXX. At the same time on MacOS it worked well. After I added sys.path on Ubuntu I could get import module well.

Answer 1

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

Answer 2

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.