Fill up `self._op_expr` dict.

        :param op_list:     A list of operation names.
        :param op_dict:     A dictionary of operation methods.
        :param op_class:    Where the operation method comes from.
        :return:

def _make_expr_ops(self, op_list, op_dict=None, op_class=None):
        """
        Fill up `self._op_expr` dict.

        :param op_list:     A list of operation names.
        :param op_dict:     A dictionary of operation methods.
        :param op_class:    Where the operation method comes from.
        :return:
        """
        for o in op_list:
            if op_dict is not None:
                if o in op_dict:
                    self._op_expr[o] = op_dict[o]
                else:
                    l.warning("Operation %s not in op_dict.", o)
            else:
                if hasattr(op_class, o):
                    self._op_expr[o] = getattr(op_class, o)
                else:
                    l.warning("Operation %s not in op_class %s.", o, op_class)