复制实例中的函数

下面是一些(简化的)代码,用于我要做的事情:

class a:
    pass

class b:
    def printSelf(self):
        print self


instOfA = a()
instOfB = b()
instOfA.printSelf = instOfB.printSelf
instOfA.printSelf()
  <__main__.b instance at 0x0295D238>

当我调用instofa.printself()时,它将self打印为instofb。
但我希望自己在调用instofa.printself()时成为instofa,在调用instofb.printself()时成为instofb)
如果不在类A中手动定义printself,我该怎么做呢?

对于那些想知道我为什么要这样做的人来说,下面是一个较长的例子:

#Acts as a template for aInstance. I would have several aInstances that have common rules, which are defined by an instance of the aDefinition class (though I'd have multiple rule sets too)
class aDefinitionClass: 
    def setInput(self, val):
        self.inputStr = val
    def checkInputByLength(self):
        return len(self.inputStr) < 5
    def checkInputByCase(self):
        return self.inputStr == self.inputStr.upper()
    checkInput = checkInputByLength


class aInstance(aDefinition):
    inputStr = ""
    def __init__(self, ruleDefinition):
        self.checkInput = ruleDefinition.checkInput


aDef = aDefinitionClass()
aDef.checkInput = aDef.checkInputByCase #Changing one of the rules.
aInst = aInstance(aDef)
aInst.setInput("ABC")
aInst.checkInput()
  AttributeError: aDefinitionClass instance has no attribute 'inputStr'

我意识到这有点不寻常,但我想不出另一种方式来做。我正在有效地尝试子类化一个实例。如果python允许的话,它看起来会像这样:

class aInstance(aDef):
    inputStr = ""