def __get__(self, instance, cls):
        if instance is None:
            return cls._fields[self.name]
        else:
            value = instance._data[self.name]
            if value is Undefined:
                raise UndefinedValueError(instance, self.name)
            else:
                return value
Simulates a failure
dataset/ETHPy150Open schematics/schematics/schematics/models.py/FieldDescriptor.__get__--crossover with--dataset/ETHPy150Open mozilla/app-validator/tests/test_submain.py/MockErrorHandler.fail_tier