Abstract Classes in Python

In Python, an abstract class is a class that cannot be instantiated on its own and is designed to be a blueprint for other classes. Abstract classes allow us to define methods that must be implemented by subclasses, ensuring a consistent interface while still allowing the subclasses to provide specific implementations.

Abstract Base Classes in Python

It defines methods that must be implemented by its subclasses, ensuring that the subclasses follow a consistent structure. ABCs allow you to define common interfaces that various subclasses can implement while enforcing a level of abstraction.

Python provides the abc module to define ABCs and enforce the implementation of abstract methods in subclasses.

Example:

from abc import ABC, abstractmethod

# Define an abstract class
class Animal(ABC):
    
    @abstractmethod
    def sound(self):
        pass  # This is an abstract method, no implementation here.

# Concrete subclass of Animal
class Dog(Animal):
    
    def sound(self):
        return "Bark"  # Providing the implementation of the abstract method

# Create an instance of Dog
dog = Dog()
print(dog.sound())  # Output: Bark

Code Explanation:

• Abstract Base Class: Animal is an abstract class that inherits from ABC (Abstract Base Class). This class cannot be instantiated directly because it contains an abstract method sound(). The @abstractmethod decorator is used to mark sound() as an abstract method. This means any subclass must implement this method to be instantiated.
• Concrete Subclass: Dog is a subclass of Animal that provides an implementation for the sound() method. This allows the Dog class to be instantiated and used.
• Instantiation: We create an instance of Dog and call the sound() method, which returns “Bark”.

The main components of an abstract class are:

Abstract Methods

Abstract methods are methods that are defined in an abstract class but do not have an implementation. They serve as a blueprint for the subclasses, ensuring that they provide their own implementation.

Example:

from abc import ABC, abstractmethod

class Animal(ABC):
    @abstractmethod
    def make_sound(self):
        pass  # Abstract method, no implementation here

Explanation:

• make_sound() is an abstract method in the Animal class, so it doesn’t have any code inside it.

Concrete Methods

Concrete methods are methods that have full implementations in an abstract class. These methods can be inherited by subclasses and used directly without needing to be redefined.

Example:

from abc import ABC, abstractmethod

class Animal(ABC):
    @abstractmethod
    def make_sound(self):
        pass  # Abstract method, to be implemented by subclasses

    def move(self):
        return "Moving"  # Concrete method with implementation

Explanation:

• The move() method is a concrete method in the Animal class. It is implemented and does not need to be overridden by the Dog class.

Abstract Properties

Abstract properties work like abstract methods but are used for properties. These properties are declared with the @property decorator and marked as abstract using @abstractmethod. Subclasses must implement these properties.

Example:

from abc import ABC, abstractmethod

class Animal(ABC):
    @property
    @abstractmethod
    def species(self):
        pass  # Abstract property, must be implemented by subclasses

class Dog(Animal):
    @property
    def species(self):
        return "Canine"

# Instantiate the concrete subclass
dog = Dog()
print(dog.species)

Explanation:

• species is an abstract property in the Animal class and it is marked as @abstractmethod.
• The Dog class implements the species property, making it a concrete subclass that can be instantiated.
• Abstract properties enforce that a subclass provides the property’s implementation.

Abstract Class Instantiation

Abstract classes cannot be instantiated directly. This is because they contain one or more abstract methods or properties that lack implementations. Attempting to instantiate an abstract class results in a TypeError.

Example:

from abc import ABC, abstractmethod

class Animal(ABC):
    @abstractmethod
    def make_sound(self):
        pass

# Trying to instantiate the abstract class directly
# This will raise an error:
# animal = Animal()  # TypeError: Can't instantiate abstract class Animal with abstract methods make_sound

Explanation:

• The Animal class is abstract because it has the make_sound() method as an abstract method.
• If you try to instantiate Animal directly (e.g., animal = Animal()), Python raises a TypeError because you can’t instantiate a class with unimplemented abstract methods.
• Only subclasses that implement all abstract methods can be instantiated.

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