def sum(self, axis=None, grouping=None):
        """ Numpy-style mean function.
        """
        if grouping is None:
            return self._numpy_func("sum", axis=axis)
        elif axis == 1 or len(self.shape) == 1:
            return group_features(self, grouping)
        else:
            raise Exception("Only axis = 1 is supported for grouping right now...")