20-classes-and-objects

20 · Classes & Objects

Part 5 — Object-Oriented Programming · Estimated time: 35–45 min · Prerequisite: 13 · Functions

Object-Oriented Programming (OOP) lets you model real-world things — a dog, a bank account, a phone — in code. A class is a blueprint; an object (or instance) is a specific thing built from that blueprint. Classes bundle data (attributes) and behaviour (methods) together.

What you'll learn

• The idea of a class (blueprint) vs an object (instance)
• Defining a class with __init__, and what self means
• Attributes (data) and methods (behaviour)
• Creating many independent objects from one class
• Making objects print nicely with __str__

---

1. Your first class

class Dog:
    def __init__(self, name):    # the constructor — runs when a Dog is created
        self.name = name         # store data on THIS object via self

    def bark(self):              # a method: a function that belongs to the class
        return f"{self.name} says Woof!"

rex = Dog("Rex")        # create (instantiate) an object
fido = Dog("Fido")

print(rex.name)         # Rex
print(rex.bark())       # Rex says Woof!
print(fido.bark())      # Fido says Woof!

self is how an object refers to itself. You never pass it in by hand — Python fills it in. Class names use CamelCase by convention.

▶️ Run it: python examples/01_first_class.py

---

2. Attributes and methods

Attributes describe what an object is; methods describe what it does.

class Mobile:
    def __init__(self, brand, price):
        self.brand = brand        # attributes
        self.price = price

    def purchase(self):           # method
        discount = 10 if self.brand == "Apple" else 5
        final = self.price - self.price * discount / 100
        return f"{self.brand} costs {final} after discount"

phone = Mobile("Apple", 1000)
print(phone.brand)        # Apple
print(phone.purchase())   # Apple costs 900.0 after discount

▶️ Run it: python examples/02_attributes_and_methods.py

💡 Try it yourself: add a describe() method to Mobile that returns "Apple phone, $1000".

---

3. Many independent objects

Each object built from a class has its own data. Changing one doesn't affect the others.

class Counter:
    def __init__(self):
        self.value = 0
    def increment(self):
        self.value += 1

a = Counter()
b = Counter()
a.increment(); a.increment()
b.increment()
print("a:", a.value)   # 2
print("b:", b.value)   # 1  (completely independent)

▶️ Run it: python examples/03_multiple_objects.py

---

4. Printing objects with __str__

By default, printing an object shows something cryptic like <__main__.Book object at 0x...>. Define __str__ to control what print shows.

class Book:
    def __init__(self, title, author):
        self.title = title
        self.author = author
    def __str__(self):
        return f"{self.title} by {self.author}"

print(Book("Python 101", "Guido"))   # Python 101 by Guido

▶️ Run it: python examples/04_str_method.py

---

Common mistakes

Mistake	What happens	Fix
Forgetting self in a method's parameters	TypeError about arguments	Every method's first parameter is self.
Forgetting self. when storing data	the value is lost after __init__	Save it as self.name = name.
Calling a method without ()	you get the method object, not its result	Call it: rex.bark().
Confusing the class with an instance	Dog.bark() needs an object	Make one first: Dog("Rex").bark().

---

Recap / cheat-sheet

class Thing:
    def __init__(self, x):   # constructor
        self.x = x           # attribute on the instance

    def method(self):        # behaviour; self = this object
        return self.x

    def __str__(self):       # how print() shows it
        return f"Thing({self.x})"

obj = Thing(5)               # create an instance
obj.x                        # read an attribute
obj.method()                 # call a method

---

Exercises

Run a file with python exercises/<file>.py, then compare with the matching file in solutions/.

1. exercises/01_make_a_dog.py — define a class with a method.
2. exercises/02_rectangle_class.py — store attributes and compute from them.
3. exercises/03_book_str.py — add a __str__ method.

Try each yourself before opening the solution.

---

Next → 21 · Encapsulation (coming soon)