Python Classes and Objects for Beginners

1Classes and Objects Explained

In Python, a class is a blueprint for creating objects, and an object is a specific instance built from that blueprint. The class defines what data an object holds and what it can do, while each object gets its own copy of that data. Think of a class as a cookie cutter and objects as the individual cookies.

This pairing is the heart of object-oriented programming. By bundling data (attributes) and behavior (methods) together, classes let you model real-world things — a user, a bank account, a game character — as self-contained units that are easy to reuse and reason about.

• class Dog:
• def __init__(self, name):
• self.name = name
• buddy = Dog('Buddy') # create an object
• print(buddy.name) # Buddy

2Defining a Class

You define a class with the class keyword followed by a name in CapWords style. Inside, you define methods — functions that belong to the class. To create an object, you call the class as if it were a function, and Python returns a new instance.

• class Car:
• def __init__(self, brand, speed=0):
• self.brand = brand
• self.speed = speed
• my_car = Car('Toyota') # create an instance
• your_car = Car('Honda', 60) # another independent instance

3The __init__ Method and self

The __init__ method, called the constructor, runs automatically the moment an object is created. Its job is to set up the object's starting attributes. The first parameter, self, refers to the specific instance being created, which is how each object stores its own data.

self is not a keyword — it is a naming convention — but every instance method must take it as the first parameter. When you call buddy.bark(), Python passes buddy in as self behind the scenes, so the method knows which object it is working on.

• class Account:
• def __init__(self, owner, balance=0):
• self.owner = owner # self ties data to this instance
• self.balance = balance
• a = Account('Ava', 100)
• print(a.owner, a.balance) # Ava 100

4Attributes and Methods

Attributes are the variables attached to an object that hold its data; methods are the functions defined inside the class that define its behavior. Together they keep related state and logic in one place — a bank account's balance lives next to the deposit and withdraw operations that change it.

• class Account:
• def __init__(self, owner, balance=0):
• self.owner = owner
• self.balance = balance
• def deposit(self, amount):
• self.balance += amount
• def withdraw(self, amount):
• if amount <= self.balance: self.balance -= amount

a = Account('Ava', 100)
a.deposit(50)  # balance is now 150
a.withdraw(30)  # balance is now 120
print(a.balance)  # 120

5Class vs Instance Attributes

Instance attributes are unique to each object and are usually set in __init__ with self. Class attributes are defined directly in the class body and shared by every instance — useful for constants or defaults that all objects have in common.

• class Dog:
• species = 'Canis familiaris' # class attribute, shared
• def __init__(self, name):
• self.name = name # instance attribute, unique
• print(Dog.species) # shared by all dogs
• print(Dog('Rex').name) # unique to this dog

6Useful Special Methods

Python classes can define dunder (double-underscore) methods that hook into built-in behavior. The most useful for beginners is __str__, which controls what print() shows for your object, turning a cryptic default into something readable.

• class Point:
• def __init__(self, x, y):
• self.x = x; self.y = y
• def __str__(self):
• return f'Point({self.x}, {self.y})'
• print(Point(2, 3)) # Point(2, 3) instead of a memory address

7Common Mistakes to Avoid

Beginners hit the same few snags when learning classes.

• Forgetting self as the first parameter of a method, causing a TypeError.
• Forgetting self. when accessing an attribute inside a method, so it reads a local variable instead.
• Using a mutable class attribute for per-instance data, accidentally sharing it across objects.
• Confusing the class (the blueprint) with an instance (a built object).
• Calling a method without parentheses, which references the method instead of running it.
• Not defining __str__, leaving print output as an unhelpful memory address.

8Key Takeaways

The fundamentals of Python classes rest on a handful of ideas.

• A class is a blueprint; an object is an instance made from it.
• __init__ runs on creation and sets up the object's attributes.
• self refers to the current instance and is the first method parameter.
• Attributes hold data; methods define behavior.
• Class attributes are shared; instance attributes are per-object.

9Frequently Asked Questions

Q: What is the difference between a class and an object? A: A class is a blueprint that defines what data and behavior objects will have. An object is a concrete instance created from that class. One class can produce many objects, each with its own independent data.

Q: What does the __init__ method do? A: __init__ is the constructor. It runs automatically whenever you create a new object and is where you set up the object's initial attributes, such as self.name = name. It lets each new instance start with the right data.

Q: What is self in Python classes? A: self refers to the specific instance a method is operating on. It must be the first parameter of every instance method, and you use self. to read and set that object's attributes. Python passes it automatically when you call a method.

Q: Why is my attribute shared across all objects? A: You likely defined it as a class attribute in the class body rather than an instance attribute in __init__. Class attributes are shared by every instance, so per-object data — especially mutable data like lists — must be created inside __init__ with self.