nixpkgs/pkgs/development/interpreters/python/sitecustomize.py

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"""
This is a Nix-specific module for discovering modules built with Nix.
The module recursively adds paths that are on `NIX_PYTHONPATH` to `sys.path`. In
order to process possible `.pth` files `site.addsitedir` is used.
The paths listed in `PYTHONPATH` are added to `sys.path` afterwards, but they
will be added before the entries we add here and thus take precedence.
Note the `NIX_PYTHONPATH` environment variable is unset in order to prevent leakage.
Similarly, this module listens to the environment variable `NIX_PYTHONEXECUTABLE`
and sets `sys.executable` to its value.
"""
import site
import sys
import os
import functools
paths = os.environ.pop('NIX_PYTHONPATH', None)
if paths:
functools.reduce(lambda k, p: site.addsitedir(p, k), paths.split(':'), site._init_pathinfo())
executable = os.environ.pop('NIX_PYTHONEXECUTABLE', None)
if 'PYTHONEXECUTABLE' not in os.environ and executable:
sys.executable = executable