Inheritance and polymorphism — Inheritance is a pillar of OOP

Inheritance is one of the fundamental concepts of object oriented programming, and expresses the fundamental relationships between classes: superclasses (parents) and their subclasses (descendants). Inheritance creates a class hierarchy. Any object bound to a specific level of class hierarchy inherits all the traits (methods and attributes) defined inside any of the superclasses.

This means that inheritance is a way of building a new class, not from scratch, but by using an already defined repertoire of traits. The new class inherits (and this is the key) all the already existing equipment, but is able to add some new features if needed.

Each subclass is more specialized (or more specific) than its superclass. Conversely, each superclass is more general (more abstract) than any of its subclasses. Note that we've presumed that a class may only have one superclass — this is not always true, but we'll discuss this issue more a bit later.

A very simple example of two-level inheritance is presented here:

class Vehicle:
    pass

class LandVehicle(Vehicle):
    pass

class TrackedVehicle(LandVehicle):
    pass

All the presented classes are empty for now, as we're going to show you how the mutual relations between the super- and subclasses work.

We can say that:

the Vehicle class is the superclass for both the LandVehicle and TrackedVehicle classes;
the LandVehicle class is a subclass of Vehicle and a superclass of TrackedVehicle at the same time;
the TrackedVehicle class is a subclass of both the Vehicle and LandVehicle classes.

Inheritance and polymorphism — Inheritance is a pillar of OOP

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