类方法，继承小问题

sunmichace

#类方法：
class Spam:
    numInstance = 0
    def __init__(self):
        Spam.numInstance += 1
    def printNumInstance(aclass):
        print('Number of instances:', aclass.numInstance, aclass)
    printNumInstance = classmethod(printNumInstance)

class Sub(Spam):
    def printNumInstance(aclass):
        print('Extra stuff ...', aclass)
        Spam.printNumInstance()
    printNumInstance = classmethod(printNumInstance)

class Other(Spam):pass

>>> x, y = Sub(), Spam()
>>> x.printNumInstance()
Extra stuff ... <class '__main__.Sub'>
Number of instances: 2 <class '__main__.Spam'>
>>> Sub.printNumInstance()
Extra stuff ... <class '__main__.Sub'>
Number of instances: 2 <class '__main__.Spam'>
>>> y.printNumInstance()
Number of instances: 2 <class '__main__.Spam'>
>>> z = Other()
>>> z.printNumInstance()
Number of instances: 3 <class '__main__.Other'>
#x,z 均继承自Spam，为什么调用Spam.printNumInstance()时，
#aclass返回<class '__main__.Spam'>,而另一个却返回<class '__main__.Other'>

x,z 均继承自Spam，为什么调用Spam.printNumInstance()时，aclass返回<class '__main__.Spam'>,而另一个却返回<class '__main__.Other'>

醉酒青牛

x和z分别是类Sub和Other的实例化对象。而Sub和Other类均是类Spam的子类。
但是，Sub类中重新定义了printNumInstance函数，且其函数体内由对父类函数printNumInstance的调用语句
Spam.printNumInstance()，因此x.printNumInstance()会返回Number of instances: 2 <class '__main__.Spam'>。
而对于Other类，其类内并没有定义任何函数，所以会直接继承父类Spam的printNumInstance的函数，当其调用z.printNumInstance()时，会将z所述类Other输出，即Number of instances: 3 <class '__main__.Other'>。