Object-Oriented Fundamentals

Object-oriented programming revolves around creating blueprints called classes that define the structure and behavior of objects. A class serves as a template for instantiating individual objects, each with its own distinct state.

Key components of a class definition:

Component	Description
class keyword	Declares the class
Class name	Follows PascalCase convention
Constructor	__init__ method for initialization
Attributes	Instance variables storing state
Methods	Functions defining behavior

Class Declaration

class ClassName:
    def __init__(self, param1, param2, param3):
        self.field1 = param1
        self.field2 = param2
        self.field3 = param3
    
    def operation_a(self, arg):
        # method body
        pass
    
    def operation_b(self):
        # method body
        pass

Instantiating a class triggers the constructor automatical.

Working Example: Creating a Vehicle Class

class Vehicle:
    def __init__(self, brand, year):
        self.brand = brand
        self.year = year
    
    def show_details(self):
        print(f"Brand: {self.brand}, Year: {self.year}")
    
    def start_engine(self):
        print(f"The {self.brand} engine is now running.")

# Creating instances
car1 = Vehicle("Toyota", 2022)
car2 = Vehicle("Honda", 2021)

# Invoking methods
car1.show_details()  # Output: Brand: Toyota, Year: 2022
car2.start_engine()  # Output: The Honda engine is now running.

In this implementation, Vehicle serves as the blueprint. The __init__ constructor accepts brand and year parameters to set up instance attributes. Two instances—car1 and car2—are created with different values.

The Constructor Method

The constructor, defined as __init__, executes immediately when an object is instantiated. Its primary role is establishing the initial state of the newly created object.

Constructor guidelines:

Guideline	Explanation
Naming convention	Must be named exactly __init__
First parameter	Always self, referencing the current instance
Additional parameters	Used to populate instance attributes

class Product:
    def __init__(self, name, price, quantity):
        self.name = name
        self.price = price
        self.quantity = quantity
    
    def get_total_value(self):
        return self.price * self.quantity

# Object creation via constructor
item = Product("Laptop", 999.99, 3)
print(item.get_total_value())  # Output: 2999.97

When Product("Laptop", 999.99, 3) executes, Python invokes __init__, which binds the passed arguments to instance variables.

Dunder Methods

Dunder methods (short for "double underscore") enable custom behavior when Python operators or built-in functions interact with class instances.

Method	Trigger	Purpose
__init__	Object creation	Initialize instance state
__str__	print(), str()	Human-readable output
__repr__	Debugging, REPL	Unambiguous representation
__del__	Garbage collection	Cleanup operations
__eq__	== comparison	Define equality logic
__lt__, __gt__	<, > comparison	Ordering comparisons
__add__	+ operator	Addition behavior
__sub__	- operator	Subtraction behavior
__mul__	* operator	Multiplication behavior
__len__	len() functon	Return length value

Enhanced Example with Dunder Methods

class Student:
    def __init__(self, name, score):
        self.name = name
        self.score = score
    
    def __str__(self):
        return f"Student: {self.name}, Score: {self.score}"
    
    def __repr__(self):
        return f"Student('{self.name}', {self.score})"
    
    def __eq__(self, other):
        if isinstance(other, Student):
            return self.score == other.score
        return NotImplemented
    
    def __lt__(self, other):
        if isinstance(other, Student):
            return self.score < other.score
        return NotImplemented

# Instantiation
student_a = Student("Emma", 85)
student_b = Student("Liam", 92)
student_c = Student("Emma", 85)

# Output demonstrations
print(str(student_a))      # Output: Student: Emma, Score: 85
print(repr(student_b))     # Output: Student('Liam', 92)
print(student_a == student_c)  # Output: True
print(student_a < student_b)   # Output: True

The __str__ method provides a formatted string suitable for end users. The __repr__ method returns a representation that could theoretically recreate the object. The __eq__ method compares only the score attribute, while __lt__ enables sorting students by their scores.