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Introduction

In Python, @abstractclassmethod is a decorator that can be used with the abc module (Abstract Base Classes) to create abstract class methods. An abstract class method is a method that is declared in an abstract class but doesn't have an implementation in that class. Instead, its implementation is provided by its subclasses.

To use @abstractclassmethod, you need to import it from the abc module and then use it as a decorator on top of a method in your abstract class. Here's an example:

from abc import ABC, abstractclassmethod

class Shape(ABC):
    @abstractclassmethod
    def calculate_area(cls):
        pass

class Circle(Shape):
    def __init__(self, radius):
        self.radius = radius

    @classmethod
    def calculate_area(cls):
        return 3.14 * cls.radius ** 2

class Square(Shape):
    def __init__(self, side):
        self.side = side

    @classmethod
    def calculate_area(cls):
        return cls.side ** 2

In this example, Shape is an abstract class with the abstract class method calculate_area(). The Circle and Square classes inherit from Shape and provide their own implementation for the calculate_area() method. The calculate_area method in the child classes uses the cls parameter instead of self. This means that calculate_area is now a class method in the child classes, as required by the @abstractclassmethod decorator in the abstract base class (Shape).

Note that if a subclass doesn't provide an implementation for an abstract class method, attempting to create an instance of that subclass will raise an error indicating that the class is still abstract. Abstract class methods help ensure that certain methods are implemented by subclasses, enforcing a contract for the subclasses to follow.

Significance

The significance of @abstractclassmethod in the example is to declare the method calculate_area() as an abstract class method in the Shape class. By using this decorator, you are telling Python that any class inheriting from Shape must implement its own version of calculate_area(), or else it will be considered abstract and cannot be instantiated directly.

In other words, @abstractclassmethod enforces a contract on the subclasses, ensuring that they provide their own implementation of the calculate_area() method. If a subclass forgets to implement this method, Python will raise an error when trying to create an instance of that subclass.

Using abstract class methods is helpful when you want to define a common interface for a group of classes but leave the implementation details to the individual subclasses. It's a way to define a blueprint for methods that must be implemented in all the derived classes to make them fully functional.

In the example I provided earlier, Shape serves as an abstract base class, defining the method calculate_area(), but without a specific implementation. The Circle and Square classes inherit from Shape and provide their own implementation of calculate_area(), satisfying the requirement set by the abstract class method.

If you use @abstractcalssmethod in the abstract class, you HAVE to use the @classmethod decorator in the child class

When you use @abstractclassmethod in an abstract base class, any subclass inheriting from that abstract base class must also use the @classmethod decorator on the method implementing the abstract class method.

The reason for this is that the @abstractclassmethod decorator enforces the child classes to implement the method as a class method, and therefore, you should use @classmethod in the child class to make sure it follows the contract set by the abstract base class.

In summary, if you have an abstract class with an abstract class method decorated with @abstractclassmethod, the child classes must implement that method using @classmethod to ensure that it is indeed a class method in each of the subclasses. This ensures consistency and adherence to the abstract class's interface across all derived classes.