Python Inheritance - Part 2

 

Inheritance - Part 2

How would you design and implement placing order with the details of all the products bought?

Composition

Modelling instances of one class as attributes of another class is called Composition

Code

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class Product:

def __init__(self, name, price, deal_price, ratings):

self.name = name

self.price = price

self.deal_price = deal_price

self.ratings = ratings

self.you_save = price - deal_price

def display_product_details(self):

print("Product: {}".format(self.name))

print("Price: {}".format(self.price))

print("Deal Price: {}".format(self.deal_price))

print("You Saved: {}".format(self.you_save))

print("Ratings: {}".format(self.ratings))

def get_deal_price(self):

return self.deal_price

class ElectronicItem(Product):

def set_warranty(self, warranty_in_months):

self.warranty_in_months = warranty_in_months

def get_warranty(self):

return self.warranty_in_months

class GroceryItem(Product):

pass

class Order:

def __init__(self, delivery_speed, delivery_address):

self.items_in_cart = []

self.delivery_speed = delivery_speed

self.delivery_address = delivery_address

def add_item(self, product, quantity):

self.items_in_cart.append((product, quantity))

def display_order_details(self):

for product, quantity in self.items_in_cart:

product.display_product_details()

print("Quantity: {}".format(quantity))

def display_total_bill(self):

total_bill = 0

for product, quantity in self.items_in_cart:

price = product.get_deal_price() * quantity

total_bill += price

print("Total Bill: {}".format(total_bill))

milk = GroceryItem("Milk",40, 25, 3.5)

tv = ElectronicItem("TV",45000, 40000, 3.5)

order = Order("Prime Delivery", "Hyderabad")

order.add_item(milk, 2)

order.add_item(tv, 1)

order.display_order_details()

order.display_total_bill()

PYTHON

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Output

Product: Milk

Price: 40

Deal Price: 25

You Saved: 15

Ratings: 3.5

Quantity: 2

Product: TV

Price: 45000

Deal Price: 40000

You Saved: 5000

Ratings: 3.5

Quantity: 1

Total Bill: 40050

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In the above example, we are modelling Product as attribute of Order

Overriding Methods

Sometimes, we require a method in the instances of a sub class to behave differently from the method in instance of a superclass.

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class Product:

def __init__(self, name, price, deal_price, ratings):

self.name = name

self.price = price

self.deal_price = deal_price

self.ratings = ratings

self.you_save = price - deal_price

def display_product_details(self):

print("Product: {}".format(self.name))

print("Price: {}".format(self.price))

print("Deal Price: {}".format(self.deal_price))

print("You Saved: {}".format(self.you_save))

print("Ratings: {}".format(self.ratings))

def get_deal_price(self):

return self.deal_price

class ElectronicItem(Product):

def display_product_details(self):

self.display_product_details()

print("Warranty {} months".format(self.warranty_in_months))

def set_warranty(self, warranty_in_months):

self.warranty_in_months = warranty_in_months

def get_warranty(self):

return self.warranty_in_months

e = ElectronicItem("Laptop",45000, 40000,3.5)

e.set_warranty(10)

e.display_product_details()

PYTHON

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Output

RecursionError: maximum recursion depth exceeded

Because

self.display_product_details()

in ElectronicItem class does not call the method in the superclass.

Super

Accessing Super Class’s Method

super()

allows us to call methods of the superclass (Product) from the subclass.

Instead of writing and methods to access and modify warranty we can override

__init__

Let's add warranty of ElectronicItem.

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class Product:

def __init__(self, name, price, deal_price, ratings):

self.name = name

self.price = price

self.deal_price = deal_price

self.ratings = ratings

self.you_save = price - deal_price

def display_product_details(self):

print("Product: {}".format(self.name))

print("Price: {}".format(self.price))

print("Deal Price: {}".format(self.deal_price))

print("You Saved: {}".format(self.you_save))

print("Ratings: {}".format(self.ratings))

def get_deal_price(self):

return self.deal_price

class ElectronicItem(Product):

def display_product_details(self):

super().display_product_details()

print("Warranty {} months".format(self.warranty_in_months))

def set_warranty(self, warranty_in_months):

self.warranty_in_months = warranty_in_months

def get_warranty(self):

return self.warranty_in_months

e = ElectronicItem("Laptop",45000, 40000,3.5)

e.set_warranty(10)

e.display_product_details()

PYTHON

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Output

Product: Laptop

Price: 45000

Deal Price: 40000

You Saved: 5000

Ratings: 3.5

Warranty 10 months

MultiLevel Inheritance

We can also inherit from a subclass. This is called MultiLevel Inheritance.

We can continue such inheritance to any depth in Python.

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class Product:

pass

class ElectronicItem(Product):

pass

class Laptop(ElectronicItem):

pass

PYTHON

Inheritance & Composition

When to use Inheritance?

Prefer modeling with inheritance when the classes have an IS-A relationship.

When to use Composition?

Prefer modeling with inheritance when the classes have an HAS-A relationship.