How to Use Class Objects in Python

I am used to statistical programming and use self-defined functions a lot in R. In my research work, a self-defined function works well in most cases since the scalability and flexibility of codes are less of concern. However, as using more Python and reading more source codes of Python packages, I find class is an important object and deserves a brief study on it.

A class is a prototype for an object, from which new instances can be created. An instance is a copy of the class with actual values.

Class Object

A given class object consists of two components:

• State: Attributes/properties of an object
• Behavior: Methods of an object

All the instances share the attributes and the methods of the class. But the values of attributes are unique for each instance. Let's see an example as follows.

# Define a python class named 'makecake'
class makecake:
    
    # Define class variables
    cook = 'Sharon'
    price_lb = 5    # unit price; dollars per lb
    
    # Initialize
    def __init__(self, weight):
        # Define instance variables
        self.weight = weight
       
    # Define a method for an instance
    def price(self):
        return self.weight*self.price_lb

makecake is the class name.

cook and price_lb are the class attributes, representing the cook and the unit price of a cake.

price() is the class method used to calculate the cake price.

__init__ method is a preserved method for a class object, used to initializing its state. The __init__ method is executed at the time the class is instantiated (i.e., the creation of an instance).

self is a must-have parameter in each class method, though it might take no arguments. When we call a method of an object as myobject.method(arg1, arg2), it is equivalent to the command that MyClass.method(myobject, arg1, arg2).

Also, note that weight is a variable unique to each instance, while cook, price_lb, price() are class variables shared by all instances of the class.

cake = makecake(weight=2)
print(cake.cook)
print(cake.price_lb)
cake.price()

Sharon
5
10

Inheritance

Inheritance allows us to define a class that inherits all the methods and properties from another class. For example, inherited from makecake, I declare a new class ordercake as follows.

# Define a python class based on 'makecake'
class ordercake(makecake):
    
    # Initialize
    def __init__(self, weight, tax_rate=None):
        # Define instance variables
        self.weight = weight
        self.tax_rate = tax_rate if tax_rate is not None else 0.06
    
    def checkout(self, tip=None):
        if tip is None:
            tip = 0
        
        # self.price() is inherited from 'makecake'
        print('Receipt', '\n--------------------'
              '\nCake Price:', round(self.price(), 2),
              '\nTax:', round(self.price()*self.tax_rate, 2),
              '\nTips:', tip, '\n--------------------',
              '\nTotal:', round(self.price()*(1+self.tax_rate)+tip, 2))

myorder0 = ordercake(weight=2)
myorder0.checkout()

Receipt 
--------------------
Cake Price: 10 
Tax: 0.6 
Tips: 0 
-------------------- 
Total: 10.6

In addtion to the inherited attributes, I add another attribute tax_rate in the ordercake class to calculate the cake price after tax such that

myorder1 = ordercake(weight=2, tax_rate = 0.08)
myorder1.checkout()

Receipt 
--------------------
Cake Price: 10 
Tax: 0.8 
Tips: 0 
-------------------- 
Total: 10.8

myorder1.checkout(tip=2)

Receipt 
--------------------
Cake Price: 10 
Tax: 0.8 
Tips: 2 
-------------------- 
Total: 12.8

Use Self-defined Function in Class

To enhance the ordercake class, I add an additional function to determine the delivery fee as follows.

# Define a free-delivery function
def free_delivery(order_price):
    # Define the delivery fee
    delivery_fee = 5
    
    # Waive delivery fee if order price >= 15 dollars
    if order_price >= 15:
        delivery_fee = 0
    
    return delivery_fee

# Define a python class based on 'makecake'
class ordercake_delivery(makecake):
        
    def __init__(self, weight, delivery, tax_rate=None):
        # Define instance variables
        self.weight = weight
        self.delivery_fcn = delivery
        self.tax_rate = tax_rate if tax_rate is not None else 0.06
        
    def checkout(self, tip=None):
        d_fee = self.delivery_fcn(self.price())
        if tip is None:
            tip = 0
            
        print('Receipt', '\n--------------------'
              '\nCake Price:', round(self.price(), 2),
              '\nTax:', round(self.price()*self.tax_rate, 2),
              '\nTips:', tip,
              '\nDelivery:', d_fee, '\n--------------------',
              '\nTotal:', round(self.price()*(1+self.tax_rate)+tip+d_fee, 2))

A delivery fee of 5 dollars is added to the receipt if the cake price is less than 15 dollars.

myorder2 = ordercake_delivery(weight=2, delivery=free_delivery)
myorder2.checkout()

Receipt 
--------------------
Cake Price: 10 
Tax: 0.6 
Tips: 0 
Delivery: 5 
-------------------- 
Total: 15.6

In contrast, the delivery fee is waived if the cake price is 15 dollars or above.

myorder3 = ordercake_delivery(weight=3, delivery=free_delivery)
myorder3.checkout(tip=2)

Receipt 
--------------------
Cake Price: 15 
Tax: 0.9 
Tips: 2 
Delivery: 0 
-------------------- 
Total: 17.9

Reference

Python Classes and Objects at GeeksforGeeks: https://www.geeksforgeeks.org/python-classes-and-objects/