如何让Python @dataclass动态计算派生属性值？

问题根源 #

你当前在类定义阶段直接赋值satisfied和loud，这两个是类变量，只会在类加载时执行一次，使用的是字段的默认值（food_tasty=False、restaurant_noisiness_dB=None），不会随实例化时传入的参数更新。

解决方案1：使用__post_init__方法（推荐） #

在dataclass初始化完成后，基于实例的实际属性值计算这两个字段：

from dataclasses import dataclass, astuple

class BaseSatisfaction():
    satisfied: list[bool]
    loud: bool | int

@dataclass
class ClientSatisfaction(BaseSatisfaction):
    food_tasty: bool = False
    served_quickly: bool = False
    restaurant_noisiness_dB: None | float = None

    def __post_init__(self):
        # 基于当前实例的属性动态计算
        self.satisfied = [self.food_tasty, self.served_quickly]
        self.loud = 0 if self.restaurant_noisiness_dB is None else self.restaurant_noisiness_dB > 100
    
happy_client = ClientSatisfaction(food_tasty=True, served_quickly=True)
rage_client = ClientSatisfaction(restaurant_noisiness_dB=120)

print("======== Happy Client =========")
print(happy_client.__dict__.items())
print(f"Happy client is satisfied: {happy_client.satisfied}")
print(f"How loud was it?: {happy_client.loud}")
print("======== Rage Client =========")
print(rage_client.__dict__.items())
print(f"Rage client is satisfied: {rage_client.satisfied}")
print(f"How loud was it?: {rage_client.loud}")

运行结果 #

======== Happy Client =========
dict_items([('food_tasty', True), ('served_quickly', True), ('restaurant_noisiness_dB', None), ('satisfied', [True, True]), ('loud', 0)])
Happy client is satisfied: [True, True]
How loud was it?: 0
======== Rage Client =========
dict_items([('food_tasty', False), ('served_quickly', False), ('restaurant_noisiness_dB', 120), ('satisfied', [False, False]), ('loud', True)])
Rage client is satisfied: [False, False]
How loud was it?: True

解决方案2：使用@property装饰器（动态更新场景） #

如果需要在实例属性修改后，satisfied和loud能自动更新，可将它们定义为属性：

from dataclasses import dataclass, astuple

class BaseSatisfaction():
    @property
    def satisfied(self) -> list[bool]:
        ...
    @property
    def loud(self) -> bool | int:
        ...

@dataclass
class ClientSatisfaction(BaseSatisfaction):
    food_tasty: bool = False
    served_quickly: bool = False
    restaurant_noisiness_dB: None | float = None

    @property
    def satisfied(self) -> list[bool]:
        return [self.food_tasty, self.served_quickly]
    
    @property
    def loud(self) -> bool | int:
        return 0 if self.restaurant_noisiness_dB is None else self.restaurant_noisiness_dB > 100

优势 #

当你修改实例属性时，比如happy_client.food_tasty = False，再次访问happy_client.satisfied会自动返回[False, True]，无需手动重新计算。

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