Question

I have been working on a multiple inheritance code which looks similar to this:

class Document():
    def save(self, x):
        print "inside Document from " + x

class Cdm(object):
    def save(self,x):
        print "inside Cdm from " + x
        super(Cdm,self).save('Cdm')

class Contacts(Cdm, Document):
    def __init__(self):
        self.save('Contacts')  

This is the result I get when I create an instance of Contacts.

> c = Contacts()
< inside Cdm from Contacts
< inside Document from Cdm

Now this is just weird, possibly to my eye. I might have misunderstood something. As you can see, the class Contacts inherits from cdm and Document. I am trying to use the save method of Cdm from Contacts. But Cdm inherits from object and does not have a super class method called save yet it calls the save function from Document class. In the program what I am working this is the behaviour I want but I am worried as theoretically it should not work or at least I think.

Do you have any comments on this. Or have I misunderstood Python’s inheritance.

Answer 1

As Python documentation says:

super(): Return a proxy object that delegates method calls to a parent or sibling class of type.

In that situation, super(Cdm,self).save('Cdm') will call the version of save() which is next in method resolution order. In that case it is the save() method in the Document class because it is a sibling class of Cdm (i.e. it is the second base class of Contacts).

A great practical explanation of super() can be found here: Python’s super() considered super!