Reflected dunder 方法不适用于相同类型的实例

Question

这是一个小的重现 - class A 和 B 完全相同，为反射右移运算符实现了 dunder 方法。因此，x >> y 有望解析为 y.__rrshift__(x)。

class A:
    def __init__(self, x):
        self.x = x
        
    def __rrshift__(self, other):
        return self.__class__(self.x + other.x)

class B:
    def __init__(self, x):
        self.x = x
        
    def __rrshift__(self, other):
        return self.__class__(self.x + other.x)

但是，dunder 方法不适用于相同 class 的实例。

A(1) >> A(2)

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for >>: 'A' and 'A'

虽然它适用于来自不同 classes 的实例。

A(1) >> B(2)

<__main__.B object at 0x7f6ca8294f40>

这真是令人惊讶。并且似乎也是其他反映运算符的常见行为（例如 radd）。这是行不通的原因吗？

Answer 1

同doc states一样，操作数不同类型时有效

These functions are only called if the left operand does not support the corresponding operation and the operands are of different types

看来你只需要 __rshift__ 两个类（只有 A 对你的例子有效）

def __rshift__(self, other):
    return self.__class__(self.x + other.x)

Answer 2

来自文档（Python 语言参考手册中的 3.3.8. Emulating numeric types 部分）：

These functions are only called if the left operand does not support the corresponding operation [3] and the operands are of different types. [4]

其中 footnote 4 显示：

[4] For operands of the same type, it is assumed that if the non-reflected method – such as __add__() – fails then the overall operation is not supported, which is why the reflected method is not called.

因此，无论喜欢与否，该行为都是有意为之并记录在案的。

Reflected dunder 方法不适用于相同类型的实例

Reflected dunder method doesn't work for instances of the same type

python

class

python-class