Introduction to Object-Oriented Programming (OOP)

Overview

Object-Oriented Programming (OOP) is a style of programming, not a language-specific feature. It is implemented in many languages including Python, Java, and C++. The core principles are transferable across languages — only the syntax changes.

Key Insight: You've Already Been Using OOP

Everything in Python is an object. Even basic data types like integers, strings, and lists are objects — instances of their respective classes.

x = 1          # Object of type int
y = "hello"    # Object of type str
z = [1, 2, 3]  # Object of type list

Core Principles

1. Objects

An object is a piece of data in your program. Every value you create is an object.

• Objects have a type (also called a class)
• The type defines how the object can behave and interact with other objects

print(type(x))  # <class 'int'>
print(type(y))  # <class 'str'>
print(type(z))  # <class 'list'>

Even functions are objects! ```python def funk(): print("Hello")

print(type(funk)) # ```

2. Classes

A class is a blueprint that defines the behavior of all objects of that type.

• int, float, str, list, dict, set, bool — these are all classes
• The class determines what operations and methods are valid for its instances

# Why this works:
x + 2       # int + int ✅

# Why this fails:
x + y       # int + str ❌  → TypeError: unsupported operand type(s)

3. Instances

An instance is a specific object created from a class. Every time you create a value, you're creating an instance.

x  = 1    # Instance of class int, value = 1
x2 = 2    # Another instance of class int, value = 2
# x and x2 are separate, distinct objects

Multiple instances of the same class can exist independently of each other.

4. Methods

A method is a function that belongs to a class and is called on an instance using the dot (.) operator.

st = "hello"
st.upper()       # Method on str instance → "HELLO"
st.lower()       # Method on str instance → "hello"

nums = [3, 1, 2]
nums.index(1)    # Method on list instance → 1

Why does .upper() work on strings but not ints? Because the str class defines the .upper() method, while the int class does not.

x = 1
x.upper()  # ❌ AttributeError: 'int' object has no attribute 'upper'

Terminology Summary

Key Takeaways & Recap

• OOP is a programming paradigm — concepts apply across many languages
• In Python, everything is an object (numbers, strings, lists, even functions)
• The class of an object defines what it can do
• You create instances of classes whenever you define a variable
• Methods are functions tied to specific class types