详解Python中内置的NotImplemented类型的用法
它是什么？
>>> type(NotImplemented) <type 'NotImplementedType'>
NotImplemented 是Python在内置命名空间中的六个常数之一。其他有False、True、None、Ellipsis 和 __debug__。和 Ellipsis很像，NotImplemented 能被重新赋值（覆盖）。对它赋值，甚至改变属性名称， 不会产生 SyntaxError。所以它不是一个真正的“真”常数。当然，我们应该永远不改变它。 但是为了完整性：
>>> None = 'hello' ... SyntaxError: can't assign to keyword >>> NotImplemented NotImplemented >>> NotImplemented = 'do not' >>> NotImplemented 'do not'
它有什么用？什么时候用？
NotImplemented 是个特殊值，它能被二元特殊方法返回（比如__eq__() 、 __lt__() 、 __add__() 、 __rsub__() 等），表明某个类型没有像其他类型那样实现这些操作。同样，它或许会被原地处理（in place）的二元特殊方法返回（比如__imul__()、__iand__()等）。还有，它的实际值为True：
>>> bool(NotImplemented) True
你也许会问自己，“但我认为当这个操作没有实现时，我应该产生个NotImpementedError”。我们会看些例子，关于为什么当实现二元特殊方法时不是这么回事儿。
让我们看看NotImplemented常数的用法，通过__eq__()对于两个非常基本（且没用）的类 A 和 B 的编码。[对于这个简单的例子，为了避免干扰，不会实现__ne__() ，但是总的说来，每次实现__eq__() 时， __ne__()也应该被实现，除非，有个足够充分的理由去不实现它。]
# example.py
class A(object):
def __init__(self, value):
self.value = value
def __eq__(self, other):
if isinstance(other, A):
print('Comparing an A with an A')
return other.value == self.value
if isinstance(other, B):
print('Comparing an A with a B')
return other.value == self.value
print('Could not compare A with the other class')
return NotImplemented
class B(object):
def __init__(self, value):
self.value = value
def __eq__(self, other):
if isinstance(other, B):
print('Comparing a B with another B')
return other.value == self.value
print('Could not compare B with the other class')
return NotImplemented
现在，在解释器中：
>>> from example import A, B >>> a1 = A(1) >>> b1 = B(1)
我们现在可以实验下对于 __eq__() 不同的调用，看看发生了什么。作为提醒，在Python中，a == b会调用a.__eq__(b)：
>>> a1 == a1 Comparing an A with an A True
正如所望，a1等于a1（自己），使用类A中的__eq__()来进行这个比较的。比较b1和它自己也会产生类似结果：
>>> b1 == b1 Comparing a B with another B True
现在，那要是我们比较a1和b1呢？由于在A的__eq__()会检查other是不是B的一个实例，我们想要a1.__eq__(b1)去处理这个比较并返回True：
>>> a1 == b1 Comparing an A with a B True
就是这样。现在，如果我们比较b1和a1（即调用b1.__eq__(a1)），我们会想要返回NotImplemented。这是因为B的__eq__()只和其他B的实例进行比较。来看看发生了什么：
>>> b1 == a1 Could not compare B against the other class Comparing an A with a B True
聪明！b1.__eq__(a1)方法返回NotImplemented，这样会导致调用A中的__eq__()方法。而且由于在A中的__eq__()定义了A和B之间的比较，所以就得到了正确的结果（True）。
这就是返回了NotImplemented的所做的。NotImplemented告诉运行时，应该让其他对象来完成某个操作。在表达b1 == a1中，b1.__eq__(a1)返回了NotImplemented，这说明Python试着用a1.__eq__(b1)。由于a1足够可以返回True，因此这个表达可以成功。如果A中的__eq__()也返回NotImplemented，那么运行时会退化到使用内置的比较行为，即比较对象的标识符（在CPython中，是对象在内存中的地址）。
注意：如果在调用b1.__eq__(a1)时抛出NotImpementedError，而不进行处理，就会中断代码的执行。而NotImplemented无法抛出，仅仅是用来进一步测试是否有其他方法可供调用。
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