Add derivations to PATH at runtime from python

nix-path.py

#!/usr/bin/env python
#
# Add binaries to PATH at runtime

import subprocess
import nix
import os


def realise_path(store_path: str):
    if not os.path.exists(store_path):
        # Abuse IFD to realise store path
        # Try to import a file that will never exist
        dummy_file = hashlib.sha1(store_path.encode()).hexdigest()
        try:
            nix.eval("""
              import "${%s}/%s.nix"
            """ % (store_path, dummy_file))
        except nix.NixError as ex:
            # This is expected.
            # What really matters now is whether the store path exists.
            # If it does, the realization was a success.
            # Otherwise, it's likely a missing/misspelt derivation name
            if not os.path.exists(store_paths[0]):
                raise ex


def add_path(drv_name: str):
    store_path = nix.eval("""
      with import <nixpkgs> {};
      "${lib.getBin %s}/bin"
    """ % drv_name)

    realise_path(store_path)

    paths = os.environ['PATH'].split(':')
    if store_path not in paths:
        paths.insert(0, store_path)
        os.environ['PATH'] = ':'.join(paths)


if __name__ == '__main__':
    add_path('openssl')
    subprocess.run(['openssl'])