@property

    def _constraint_methods(self):

        """ Return a list of methods implementing Python constraints. """

        def is_constraint(func):

            return callable(func) and hasattr(func, '_constrains') // 判断条件

​

        cls = type(self)

        methods = []

        for attr, func in getmembers(cls, is_constraint):

            for name in func._constrains:

                field = cls._fields.get(name)

                if not field:

                    _logger.warning("method %s.%s: @constrains parameter %r is not a field name", cls._name, attr, name)

                elif not (field.store or field.inverse or field.inherited):

                    _logger.warning("method %s.%s: @constrains parameter %r is not writeable", cls._name, attr, name)

            methods.append(func)

​

        # optimization: memoize result on cls, it will not be recomputed

        cls._constraint_methods = methods

        return methods