Lesson 13, by Codesarray, 13 October 2026, 4 min read

Python classes for beginners: init, self, methods and @dataclass

Build your own type with class, store data on it with __init__ and self, give it behaviour with methods, print it clearly with __repr__, and see the @dataclass shortcut.

So far you have used types that Python gave you: int, str, list, dict. By the end of this page you will build your own type with class, give it data and behaviour, and know the shortcut Python offers for simple classes.

What a class is

A class is a blueprint for your own type. class Dog: creates a new kind of thing. Calling Dog("Rex", 3) builds one real dog from the blueprint, and that dog is called an instance. You can build as many instances as you like, and each one keeps its own data.

init, self and attributes

When you call Dog("Rex", 3), Python builds an empty object and then runs a special method named __init__ to set it up. Its first parameter is self, the new object. Inside it, self.name = name stores the name on that object. Values stored on an object this way are its attributes.

class Dog:
    def __init__(self, name, age):
        self.name = name
        self.age = age

Because every instance has its own self, two dogs from the same class do not share their names or ages.

Methods

A method is a function that lives inside a class. It always takes self first, so it knows which object it was called on. When you write rex.bark(), Python passes rex in as self.

    def bark(self):
        return f"{self.name} says woof"

repr

By default, printing an object shows something unhelpful, such as <__main__.Dog object at 0x...>, which is a memory address. Add a __repr__ method to decide how the object prints. Inside the f-string, {self.name!r} formats the name with repr(), so a string keeps its quotes and the result looks like the code that would build the object.

    def __repr__(self):
        return f"Dog({self.name!r}, {self.age})"

The @dataclass shortcut

Writing __init__ and __repr__ by hand gets repetitive. For classes that mostly hold data, Python has a decorator in the standard library. List the fields with their types, and Python writes __init__ and __repr__ for you.

from dataclasses import dataclass


@dataclass
class Dog:
    name: str
    age: int

Dog("Rex", 3) now prints as Dog(name='Rex', age=3). That is four lines instead of seven for the same object.

The whole program, line by line

class Dog:
    def __init__(self, name, age):
        self.name = name
        self.age = age

    def bark(self):
        return f"{self.name} says woof"

    def __repr__(self):
        return f"Dog({self.name!r}, {self.age})"


rex = Dog("Rex", 3)
print(rex.bark())
print(rex.age)
print(rex)

Rex says woof
3
Dog('Rex', 3)

Line 1 defines the Dog class. Line 2 is __init__, taking a name and an age. Lines 3 and 4 store them on self. Line 6 is the bark method, and line 7 builds a string from the name. Line 9 is __repr__, and line 10 returns how a dog should print. Line 13 creates a dog called Rex, aged 3. Line 14 calls bark. Line 15 prints the age attribute. Line 16 prints the dog itself, using __repr__.

What you learned

A class is a blueprint, and calling it builds an instance.
__init__ sets up each new object, and self is the object itself.
Values stored with self.name = name are attributes.
A method is a function inside a class that takes self first.
__repr__ decides how an object prints, and @dataclass writes __init__ and __repr__ for you.

Common questions

What is the difference between a class and an instance? The class is the blueprint, Dog. An instance is one object built from it, such as rex = Dog("Rex", 3). One class can have many instances.

Why does every method take self? Python passes the object you called the method on as the first argument. In rex.bark(), self is rex, which is how the method finds self.name.

What happens if I forget an argument? __init__ is called with whatever you pass, so a missing one is an error. Dog() raises TypeError: Dog.__init__() missing 2 required positional arguments: 'name' and 'age'.

**What does !r do in an f-string?** It formats the value with repr() instead of str(). For a string, that keeps the quotes: 'Rex' instead of Rex.

When should I use @dataclass? When a class mostly holds values. It writes __init__ and __repr__ for you, and two dataclass instances with the same values compare as equal with ==.

This lesson follows on from Python modules, import and uv. Next in the series: the beginner capstone, a to-do list command line app.

This article goes with the video Classes. Watch it on YouTube.

Python classes for beginners: __init__, self, methods and @dataclass