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Python looks up modules in many places. Hooking importers will not be covered here (see RealPython or importlib.abc ).

The effective search path is in sys.path . An easy way to list all entries in the search path is:

% python -m site
$PYTHONPATH
The most known module search path is the environment variable PYTHONPATH.
It’s easy to modify it for a command only: PYTHONPATH=/some/where python ....
Sites
Third-party (non standard library) packages are installed in Python “site” directories.
The most common sites are:
  system site
      
often located in something like /usr/lib/python3/site-packages
      where pip install will write (except on Debian where --system is required)
  user site
      
often located in ~/.local/lib/python3.<minor>/site-packages
      where pip install --user will write
>>> import site
>>> site.getsitepackages()
['/usr/local/lib/python3.9/site-packages', '/usr/lib/python3/site-packages', '/usr/lib/python3.9/site-packages']
>>> site.getusersitepackages()
'/home/xxx/.local/lib/python3.9/site-packages'
Debian specificity
Debian packages a modified Python interpreter
which looks system site in /usr/lib/python3/dist-packages instead of /usr/lib/python3/site-packages.
Also, pip will install with --user by default.
Example
% pwd
/tmp/plop-2335880
% PYTHONPATH=/foo:/bar python -m site
sys.path = [
    '/tmp/plop-2335880',				← current directory
    '/foo',						← PYTHONPATH
    '/bar',						← PYTHONPATH
    '/usr/lib/python39.zip',				← standard lib
    '/usr/lib/python3.9',				← standard lib
    '/usr/lib/python3.9/lib-dynload',			← standard lib
    '/home/xxx/.local/lib/python3.9/site-packages',	← user site
    '/usr/local/lib/python3.9/dist-packages',		← system-local site
    '/usr/lib/python3/dist-packages',			← system site
    '/usr/lib/python3.9/dist-packages',			← system site
USER_BASE: '/home/xxx/.local' (exists)
USER_SITE: '/home/xxx/.local/lib/python3.9/site-packages' (exists)
ENABLE_USER_SITE: True
Limiting options
The Python interpreter has options.
python -s (equivalent to setting $PYTHONNOUSERSITE) disables the user site:
% PYTHONPATH=/foo:/bar python -s -m site
sys.path = [
    '/tmp/plop-2335880',
    '/foo',
    '/bar',
    '/usr/lib/python39.zip',
    '/usr/lib/python3.9',
    '/usr/lib/python3.9/lib-dynload',
    '/usr/local/lib/python3.9/dist-packages',
    '/usr/lib/python3/dist-packages',
    '/usr/lib/python3.9/dist-packages',
USER_BASE: '/home/xxx/.local' (exists)
USER_SITE: '/home/xxx/.local/lib/python3.9/site-packages' (exists)
ENABLE_USER_SITE: False
python -S disables all sites:
% PYTHONPATH=/foo:/bar python -S -m site
sys.path = [
    '/tmp/plop-2335880',
    '/foo',
    '/bar',
    '/usr/lib/python39.zip',
    '/usr/lib/python3.9',
    '/usr/lib/python3.9/lib-dynload',
USER_BASE: '/home/xxx/.local' (exists)
USER_SITE: '/home/xxx/.local/lib/python3.9/site-packages' (exists)
ENABLE_USER_SITE: None
python -E disables $PYTHONPATH and $PYTHONHOME:
sys.path = [
    '/tmp/plop-2335880',
    '/usr/lib/python39.zip',
    '/usr/lib/python3.9',
    '/usr/lib/python3.9/lib-dynload',
    '/home/xxx/.local/lib/python3.9/site-packages',
    '/usr/local/lib/python3.9/dist-packages',
    '/usr/lib/python3/dist-packages',
    '/usr/lib/python3.9/dist-packages',
USER_BASE: '/home/xxx/.local' (exists)
USER_SITE: '/home/xxx/.local/lib/python3.9/site-packages' (exists)
ENABLE_USER_SITE: True
python -I combines -s and -E but also prevents current directory from being added:
% PYTHONPATH=/foo:/bar python -I -m site
sys.path = [
    '/usr/lib/python39.zip',
    '/usr/lib/python3.9',
    '/usr/lib/python3.9/lib-dynload',
    '/usr/local/lib/python3.9/dist-packages',
    '/usr/lib/python3/dist-packages',
    '/usr/lib/python3.9/dist-packages',
USER_BASE: '/home/xxx/.local' (exists)
USER_SITE: '/home/xxx/.local/lib/python3.9/site-packages' (exists)
ENABLE_USER_SITE: False
*.pth
When Python loads a site, it will load files with pth extension to try to find additional paths.
Those files can either contain:
  paths (or names) of more folders to add to path
      
example: ~/.local/lib/python3.9/site-packages/easy-install.pth
  Python source code which may add entries to the path
      
example: <virtualenv>/lib/python3.9/site-packages/_virtualenv.pth
This lets the possibility to have arbitrary code run when the Python interpreter is started, by default, unless restrictive options are given (see above).
This can be a security threat since it makes it easy to inject malicious code into every Python program.
virtualenv
In a virtualenv created with --system-site-packages:
% virtualenv --system-site-packages env-system
[...]
% . env-system/bin/activate
% PYTHONPATH= python -s -m site
sys.path = [
    '/tmp/plop-2335880',
    '/usr/lib/python39.zip',
    '/usr/lib/python3.9',
    '/usr/lib/python3.9/lib-dynload',
    '/tmp/plop-2335880/env-system/lib/python3.9/site-packages',
    '/usr/local/lib/python3.9/dist-packages',
    '/usr/lib/python3/dist-packages',
    '/usr/lib/python3.9/dist-packages',
USER_BASE: '/home/xxx/.local' (exists)
USER_SITE: '/home/xxx/.local/lib/python3.9/site-packages' (exists)
ENABLE_USER_SITE: False
The virtualenv has its own site dir (/tmp/plop-2335880/env-system/lib/python3.9/site-packages),
in addition to the system site dirs (as --system-site-packages let it point to the system).
In a virtualenv created without:
% virtualenv env-no-system
[...]
% . env-no-system/bin/activate
% PYTHONPATH= python -s -m site
sys.path = [
    '/tmp/plop-2335880',
    '/usr/lib/python39.zip',
    '/usr/lib/python3.9',
    '/usr/lib/python3.9/lib-dynload',
    '/tmp/plop-2335880/env-no-system/lib/python3.9/site-packages',
USER_BASE: '/home/xxx/.local' (exists)
USER_SITE: '/home/xxx/.local/lib/python3.9/site-packages' (exists)
ENABLE_USER_SITE: False
See this PyCon 2011 video
on the internals of virtualenv.

`$PYTHONPATH`

Sites

Debian specificity

Example

Limiting options

`*.pth`

virtualenv