Abstract Classes in Python - GeeksforGeeks

19 Mar, 2021
An abstract class can be considered as 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 which 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.   Why use Abstract Base Classes : 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.   How Abstract Base classes work : 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 abstract and then registering concrete classes as implementations of the abstract base. A method becomes abstract when decorated with the keyword @abstractmethod. For Example – 
Code 1:
Python3
# Python program showing# abstract base class work from abc import ABC, abstractmethod class Polygon(ABC):     @abstractmethod    def noofsides(self):        pass class Triangle(Polygon):     # overriding abstract method    def noofsides(self):        print("I have 3 sides") class Pentagon(Polygon):     # overriding abstract method    def noofsides(self):        print("I have 5 sides") class Hexagon(Polygon):     # overriding abstract method    def noofsides(self):        print("I have 6 sides") class Quadrilateral(Polygon):     # overriding abstract method    def noofsides(self):        print("I have 4 sides") # Driver codeR = Triangle()R.noofsides() K = Quadrilateral()K.noofsides() R = Pentagon()R.noofsides() K = Hexagon()K.noofsides()
Output:  
I have 3 sides
I have 4 sides
I have 5 sides
I have 6 sides
  Code 2: 
Python3
# Python program showing# abstract base class work from abc import ABC, abstractmethodclass Animal(ABC):     def move(self):        pass class Human(Animal):     def move(self):        print("I can walk and run") class Snake(Animal):     def move(self):        print("I can crawl") class Dog(Animal):     def move(self):        print("I can bark") class Lion(Animal):     def move(self):        print("I can roar")         # Driver codeR = Human()R.move() K = Snake()K.move() R = Dog()R.move() K = Lion()K.move()
Output:  
I can walk and run
I can crawl
I can bark
I can roar
  Implementation Through Subclassing : 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 PluginImplementation as implementing the abstract PluginBase.  
Python3
# Python program showing# implementation of abstract# class through subclassing import abc class parent:          def geeks(self):        pass class child(parent):    def geeks(self):        print("child class") # Driver codeprint( issubclass(child, parent))print( isinstance(child(), parent))
Output:  
True
True
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.   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():  
Python3
# Python program invoking a# method using super() import abcfrom abc import ABC, abstractmethod class R(ABC):    def rk(self):        print("Abstract Base Class") class K(R):    def rk(self):        super().rk()        print("subclass ") # Driver coder = K()r.rk()
Output:  
Abstract Base Class
subclass
In the above program, we can invoke the methods in abstract classes by using super().   Abstract Properties : Abstract classes include attributes in addition to methods, you can require the attributes in concrete classes by defining them with @abstractproperty.  
Python3
# Python program showing# abstract properties import abcfrom abc import ABC, abstractmethod class parent(ABC):    @abc.abstractproperty    def geeks(self):        return "parent class"class child(parent):          @property    def geeks(self):        return "child class"    try:    r =parent()    print( r.geeks)except Exception as err:    print (err)  r = child()print (r.geeks)
Output:  
Can't instantiate abstract class parent with abstract methods geeks
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.  
Python3
# Python program showing# abstract class cannot# be an instantiationfrom abc import ABC,abstractmethod class Animal(ABC):    @abstractmethod    def move(self):        passclass Human(Animal):    def move(self):        print("I can walk and run") class Snake(Animal):    def move(self):        print("I can crawl") class Dog(Animal):    def move(self):        print("I can bark") class Lion(Animal):    def move(self):        print("I can roar") c=Animal()
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
 
OmriLevin
zack_aayush
Picked
python-oop-concepts
Python
Writing code in comment?
Please use ide.geeksforgeeks.org,
generate link and share the link here.
Comments
Old Comments
Adding new column to existing DataFrame in Pandas
Python map() function
Read JSON file using Python
How to get column names in Pandas dataframe
Enumerate() in Python
How to Install PIP on Windows ?
Read a file line by line in Python
Python OOPs Concepts
Iterate over a list in Python
Different ways to create Pandas Dataframe