code: def _build_paths(self, name, spec_path_lists, exists):
          
          # flatten spec_path_lists
          spec_paths = itertools.chain.from_iterable(spec_path_lists)
          env_paths = safe_env.get(name, '').split(os.pathsep)
          paths = itertools.chain(spec_paths, env_paths)
          extant_paths = list(filter(os.path.isdir, paths)) if exists else paths
          if not extant_paths:
              msg = "%s environment variable is empty" % name.upper()
              raise distutils.errors.DistutilsPlatformError(msg)
          unique_paths = self._unique_everseen(extant_paths)
          return os.pathsep.join(unique_paths)
docs: Given an environment variable name and specified paths,
          return a pathsep-separated string of paths containing
          unique, extant, directories from those paths and from
          the environment variable. Raise an error if no paths
          are resolved.