Python NotImplemented常量

Python NotImplemented constant

从decimal.py来看，它在许多特殊的方法中使用NotImplemented。例如

1
2
3
4
5
6

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

def __add__(self, a):
return NotImplemented

python文档说：

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.

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它不讨论其他特殊的方法，也不描述行为。

它似乎是一个神奇的对象，如果从其他特殊方法返回，则会引发TypeError，而在"丰富的比较"中，特殊方法则毫无用处。

例如

1	print A() < A()

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打印True，但

1	print A() + 1

提出TypeError，所以我很好奇发生了什么，未实现的用法/行为是什么。

相关讨论

NotImplemented允许您指示尚未实现两个给定操作数之间的比较(而不是指示比较有效，而是为两个操作数生成False)。

从python语言参考：

For objects x and y, first x.__op__(y)
is tried. If this is not implemented
or returns NotImplemented,
y.__rop__(x) is tried. If this is also
not implemented or returns
NotImplemented, a TypeError exception
is raised. But see the following
exception:

Exception to the previous
item: if the left operand is an
instance of a built-in type or a
new-style class, and the right operand
is an instance of a proper subclass of
that type or class and overrides the
base's __rop__() method, the right
operand's __rop__() method is tried
before the left operand's __op__()
method. This is done so that a
subclass can completely override
binary operators. Otherwise, the left
operand's __op__() method would always
accept the right operand: when an
instance of a given class is expected,
an instance of a subclass of that
class is always acceptable.

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