如何在Python attrs包定义的类中调用默认方法完成实例初始化

Hey there! Great question about using attrs—let me walk you through how to get that auto-set sum behavior working just like your vanilla Python class.

First, a quick note on your existing code: your get_sum and set_sum methods are missing the self parameter, which will throw errors when you try to call them. Plus, get_sum doesn’t have a return statement—without that, your set_sum would end up assigning None to sum! Those are easy fixes.

Now, for the attrs-specific solution: attrs provides a special hook called __attrs_post_init__ that runs automatically right after the generated __init__ method finishes setting up all your attributes. This is exactly where you want to call your set_sum method, just like you did in the __init__ of your vanilla class.

Here's the corrected, working version of your Calculator class:

from attrs import define, validators, field
from typing import Union

@define
class Calculator:
    var_1: float = field(validator=[validators.instance_of(float), validators.ge(0.)])
    var_2: float = field(validator=[validators.instance_of(float), validators.ge(0.)])
    sum: Union[float, None] = field(default=None, init=False)

    def get_sum(self):  # Added required self parameter
        return self.var_1 + self.var_2  # Added return statement

    def set_sum(self):  # Added required self parameter
        self.sum = self.get_sum()

    def __attrs_post_init__(self):
        # This runs immediately after instance initialization
        self.set_sum()

When you create a new Calculator instance, the __attrs_post_init__ method will automatically trigger set_sum, so your sum property gets populated right away:

calc = Calculator(2.5, 3.5)
print(calc.sum)  # Output: 6.0

As an extra tip: If you don’t need separate get_sum and set_sum methods long-term, you could even calculate sum directly in __attrs_post_init__ to keep things concise. But since you wanted to reuse your existing method structure, the above approach fits perfectly.

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