code: def _make_expr_ops(self, op_list, op_dict=None, op_class=None):
          
          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)
docs: 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: