<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def __frontend_limit_rules_descriptor(self, api_info):
 """Builds a frontend limit rules descriptor from API info. Args: api_info: An _ApiInfo object. Returns: A list of dictionaries with frontend limit rules information. """

    if not api_info.frontend_limits.rules:
      return None

    rules = []
    for rule in api_info.frontend_limits.rules:
      descriptor = {}
      for propname, descname in (('match', 'match'),
                                 ('qps', 'qps'),
                                 ('user_qps', 'userQps'),
                                 ('daily', 'daily'),
                                 ('analytics_id', 'analyticsId')):
        if getattr(rule, propname) is not None:
          descriptor[descname] = getattr(rule, propname)
      if descriptor:
        rules.append(descriptor)

    return rules