docs: def locate_file(filename, env_var='', directory=''):
      """
      Locates a file given an environment variable or directory
      :param filename: filename to search for
      :param env_var: environment variable to look under
      :param directory: directory to look in
      :return: (string) absolute path to filename or None if not found
      """
      f = locate_by_env(filename, env_var) or locate_by_dir(filename, directory)
      return os.path.abspath(f) if can_locate(f) else None