英文:
mypy error: Unsupported operand types for + ("Self" and "A") [operator] in Python dataclasses
问题
我正在研究一个依赖于严格类型提示的项目。我正在处理的最小代码示例将返回此mypy错误：
error.py:15: error: 不支持的操作数类型+（“Self”和“A”）[operator]
return self + rhs.to_A()
^~~~~~~~~~
可以有人解释一下为什么会出现这种情况吗？
英文:
I'm working on a project that relies on strict type hinting. The minimal code example that I'm working on will return this mypy error:
error.py:15: error: Unsupported operand types for + ("Self" and "A") [operator]
return self + rhs.to_A()
^~~~~~~~~~
from __future__ import annotations
from dataclasses import dataclass
from typing_extensions import Self
@dataclass
class A:
num: int = 0
def __add__(self, rhs: Self) -> Self:
return type(self)(self.num + rhs.num)
def add_B(self, rhs: B) -> Self:
return self + rhs.to_A()
@dataclass
class B:
num: int
def to_A(self) -> A:
return A(self.num)
Can someone explain to me why this is the case?
答案1
得分: 1
在你的类A中，你定义了只能添加Self和Self。你的add_B方法试图添加Self和A。
想象一个A的子类A2。现在，add_B将尝试将它使用to_A构建的A对象传递给一个期望A2的方法。这是被禁止的，因为A不是A2的子类。
这意味着根本不可能对A进行子类化。Mypy可能会报告这个问题，因为它不能安全地知道是否有子类，或者是否将来会有A的子类。
修复方法很明显：在__add__中只接受A作为rhs。
...
def __add__(self, rhs: 'A') -> Self:
...
英文:
In your class A you define that you can only add Self and Self. Your add_B method tries to add Self and A.
Imagine a subclass A2 of A. Now add_B will try to pass the A object it built with to_A into a method that expects an A2. That is forbidden, because A is not a subclass of A2.
That means it is impossible to subclass A at all. Mypy probably reports this because it cannot safely know that there are no subclasses or that there will never be subclasses of A.
The fix is obvious: accept just A for rhs in __add__.
...
def __add__(self, rhs: 'A') -> Self:
...
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