Question

I have a list of Spam objects:

class Spam:
    def update(self):
        print('updating spam!')

some of them might be SpamLite objects:

class SpamLite(Spam):
    def update(self):
        print('this spam is lite!')
        Spam.update(self)

I would like to be able to take an arbitrary object from the list, and add something to it’s update method, something like:

def poison(spam):
    tmp = spam.update 
    def newUpdate(self):
        print 'this spam has been poisoned!'
        tmp(self)
    spam.update = newUpdate

I want spam.update() to now either print:

this spam has been poisoned!
updating spam!

or

this spam has been poisoned!
this spam is lite!
updating spam!

depending on whether it was a SpamLite or just a Spam.

But that doesn’t work, because spam.update() won’t pass in the self argument automatically, and because if tmp leaves scope or changes then it won’t call the old update. Is there a way I can do this?

Answer 1

def poison(spam):
    tmp = spam.update
    def newUpdate():
        print 'this spam has been poisoned!'
        tmp()
    spam.update = newUpdate

Full Script:

class Spam:
    def update(self):
        print('updating spam!')

class SpamLite(Spam):
    def update(self):
        print('this spam is lite!')
        Spam.update(self)

def poison(spam):
    tmp = spam.update # it is a bound method that doesn't take any arguments
    def newUpdate():
        print 'this spam has been poisoned!'
        tmp()
    spam.update = newUpdate


from operator import methodcaller    
L = [Spam(), SpamLite()]
map(methodcaller('update'), L)
map(poison, L)
print "*"*79
map(methodcaller('update'), L)

Output:

updating spam!
this spam is lite!
updating spam!
*******************************************************************************
this spam has been poisoned!
updating spam!
this spam has been poisoned!
this spam is lite!
updating spam!