docs: def init(self, key_value_pairs=None, **kwargs):
          """Initialize datastore.
  
          Only sets values for keys that are not in the datastore already.
  
          :param dict key_value_pairs:
              A set of key value pairs to use to initialize the datastore.
  
          :rtype: Iterable[tornado.concurrent.Future]
          """
          if key_value_pairs is None:
              key_value_pairs = kwargs
          return [self.set(k, v)
                  for k, v in key_value_pairs.items()
                  if k not in self]