def _check_operators(self):
        """Check Operators

        This method checks if the input operators have a "cost" method

        Raises
        ------
        ValueError
            For invalid operators type
        ValueError
            For operators without "cost" method

        """

        if not isinstance(self._operators, (list, tuple, np.ndarray)):
            raise TypeError(('Input operators must be provided as a list, '
                             'not {}').format(type(self._operators)))

        for op in self._operators:
            if not hasattr(op, 'cost'):
                raise ValueError('Operators must contain "cost" method.')
            op.cost = check_callable(op.cost)
Check Operators

        This method checks if the input operators have a "cost" method

        Raises
        ------
        ValueError
            For invalid operators type
        ValueError
            For operators without "cost" method