Object Oriented Programming (OOP) in Python: Inheritance. Part 2

Inheritance as a concept refers to the ability of python child classes to acquire the data attributes and methods of python parent classes. Inheritance occurs many times in programming. We might find two objects that are related but one of the objects has a functionality that is specialized. What we might do is take the common attributes and functions and put them in a class and then take the special attributes and functions and put them in another class, then the objects with the special functions inherit the common attributes and functions.

 

For example we have persons and dogs. We know that all persons and dogs are animals with names and they can walk but dogs bark and persons speak words. So the commonality here is being animals. We can create a different animal class for the common features and then create separate dog and person classes for the special features.

The python class which inherits from another python class is called the python child class or derived class while the class from which other classes inherit is called the python parent class or base class.

The syntax for a python class inheritance from a parent class is as stated below:

    
class ChildClassName(ParentClassName):

    statement 1

    statement 2

Let’s take an example from the dog and person objects above. We could create a class for dog objects and another class for person objects and then make the dog and person objects inherit from the animal class since that is their common features. The code could run like the one below:

You can see from the code below that the python child classes, Dog and Person, call the python parent class constructor, Animal.__init__() to initialize their names because name is a common feature for both of them. But Dog and Person have special features like bark and words that they use to talk differently. Also, notice that there is a class variable, legs, for Dog and Person that have different values. This is to emphasize their different legs and these class variables apply to all their objects.

When a derived class definition is executed, it is executed the same way as the parent class and when the python child class is constructed through the __init__() method, the parent class is also remembered. When attribute references are being resolved, the parent class will also be included in the hierarchy of classes to check for such attributes. Just like the self.name calls we had for Dog and Person objects above. When the attribute is not found in the child class, it searches for it in the parent class. The same process occurs for method references. Notice that when I called bingo.talk() and Michael.talk() above, the program first searched the class definitions of the objects to find if there was a talk method defined therein. If there were none, it would have gone on to search the parent classes until the specified method is found.

Another python class inheritance feature you have to notice is that the python child classes in this example override the methods of the python parent classes. This feature is allowed in python class inheritance mechanism. In the example above, the Animal class has a talk and walk method but the child classes also implement a talk and walk method, overriding the talk and walk methods of the parent classes.

You can access the python parent class attributes and functions in a python child class by calling parentclassname.attribute or parentclassname.function. Try it out yourself in code and see. That is what python class inheritance is all about. Child classes own the artefacts of their parents.

Also, another feature you have to notice is that overriding methods may not just want to override the parent methods but they want to extend the parent methods by adding more functionality to what the parent can do. Let me give an example of how python extends classes in inheritance.

You can see that the worker class defines a worker by name and the company he works in. The CEO class inherits from the worker class and also defines the company he works in. But in the works() method, the CEO class calls the works method of Worker using Worker.works(self) and then extends it by printing out that the worker CEO also owns the company. So, you can see how one can extend a method from a child class. You call the method of the parent and add further functionality.

Python functions used to check python class inheritance.

Python has two built-in functions that you can use to check for python class inheritance in your objects. They are isinstance() and issubclass().

isinstance(): The syntax is isinstance(object, classinfo) and it checks if object is an instance of classinfo. That is, it checks for the type of the object. It returns True if object is an instance of classinfo and false otherwise. Examine the code below and run it to see how it works for our inheriting classes.

You will notice that since a dog is a childclass of an animal, it is also an instance of an animal, or it is of type Animal. So, a dog object is an instance (type) of Dog and also an instance (type) of Animal. But a dog is not an instance of a person because it is not inheriting anything from class Person. Please, note these differences.

issubclass(): This function is used to check for python class inheritance. The syntax is issubclass(class, classinfo) where class and classinfo are classes. The function returns True if class is a subclass of classinfo but False otherwise. Let’s use our example classes to check for inheritance.

You will notice that since Dog and Person classes are inheriting from Animal class, they are subclasses of the Animal class but Dog or Person classes are not subclasses because there is no inheritance relationship.

Types of python class inheritance: multilevel inheritance and multiple inheritance.

Python multilevel inheritance: Here we inherit the classes at separate multiple levels. C inherits from B and B inherits from A. For example, consider an example where Person inherits from Animal and Student inherits from Person. If you use isinstance() and issubclass() functions, you can see that Student by this multilevel class inheritance mechanism acquires the data attributes and methods of Animal. Let’s show this with example.

You can see that the Student has a name attribute that is inherited from Animal even though it is directly inheriting from Person, this is because in the inheritance tree it is also inheriting from animal. We show this when we call isinstance() method at the last line.

This is an example of multilevel inheritance.

Python multiple inheritance: In multiple inheritance, a python child class can inherit from more than one python parent class. The syntax for multiple inheritance is:

    
class ChildClass(ParentClass1, ParentClass2, etc):

    statement 1

    statement 2
    

The way python works is that when the object of the child class is searching for an attribute it first looks for it in its own name space, and when not found in that of parentclass1 and in all the parent classes of that class and if it is not found, it then goes to parentclass2 and so on in the order they are written.

Benefits of using python class Inheritance.

Inheritance as an OOP concept has many advantages that it gives to the programmer.

1. It makes the programmer to write less code and to avoid repeating himself. Any code that is written in the parent class is automatically available to the child classes.

2. It also makes for more structured code. When code is divided into classes, the structure of the software is better as each class represents a separate functionality.

3. The code is more scalable.

You can check out my other posts about OOP concepts in python like that about classes and objects, as well as that on python polymorphism.

Happy pythoning.