code: def maybe_encode_keys(self, keys):
          
          ret = {}
          for k, v in six.iteritems(keys):
              if is_dynamo_value(v):
                  return keys
              elif not is_null(v):
                  ret[k] = self.encode(v)
          return ret
docs: Same as encode_keys but a no-op if already in Dynamo format