How to Create a Class in Python

What you’ll build or solve

You’ll create a Python class with attributes and methods, then instantiate it to make objects you can use in your code.

When this approach works best

This approach works best when you:

• Keep passing around related data that belongs together, like a user’s name, email, and role.
• Need reusable behavior tied to data, like Cart.add_item() or Timer.start().
• Want a clean way to model real things in your app, like Student, Invoice, or GameCharacter.

Avoid this approach when:

• You only need a simple container for values. A dictionary or tuple may be simpler.

Prerequisites

• Python installed

Step-by-step instructions

1) Define a class with class

A class definition starts with class, a name in CapWords style, and a colon. The class body is indented.

classUser:
pass

Create an object (an instance) by calling the class:

user=User()
print(user)

What to look for: if the class body is not indented, you will get an IndentationError.

2) Add __init__ to store data on the instance

__init__ runs when you create a new instance. Use it to set attributes on self.

classUser:
def__init__(self,name,email):
self.name=name
self.email=email

user=User("Mina","mina@example.com")
print(user.name)
print(user.email)

What to look for: you do not pass self when you call User(...). Python passes it automatically.

3) Add methods for behavior and state changes

Methods are functions defined inside a class. They take self first so they can access and update instance data.

classCounter:
def__init__(self,start=0):
self.value=start

defincrement(self):
self.value+=1
returnself.value

counter=Counter()
print(counter.increment())
print(counter.increment())

You can still update attributes directly, but methods help keep changes consistent:

counter.value=10
print(counter.increment())

What to look for: if you forget self in a method definition, calling it usually raises a TypeError.

4) Create and use multiple instances

Each instance has its own separate data. Creating more than one instance is a quick way to confirm your class works as intended.

classUser:
def__init__(self,name):
self.name=name

defgreet(self):
returnf"Hi,{self.name}!"

a=User("Mina")
b=User("Sam")

print(a.greet())
print(b.greet())

Each object keeps its own name.

Examples you can copy

Example 1: A simple class for holding data

classProduct:
def__init__(self,name,price_eur):
self.name=name
self.price_eur=price_eur

p=Product("Notebook",4.5)
print(p.name,p.price_eur)

Example 2: A class with methods that update state

classBankAccount:
def__init__(self,owner,balance=0):
self.owner=owner
self.balance=balance

defdeposit(self,amount):
self.balance+=amount
returnself.balance

defwithdraw(self,amount):
ifamount>self.balance:
return"Insufficient funds"
self.balance-=amount
returnself.balance

acct=BankAccount("Mina",50)
print(acct.deposit(25))
print(acct.withdraw(60))
print(acct.withdraw(10))

Example 3: A class that validates input

classTemperature:
def__init__(self,celsius):
ifcelsius<-273.15:
raiseValueError("Temperature cannot be below absolute zero.")
self.celsius=celsius

deffahrenheit(self):
return (self.celsius*9/5)+32

t=Temperature(20)
print(t.fahrenheit())

Example 4: Use inheritance to extend a class

Use inheritance when you have a clear “is-a” relationship.

classAnimal:
def__init__(self,name):
self.name=name

defspeak(self):
return"..."

classDog(Animal):
defspeak(self):
return"Woof!"

pet=Dog("Rex")
print(pet.name,pet.speak())

Dog is an Animal, so it inherits name and overrides speak().

Common mistakes and how to fix them

Mistake 1: Forgetting self in a method

You might write:

classGreeter:
defhello():
return"Hello"

Why it breaks: Python passes the instance automatically when you call a method, but your method does not accept it.

Correct approach:

classGreeter:
defhello(self):
return"Hello"

g=Greeter()
print(g.hello())

Mistake 2: Setting attributes without self

You might write:

classUser:
def__init__(self,name):
name=name

Why it breaks: you assign to a local variable, not to the instance. The instance never gets a name attribute.

Correct approach:

classUser:
def__init__(self,name):
self.name=name

u=User("Ada")
print(u.name)

Troubleshooting

• If you see TypeError: __init__() missing ..., pass the required arguments when creating the instance.
• If you see AttributeError: object has no attribute ..., check you set the attribute on self in __init__.
• If you see TypeError: ... takes 0 positional arguments but 1 was given, you forgot self in a method definition.
• If you see IndentationError, indent class and method bodies consistently, usually 4 spaces.
• If you see unexpected shared data between instances, avoid using mutable default values like items=[] in __init__. Use None and set a new list inside the method instead.

Quick recap

• Define a class with class Name: and indent the body.
• Add __init__ to store data on self.
• Add methods that take self first to use and update instance data.
• Create instances by calling the class like Name(...).
• Use inheritance only when you have a clear “is-a” relationship.