Python dataclass依赖注入时空列表默认值引发异常解决方法

定义了一个基础dataclass，需要将其能力注入到逻辑更复杂的类中，未做继承注入时，该dataclass可按预期正常运行：

from dataclasses import dataclass, field
from typing import NamedTuple, List


@dataclass
class Data:
    name: str
    start_url: str
    saved_urls: List[str] = field(default_factory=list)

google = Data("Google", "https://www.google.com")
print(google)

# 运行结果：Data(name='Google', start_url='https://www.google.com', saved_urls=[])

通过让Crawl类继承Data类实现能力注入时，程序抛出类型错误，异常触发与初始化阶段强制要求传入saved_urls参数有关，报错的实现代码如下：

from dataclasses import dataclass, field
from typing import NamedTuple, List


@dataclass
class Data:
    name: str
    start_url: str
    saved_urls: List[str] = field(default_factory=list)


class Crawl(Data):
    def __init__(self, name, start_url, saved_urls):
        self.name = name
        self.start_url = start_url
        self.saved_urls = saved_urls


def main():
    google = Crawl("Google", "https://www.google.com")
    
if __name__ == "__main__":
    main()

# 抛出异常：TypeError: __init__() missing 1 required positional argument: 'saved_urls'

报错根因：子类Crawl手动重写__init__方法时，将父类中配置了默认值的saved_urls定义为必传位置参数，覆盖了dataclass自动生成的带默认参数的初始化逻辑，实例化时未传入该参数就会触发参数缺失异常。

在子类__init__方法中直接调用父类构造方法，仅传入必填的两个参数即可，父类会自动为saved_urls应用配置好的默认值，实现代码如下：

@dataclass
class Data:
    name: str
    start_url: str
    saved_urls: List[str] = field(default_factory=list)

class Crawl(Data):
    def __init__(self, name, start_url):
        super().__init__(name, start_url)

def main():    
    google = Crawl("Google", "https://www.google.com")
    print(google)

if __name__ == "__main__":
    main()

# 运行结果：Data(name='Google', start_url='https://www.google.com', saved_urls=[])

内容的提问来源于stack exchange，提问作者dimButTries