Python NotImplemented 常量

Question

查看decimal.py，它在很多特殊的方法中使用了NotImplemented。例如

class A(object):
    def __lt__(self, a):
        return NotImplemented

    def __add__(self, a):
        return NotImplemented

Python docs say :

NotImplemented

Special value which can be returned by the “rich comparison” special methods (__eq__(), __lt__(), and friends), to indicate that the comparison is not implemented with respect to the other type.

它没有谈论其他特殊方法，也没有描述行为。

它似乎是一个神奇的对象，如果从其他特殊方法返回会引发 TypeError，而在“丰富的比较”中，特殊方法什么也不做。

例如

print A() < A()

打印True，但是

print A() + 1

引发 TypeError，所以我很好奇发生了什么以及 NotImplemented 的用法/行为是什么。

Answer 1

NotImplemented 允许您指示两个给定操作数之间的比较尚未实现(而不是指示比较有效，但产生 False，因为两个操作数)。

来自 Python Language Reference :

For objects x and y, first x.__op__(y)is tried. If this is not implementedor returns NotImplemented,y.__rop__(x) is tried. If this is alsonot implemented or returnsNotImplemented, a TypeError exceptionis raised. But see the followingexception:

Exception to the previousitem: if the left operand is aninstance of a built-in type or anew-style class, and the right operandis an instance of a proper subclass ofthat type or class and overrides thebase's __rop__() method, the rightoperand's __rop__() method is triedbefore the left operand's __op__()method. This is done so that asubclass can completely overridebinary operators. Otherwise, the leftoperand's __op__() method would alwaysaccept the right operand: when aninstance of a given class is expected,an instance of a subclass of thatclass is always acceptable.