Python Attributes & Methods

 

Attributes & Methods

Shopping Cart

• Users can add different items to their shopping cart and checkout.
• The total value of the cart should be more than a minimum amount (Rs. 100/-) for the checkout.
• During Offer Sales, all users get a flat discount on their cart and the minimum cart value will be Rs. 200/-.

Attributes

Broadly, attributes can be categorized as

• Instance Attributes

• Class Attributes

  

Instance Attributes

Attributes whose value can differ for each instance of class are modeled as instance attributes.

Ex: Items in Cart

Class Attributes

Attributes whose values stay common for all the objects are modelled as Class Attributes.

Ex: Minimum Cart Bill, Flat Discount

Accessing Instance Attributes

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class Cart:

flat_discount = 0

min_bill = 100

def __init__(self):

self.items = {}

def add_item(self,..):

self.items[item_name] = quantity

def display_items(self):

print(items)

a = Cart()

a.display_items()

PYTHON

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Output

NameError: name 'items' is not defined

Instance attributes can only be accessed using instance of class.

Self

self

passed to method contains the object, which is an instance of class.

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class Cart:

flat_discount = 0

min_bill = 100

def __init__(self):

self.items = {}

def add_item(self,item_name, quantity):

self.items[item_name] = quantity

def display_items(self):

print(self)

a = Cart()

a.display_items()

print(a)

PYTHON

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Output

<__main__.Cart object at 0x7f6f83c9dfd0> <__main__.Cart object at 0x7f6f83c9dfd0>

Accessing Using Self

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class Cart:

flat_discount = 0

min_bill = 100

def __init__(self):

self.items = {}

def add_item(self, item_name,quantity):

self.items[item_name] = quantity

def display_items(self):

print(self.items)

a = Cart()

a.add_item("book", 3)

a.display_items()

PYTHON

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Output

{"book": 3}

Accessing Using Object

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class Cart:

flat_discount = 0

min_bill = 100

def __init__(self):

self.items = {}

def add_item(self, item_name,quantity):

self.items[item_name] = quantity

def display_items(self):

print(self.items)

a = Cart()

a.add_item("book", 3)

print(a.items)

PYTHON

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Output

{'book': 3}

Accessing Using Class

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class Cart:

flat_discount = 0

min_bill = 100

def __init__(self):

self.items = {}

def add_item(self, item_name,quantity):

self.items[item_name] = quantity

def display_items(self):

print(self.items)

print(Cart.items)

PYTHON

Output

AttributeError: type object 'Cart' has no attribute 'items'

Accessing Class Attributes

Example 1

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class Cart:

flat_discount = 0

min_bill = 100

def __init__(self):

self.items = {}

print(Cart.min_bill)

PYTHON

Output

100

Example 2

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class Cart:

flat_discount = 0

min_bill = 100

def __init__(self):

self.items = {}

def print_min_bill(self):

print(Cart.min_bill)

a = Cart()

a.print_min_bill()

PYTHON

Output

100

Updating Class Attribute

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class Cart:

flat_discount = 0

min_bill = 100

def print_min_bill(self):

print(Cart.min_bill)

a = Cart()

b = Cart()

Cart.min_bill = 200

print(a.print_min_bill())

print(b.print_min_bill())

PYTHON

Output

200

200

Method

Broadly, methods can be categorized as

• Instance Methods
• Class Methods
• Static Methods

Instance Methods

Instance methods can access all attributes of the instance and have self as a parameter.

Example 1

Code

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class Cart:

def __init__(self):

self.items = {}

def add_item(self, item_name,quantity):

self.items[item_name] = quantity

def display_items(self):

print(self.items)

a = Cart()

a.add_item("book", 3)

a.display_items()

PYTHON

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Output

{'book': 3}

Example 2

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class Cart:

def __init__(self):

self.items = {}

def add_item(self, item_name,quantity):

self.items[item_name] = quantity

self.display_items()

def display_items(self):

print(self.items)

a = Cart()

a.add_item("book", 3)

PYTHON

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Output

{'book': 3}

Class Methods

Methods which need access to class attributes but not instance attributes are marked as Class Methods. For class methods, we send

cls

as a parameter indicating we are passing the class.

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class Cart:

flat_discount = 0

min_bill = 100

@classmethod

def update_flat_discount(cls,

new_flat_discount):

cls.flat_discount = new_flat_discount

Cart.update_flat_discount(25)

print(Cart.flat_discount)

PYTHON

Output

25

@classmethod

decorator marks the method below it as a class method.

We will learn more about decorators in upcoming sessions.

Accessing Class Method

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class Cart:

flat_discount = 0

min_bill = 100

@classmethod

def update_flat_discount(cls, new_flat_discount):

cls.flat_discount = new_flat_discount

@classmethod

def increase_flat_discount(cls, amount):

new_flat_discount = cls.flat_discount + amount

cls.update_flat_discount(new_flat_discount)

Cart.increase_flat_discount(50)

print(Cart.flat_discount)

PYTHON

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Output

50

Static Method

We might need some generic methods that don’t need access to either instance or class attributes. These type of methods are called Static Methods.

Usually, static methods are used to create utility functions which make more sense to be part of the class.

@staticmethod

decorator marks the method below it as a static method.

We will learn more about decorators in upcoming sessions.

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class Cart:

@staticmethod

def greet():

print("Have a Great Shopping")

Cart.greet()

PYTHON

Output

Have a Great Shopping

Overview of Instance, Class & Static Methods

Instance Methods	Class Methods	Static Methods
self as parameter	cls as parameter	No cls or self as parameters
No decorator required	Need decorator @classmethod	Need decorator @staticmethod
Can be accessed through object(instance of class)	Can be accessed through class	Can be accessed through class