code: def _environment_variables(**kwargs):
          # type: (Any) -> Any
          
          user_agent = os.getenv('USER_AGENT')
          if user_agent is not None:
              kwargs['user_agent'] = user_agent
          preprefix = os.getenv('PREPREFIX')
          if preprefix is not None:
              kwargs['preprefix'] = preprefix
          return kwargs
docs: Overwrite keyword arguments with environment variables
  
          Args:
              **kwargs: See below
              user_agent (str): User agent string.
  
          Returns:
              kwargs: Changed keyword arguments