Python Classes

Overview

Classes in python are relatively simple to create. However, there is one thing you need to rememeber: every method needs a self parameter:

def classMethod(self, parm1, parm2):
    # logic goes here

Also, the constructor method is called __init__.

Standard Class

Let's create a python class called Person:

class Person:

    # constructor
    def __init__(self, new_first_name, new_last_name):
        self.first_name = new_first_name
        self.last_name = new_last_name

    # set the first_name property
    def setFirstName(self, new_first_name):
        self.first_name = new_first_name
    
    # generate a username
    # first letter of the first name plus the last name
    def getUserName(self):
        user_name = self.first_name[0] + self.last_name

        return user_name

Using the Person class

# create a new Person object
person1 = Person('George', 'Clooney')
print(person1.first_name)       # George
print(person1.last_name)        # Clooney
print(person1.getUserName())    # GClooney

# change the first name
person1.setFirstName('Ryan')
print(person1.first_name)       # Ryan
print(person1.last_name)        # Clooney
print(person1.getUserName())    # RClooney

# change the last name
person1.last_name = 'Smith'
print(person1.first_name)       # Ryan
print(person1.last_name)        # Smith
print(person1.getUserName())    # RSmith

Static Methods

Static methods are the methods that can be called without creating an object of class. They are referenced by the class name itself or reference to the Object of that class. Be sure to add the @staticmethod tag to methods.

Static properties are declared inside the class definition, but not inside a method are class or "static" variables:

class Utilities:
    version_number = 11   # static property

    @staticmethod
    def printList(theList):
        for element in theList:
            print(element)

Then in your main logic:

print(Utilities.version_number)   # 11

# print a list 
myList = [4, 10, 50, 100]
Utilities.printList(myList)
# 4
# 10
# 50
# 100

Dunder Methods

In Python, dunder methods are a set of predefined methods you can use to enrich your classes. They have leading and trailing underscores surrounding their method names.

This tutorial is going to cover a few of these methods, but there are several others you can choose to implement.

Printing Objects

Using the Person class from before, if we create the __str__ method, we can override what happens when we print the object:

class Person:
    __str__(self):
        output = "Person: {}, {}".format(self.first_name, self.last_name)
        return output

Then when we call the print() function it prints whatever we returned in the __str__ method:

person2 = Person('George', 'Washington')

print(person2)    # 'Person: Washington, George'

Comparing Objects

Built in python types are easy to compare:

print(2 > 1)              # True
print('Hello' == 'bye')   # False

We can implement the __eq__ and __lt__ dunder methods to compare objects. This is called operator overloading in C++.

To not have to implement all of the comparison dunder methods, I use the functools.total_ordering decorator which allows me to take a shortcut, only implementing __eq__ and __lt__:

from functools import total_ordering

@total_ordering
class BankAccount:
    # constructor
    def __init__(self, balance):
        self.balance = balance
    
    # equal to operator overloading
    def __eq__(self, other):
        return self.balance == other.balance

    # less than operator overloading
    def __lt__(self, other):
        return self.balance < other.balance

This is how we would use these

bankAccount1 = BankAccount(100)
bankAccount2 = BankAccount(500000)
bankAccount3 = BankAccount(100)

print(bankAccount1 == bankAccount2)     # False
print(bankAccount1 < bankAccount2)      # True
print(bankAccount3 == bankAccount1)     # True
print(bankAccount2 > bankAccount3)      # True
print(bankAccount1 > bankAccount3)      # False