code: def _get_environ_vars(self):
          # type: () -> Iterable[Tuple[str, str]]
          
          for key, val in os.environ.items():
              should_be_yielded = (
                  key.startswith("PIP_") and
                  key[4:].lower() not in self._ignore_env_names
              )
              if should_be_yielded:
                  yield key[4:].lower(), val
docs: Returns a generator with all environmental vars with prefix PIP_