def add_key_filter(self, *args):
        """
        Add a single key filter to the inputs.

        :param args: a filter
        :type args: list
        :rtype: :class:`RiakMapReduce`
        """
        if self._input_mode == 'query':
            raise ValueError('Key filters are not supported in a query.')

        self._key_filters.append(args)
        return self
Add a single key filter to the inputs.

        :param args: a filter
        :type args: list
        :rtype: :class:`RiakMapReduce`