使用dataclass时子类重写父类属性setter触发AttributeError

你用dataclass定义了基类SimpleUrl，想在子类FullUrl中重写url属性的setter，但运行代码时触发AttributeError: type object 'SimpleUrl' has no attribute 'url'错误。

原代码 #

from dataclasses import dataclass, field

@dataclass(repr=False, eq=False)
class SimpleUrl:

    id: int  # pylint: disable=invalid-name
    url: str

    def __eq__(self, ins: object) -> bool:
        return isinstance(ins, SimpleUrl) and (self.url == ins.url)

    def __iter__(self) -> iter:
        return iter((self.id, self.url))


@dataclass(repr=False, eq=False)
class FullUrl(SimpleUrl):

    ssl: bool = field(init=False, default=None)

    @SimpleUrl.url.setter
    def url(self, url: str) -> None:
        self.url = url
        self.ssl = url.startswith('https://')

fu = FullUrl('https://www.google.com')
print(fu.ssl)

错误信息 #

Exception has occurred: AttributeError
type object 'SimpleUrl' has no attribute 'url'
  File "D:\projects\tests\test_urls.py", line 22, in FullUrl
    @SimpleUrl.url.setter
     ^^^^^^^^^^^^^
  File "D:\projects\tests\test_urls.py", line 18, in <module>
    class FullUrl(SimpleUrl):
AttributeError: type object 'SimpleUrl' has no attribute 'url'

错误原因 #

1. dataclass自动生成的属性是实例属性，不是类级别的property，所以SimpleUrl类本身没有url这个属性，无法通过@SimpleUrl.url.setter语法重写。
2. 即使能定义setter，直接写self.url = url会触发无限递归——赋值操作会再次调用当前setter方法。

解决方案 #

方案一：父类显式封装url为property

修改父类，将url通过property封装，用私有字段存储实际值，子类可安全重写setter：

from dataclasses import dataclass, field

@dataclass(repr=False, eq=False)
class SimpleUrl:
    id: int  # pylint: disable=invalid-name
    url: str

    def __post_init__(self):
        # 将dataclass初始化的url值存入私有字段
        self._url = self.url
        # 删除自动生成的实例属性，让property生效
        del self.url

    @property
    def url(self) -> str:
        return self._url

    @url.setter
    def url(self, value: str) -> None:
        self._url = value

    def __eq__(self, ins: object) -> bool:
        return isinstance(ins, SimpleUrl) and (self.url == ins.url)

    def __iter__(self) -> iter:
        return iter((self.id, self.url))


@dataclass(repr=False, eq=False)
class FullUrl(SimpleUrl):
    ssl: bool = field(init=False, default=None)

    def __post_init__(self):
        super().__post_init__()
        # 初始化时设置ssl状态
        self.ssl = self.url.startswith('https://')

    @SimpleUrl.url.setter
    def url(self, url: str) -> None:
        # 调用父类setter设置核心值
        super().url = url
        # 更新ssl状态
        self.ssl = url.startswith('https://')

# 正确实例化（传入id和url两个参数）
fu = FullUrl(1, 'https://www.google.com')
print(fu.ssl)  # 输出True

# 修改url时自动更新ssl
fu.url = 'http://example.com'
print(fu.ssl)  # 输出False

方案二：子类独立封装url属性（不修改父类）

如果不想改动父类，子类可自行定义url的property，覆盖父类的实例属性：

from dataclasses import dataclass, field

@dataclass(repr=False, eq=False)
class SimpleUrl:
    id: int  # pylint: disable=invalid-name
    url: str

    def __eq__(self, ins: object) -> bool:
        return isinstance(ins, SimpleUrl) and (self.url == ins.url)

    def __iter__(self) -> iter:
        return iter((self.id, self.url))


@dataclass(repr=False, eq=False)
class FullUrl(SimpleUrl):
    ssl: bool = field(init=False, default=None)
    # 子类私有字段存储url值
    _url: str = field(init=False)

    def __post_init__(self):
        # 将父类初始化的url值存入子类私有字段
        self._url = self.url
        # 删除父类的实例属性，让子类property生效
        del self.url
        # 初始化ssl状态
        self.ssl = self._url.startswith('https://')

    @property
    def url(self) -> str:
        return self._url

    @url.setter
    def url(self, url: str) -> None:
        self._url = url
        self.ssl = url.startswith('https://')

# 正确实例化（传入id和url两个参数）
fu = FullUrl(1, 'https://www.google.com')
print(fu.ssl)  # 输出True

额外提示 #

原代码中FullUrl('https://www.google.com')会触发参数错误，因为父类SimpleUrl需要id和url两个必填参数，实例化时必须同时传入。

内容的提问来源于stack exchange，提问作者Manu