Checks that the provided query mode is one of the accepted values. If
        not, raises a :obj:`ValueError`.

def _raise_on_mode(self, mode):
        """
        Checks that the provided query mode is one of the accepted values. If
        not, raises a :obj:`ValueError`.
        """
        valid_modes = [
            'random_sample',
            'random_sample_per_pix',
            'samples',
            'median',
            'mean',
            'best',
            'percentile']

        if mode not in valid_modes:
            raise ValueError(
                '"{}" is not a valid `mode`. Valid modes are:\n'
                '  {}'.format(mode, valid_modes)
            )