Realisation (Implementation)

Key characteristics of realisation:

• It represents a contract that the implementing class must fulfil.
• The class must provide implementations for all methods declared in the interface.
• It allows for polymorphism through interfaces.

Python doesn’t have a built-in interface concept like some other languages (e.g., Java), but we can simulate interfaces using abstract base classes. Here’s an example:

from abc import ABC, abstractmethod

class Drawable(ABC):
    @abstractmethod
    def draw(self):
        pass

class Circle(Drawable):
    def draw(self):
        return "Drawing a circle"

class Square(Drawable):
    def draw(self):
        return "Drawing a square"

def draw_shape(shape: Drawable):
    print(shape.draw())

# Creating instances
circle = Circle()
square = Square()

# Using polymorphism through the interface
draw_shape(circle)  # Output: Drawing a circle
draw_shape(square)  # Output: Drawing a square

In this example, Drawable is an abstract base class that acts like an interface. Both Circle and Square implement the Drawable interface by providing their own implementation of the draw method.

Here’s a UML diagram representing this realisation relationship:

classDiagram
    class Drawable {
        <<interface>>
        +draw()
    }
    class Circle {
        +draw()
    }
    class Square {
        +draw()
    }
    Drawable <|.. Circle
    Drawable <|.. Square

In this diagram, the dashed arrows pointing from Circle and Square to Drawable indicate realisation. This shows that Circle and Square implement the Drawable interface.

Realisation is a powerful concept that allows for designing loosely coupled systems. By programming to interfaces rather than concrete implementations, we can create more flexible and extensible software.

References

1. Gamma, E., Helm, R., Johnson, R., & Vlissides, J. (1994). Design Patterns: Elements of Reusable Object-Oriented Software. Addison-Wesley.
2. Martin, R. C. (2017). Clean Architecture: A Craftsman’s Guide to Software Structure and Design. Prentice Hall.
3. Fowler, M. (2002). Patterns of Enterprise Application Architecture. Addison-Wesley.
4. Bloch, J. (2018). Effective Java (3rd ed.). Addison-Wesley.
5. Phillips, D. (2018). Python 3 Object-Oriented Programming (3rd ed.). Packt Publishing.
6. Lott, S. F. (2020). Object-Oriented Python: Master OOP by Building Games and GUIs. No Starch Press.
7. Booch, G., Rumbaugh, J., & Jacobson, I. (2005). The Unified Modeling Language User Guide (2nd ed.). Addison-Wesley.