Classes Are Customized by Inheritance

Besides serving as factories for generating multiple instance objects, classes also allow us to make changes by introducing new components (called subclasses), instead of changing existing components in-place. Instance objects generated from a class inherit the class’s attributes. Python also allows classes to inherit from other classes, opening the door to coding hierarchies of classes that specialize behavior—by redefining attributes in subclasses that appear lower in the hierarchy, we override the more general definitions of those attributes higher in the tree. In effect, the further down the hierarchy we go, the more specific the software becomes. Here, too, there is no parallel with modules: their attributes live in a single, flat namespace that is not as amenable to customization.

In Python, instances inherit from classes, and classes inherit from superclasses. Here are the key ideas behind the machinery of attribute inheritance:

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  • Superclasses are listed in parentheses in a class header. To inherit attributes from another class, just list the class in parentheses in a class statement’s header. The class that inherits is usually called a subclass, and the class that is inherited from is its superclass.
  • Classes inherit attributes from their superclasses. Just as instances inherit the attribute names defined in their classes, classes inherit all the attribute names defined in their superclasses; Python finds them automatically when they’re accessed, if they don’t exist in the subclasses.
  • Instances inherit attributes from all accessible classes. Each instance gets names from the class it’s generated from, as well as all of that class’s superclasses. When looking for a name, Python checks the instance, then its class, then all superclasses.
  • Each object.attribute reference invokes a new, independent search. Python performs an independent search of the class tree for each attribute fetch expression. This includes references to instances and classes made outside class statements (e.g., X.attr), as well as references to attributes of the self instance argument in class method functions. Each self.attr expression in a method invokes a new search for attr in self and above.
  • Logic changes are made by subclassing, not by changing superclasses. By redefining superclass names in subclasses lower in the hierarchy (class tree), subclasses replace and thus customize inherited behavior.

The net effect, and the main purpose of all this searching, is that classes support factoring and customization of code better than any other language tool we’ve seen so far. On the one hand, they allow us to minimize code redundancy (and so reduce maintenance costs) by factoring operations into a single, shared implementation; on the other, they allow us to program by customizing what already exists, rather than changing it in-place or starting from scratch.