Composite Design Pattern in Python....

When I first started studying the Gang of Four book back in 2003-2004, it was an absolutely new thing to me. I must admit today that not everything became clear at that time. But gradually, the knowledge is being imbibed, and now while teaching my young son Ridit on this subject, I just love it.

Moreover, as I am now comfortable with three languages namely C++, Java, and Python, recreating the stuff for various languages looks pretty easy now.

So, today, I am rewriting the Composite Design Pattern in Python.

Here goes the Composite Design Pattern implemented in Python.

I hope you will like it.

```
from abc import ABC, abstractmethod  

class Shape(ABC):  

    def __init__(self):  
        self.parent = None  
    @abstractmethod  
    def add(self, shape):  
        pass  

    @abstractmethod  
    def remove(self, shape):  
        pass  

    def getparent(self):  
        return self.parent  

    def setparent(self, parent):  
        self.parent = parent  

    @abstractmethod  
    def display(self):  
        pass  


class CompositeShape(Shape):  
    def __init__(self):  
        self.listofShapes = []  

    def add(self, shape):  
        shape.setparent(self)  
        self.listofShapes.append(shape)  


    def remove(self, shape):  
        self.listofShapes.remove(shape)  
        shape.setparent(None)  

    def setParent(self, parent):  
        self.parent = parent  

    def display(self):  
        pass  


class Point(Shape):  
    def __init__(self, x, y):  
        self.x = x  
        self.y = y  

    def display(self):  
        print ("X = ", self.x, "Y = ", self.y)  

    def add(self, shape):  
        print("This is a leaf class.... Can't add to it...")  

    def remove(self, shape):  
        print("This is a leaf class.... Can't remove from it...")  

class Line(CompositeShape):  
    def __init__(self, p1, p2):  
        super().__init__()  
        self.add(p1)  
        self.add(p2)  

    def display(self):  
        print("The starting and end points of the line are")  
        for shape in self.listofShapes:  
            shape.display()  

class Quadrilateral(CompositeShape):  
    def __init__(self, p1,p2,p3,p4):  
        super().__init__()  
        self.add(p1)  
        self.add(p2)  
        self.add(p3)  
        self.add(p4)  

    def display(self):  
        print("The four corners of the quadrilateral are ")  
        for shape in self.listofShapes:  
            shape.display()  


# Press the green button in the gutter to run the script.  
if __name__ == '__main__':  
    p1 = Point(10,20)  
    p2 = Point(30,40)  
    p3 = Point(50,60)  
    p4 = Point(70,80)  

    p1.add(p2)  

    L1 = Line(p1,p2)  
    q1 = Quadrilateral(p1,p2,p3,p4)  

    p1.display()  
    p2.display()  
    L1.display()  

    q1.display()

If we execute the above code, the console window will display the following result

This is a leaf class... Can't add to it...

X = 10 Y = 20

X = 30 Y = 40

The starting and end points of the line are

X = 10 Y = 20

X = 30 Y = 40

The four corners of the quadrilateral are 

X = 10 Y = 20

X = 30 Y = 40

X = 50 Y = 60

X = 70 Y = 80