Overriding and super

So inheritance is great for adding new behavior to existing classes, but what about changing behavior? Our contact class allows only a name and an e-mail address. This may be sufficient for most contacts, but what if we want to add a phone number for our close friends?

As we saw in Chapter 2, we can do this easily by just setting a phone attribute on the contact after it is constructed. But if we want to make this third variable available on initialization, we have to override __init__. Overriding is altering or replacing a method of the superclass with a new method (with the same name) in the subclass. No special syntax is needed to do this; the subclass's newly created method is automatically called instead of the superclass's method. For example:

class Friend(Contact):
    def __init__(self, name, email, phone):
        self.name = name
        self.email = email
        self.phone = phone

Any method can be overridden, not just __init__. Before we go on, however, we need to correct some problems in this example. Our Contact and Friend classes have duplicate code to set up the name and email properties; this can make maintenance complicated, as we have to update the code in two or more places. More alarmingly, our Friend class is neglecting to add itself to the all_contacts list we have created on the Contact class.

What we really need is a way to call code on the parent class. This is what the super function does; it returns the object as an instance of the parent class, allowing us to call the parent method directly:

class Friend(Contact):
    def __init__(self, name, email, phone):
 super().__init__(name, email)
        self.phone = phone

This example first gets the instance of the parent object using super, and calls __init__ on that object, passing in the expected arguments. It then does its own initialization, namely setting the phone attribute.

A super() call can be made inside any method, not just __init__. This means all methods can be modified via overriding and calls to super. The call to super can also be made at any point in the method; we don't have to make the call as the first line in the method. For example, we may need to manipulate the incoming parameters before forwarding them to the superclass.