Abstract Classes in Python

An abstract class can be considered a blueprint for other classes. It allows you to create a set of methods that must be created within any child classes built from the abstract class. A class that contains one or more abstract methods is called an abstract class. An abstract method is a method that has a declaration but does not have an implementation. While we are designing large functional units we use an abstract class. When we want to provide a common interface for different implementations of a component, we use an abstract class. 

Abstract Base Classes in Python

By defining an abstract base class, you can define a common Application Program Interface(API) for a set of subclasses. This capability is especially useful in situations where a third party is going to provide implementations, such as with plugins, but can also help you when working in a large team or with a large code base where keeping all classes in your mind is difficult or not possible. 

Working on Python Abstract classes 

By default, Python does not provide abstract classes. Python comes with a module that provides the base for defining Abstract Base classes(ABC) and that module name is ABC. ABC works by decorating methods of the base class as an abstract and then registering concrete classes as implementations of the abstract base. A method becomes abstract when decorated with the keyword @abstractmethod.

Example 1:

This code defines an abstract base class called “Polygon” using the ABC (Abstract Base Class) module in Python. The “Polygon” class has an abstract method called “noofsides” that needs to be implemented by its subclasses. There are four subclasses of “Polygon” defined: “Triangle,” “Pentagon,” “Hexagon,” and “Quadrilateral.” Each of these subclasses overrides the “noofsides” method and provides its own implementation by printing the number of sides it has. In the driver code, instances of each subclass are created, and the “noofsides” method is called on each instance to display the number of sides specific to that shape.

Output:

I have 3 sides
I have 4 sides
I have 5 sides
I have 6 sides

Example 2: 

Here, This code defines an abstract base class called “Animal” using the ABC (Abstract Base Class) module in Python. The “Animal” class has a non-abstract method called “move” that does not have any implementation. There are four subclasses of “Animal” defined: “Human,” “Snake,” “Dog,” and “Lion.” Each of these subclasses overrides the “move” method and provides its own implementation by printing a specific movement characteristic.

Output:

I can walk and run
I can crawl
I can bark
I can roar

Implementation of Abstract through Subclass

By subclassing directly from the base, we can avoid the need to register the class explicitly. In this case, the Python class management is used to recognize Plugin implementation as implementing the abstract PluginBase. 

Output: 

A side-effect of using direct subclassing is, it is possible to find all the implementations of your plugin by asking the base class for the list of known classes derived from it. 

True
True

Concrete Methods in Abstract Base Classes

Concrete classes contain only concrete (normal)methods whereas abstract classes may contain both concrete methods and abstract methods. The concrete class provides an implementation of abstract methods, the abstract base class can also provide an implementation by invoking the methods via super(). Let look over the example to invoke the method using super():  

Output:

Abstract Base Class
subclass

In the above program, we can invoke the methods in abstract classes by using super(). 

Abstract Properties in Python

Abstract classes include attributes in addition to methods, you can require the attributes in concrete classes by defining them with @abstractproperty. 

Output: 

Can't instantiate abstract class parent with abstract methods Lazyroar
child class

In the above example, the Base class cannot be instantiated because it has only an abstract version of the property-getter method. 

Abstract Class Instantiation

Abstract classes are incomplete because they have methods that have nobody. If Python allows creating an object for abstract classes then using that object if anyone calls the abstract method, but there is no actual implementation to invoke. So we use an abstract class as a template and according to the need, we extend it and build on it before we can use it. Due to the fact, an abstract class is not a concrete class, it cannot be instantiated. When we create an object for the abstract class it raises an error. 

Output:

Traceback (most recent call last):
  File "/home/ffe4267d930f204512b7f501bb1bc489.py", line 19, in 
    c=Animal()
TypeError: Can't instantiate abstract class Animal with abstract methods move