docs: def keys(self, pattern=None):
          """Returns a list of keys matching ``pattern``.
          By default return all keys.
  
          >>> dc = Dictator()
          >>> dc['l0'] = [1, 2, 3, 4]
          >>> dc['s0'] = 'string value'
          >>> dc.keys()
          ['l0', 's0']
          >>> dc.keys('h*')
          []
          >>> dc.clear()
  
          :param pattern: key pattern
          :type pattern: str
          :return: list of keys in db
          :rtype: list of str
          """
          logger.debug('call pop %s', pattern)
          if pattern is None:
              pattern = '*'
          return self._redis.keys(pattern=pattern)