Previous: Static Methods & Class Methods

Class Composition, or Has-A Relationship, is different from Is-A Relationship. That is:

• Is-A Relationship: Simply one subclass inherits from a superclass, which is Inheritance. For example: Superclass Shape and subclass Circle(Shape).
• Has-A Relationship: One class is used inside another.

In the following example: class Cylinder does not inherit from class Circle. Instead, it has class Circle via the base attribute.

In essence, it can be considered Circle as a data type (similar to class/string type) used in the Cylinder Class.

class Circle:
    def __init__(self, radius=1.0, color="red"):
        self.radius = radius
        self.color = color

    def __str__(self):
        return f"Radius = {self.radius}, Color = {self.color}"

class Cylinder:  # The base: Circle
    def __init__(self, base: Circle, height=1.0):
        self.base = base
        self.height = height

    def __str__(self):
        return (f"Radius = {self.base.radius}, "
                f"Color = {self.base.color}, "
                f"Height = {self.height}")

def main():
    base_circle = Circle()  # Circle object
    # Assign to the cylinder object
    cylinder = Cylinder(base_circle, height=12)
    print("Cylinder:", cylinder)

main()

Output:

Cylinder: Radius = 1.0, Color = red, Height = 12

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