docs: def msetnx(self, mapping):
          """
          Sets key/values based on a mapping if none of the keys are already set.
          Mapping is a dictionary of key/value pairs. Both keys and values
          should be strings or types that can be cast to a string via str().
          Returns a boolean indicating if the operation was successful.
          """
          items = []
          for pair in iteritems(mapping):
              items.extend(pair)
          return self.execute_command('MSETNX', *items)