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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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