Python inheritance - calling base class methods inside child class?

Question

It baffles me how I can't find a clear explanation of this anywhere. Why and when do you need to call the method of the base class inside the same-name method of the child class?

class Child(Base):
    def __init__(self):
        Base.__init__(self)

    def somefunc(self):
        Base.somefunc(self)

I'm guessing you do this when you don't want to completely overwrite the method in the base class. is that really all there is to it?

Solution 1

Usually, you do this when you want to extend the functionality by modifiying, but not completely replacing a base class method. defaultdict is a good example of this:

class DefaultDict(dict):
    def __init__(self, default):
        self.default = default
        dict.__init__(self)

    def __getitem__(self, key):
        try:
            return dict.__getitem__(self, key)
        except KeyError:
            result = self[key] = self.default()
            return result

Note that the appropriate way to do this is to use super instead of directly calling the base class. Like so:

class BlahBlah(someObject, someOtherObject):
    def __init__(self, *args, **kwargs):
        #do custom stuff
        super(BlahBlah, self).__init__(*args, **kwargs) # now call the parent class(es)

Solution 2

It completely depends on the class and method.

If you just want to do something before/after the base method runs or call it with different arguments, you obviously call the base method in your subclass' method.

If you want to replace the whole method, you obviously do not call it.

Comments

[Amber]

As a side note, however, you should always call the parent class' __init__.

[Admin]

What's so great about this "super" functions?

[Joel Cornett]

@user1369281: Well, they're not always great. If you use them consistently however, they make inheritance trees much easier to work with. What if, for example, I wanted to change my defaultdict to a default ordered dict. If I used super(), the only thing I would have to change in my code would be the class definition. All those super's automatically refer to whatever base class my current class is inheriting from. It facilitates black box programming.

[casey]

I like this because it reminds you (or teaches) that your class can inherit multiple classes. So if your parent class is from a lib you can't modify and doesn't inherit object already, you can just add object to your class.