base.py 文件源码

def __get__(self, obj, cls: typing.Optional[type]=None):
        if obj is None:
            return self
        default, expression = self.get_raw_value(obj)
        if not default:
            if not isinstance(expression, collections.abc.Mapping):
                raise ConfigTypeError(
                    '{0!r} field must be a mapping, not {1}'.format(
                        self.key, typing._type_repr(type(expression))
                    )
                )
            elif 'class' not in expression:
                raise ConfigValueError(
                    '{0!r} field lacks "class" field'.format(self.key)
                )
            value = self.evaluate(expression)
            self.typecheck(value)
        return value