Python Classes and Objects

1Python Classes/Objects: overview

A class is a blueprint that describes what data a kind of thing holds and what it can do; an object is one concrete thing built from that blueprint, with its own copy of the data.

Before this lesson: Python Dictionaries, Python Functions

2Python Classes/Objects: diagram

Quick reference: Python Classes/Objects

• Python OOP Cheat Sheet: Classes, Inheritance, Dunder Methods — In Python OOP you define a `class` as a blueprint, build objects with it, store data in attributes set in `__…
• Python Function vs Method: What Is the Difference? — A **function** is a standalone block of code you call by name, like `len(items)`, while a **method** is a fun…
• Python @staticmethod vs @classmethod: What Is the Difference? — A `@classmethod` receives the **class itself** as its first argument (`cls`) and can create or configure inst…
• Python str() vs repr(): What Is the Difference? — `str()` returns a **readable** version of an object meant for users, while `repr()` returns an **unambiguous*…
• Python isinstance() Function — `isinstance(obj, classinfo)` returns `True` if `obj` is an instance of the class (or of any class in a tuple …

4Python Classes/Objects: syntax

class ClassName:
    shared = "value"                 # class attribute (shared by all objects)

    def __init__(self, a, b):        # runs when an object is created
        self.a = a                   # instance attribute
        self.b = b

    def method(self, x):             # self = the object it was called on
        return self.a + x

    def __str__(self):               # what print(obj) shows
        return f"ClassName({self.a}, {self.b})"

obj = ClassName(1, 2)                # create an object (an instance)
obj.a                                # read an attribute
obj.method(10)                       # call a method -> 11

5Python Classes/Objects: run the example

Press Run (or Ctrl + Enter). The output (step 6) appears directly below the code. Change anything and run it again.

class BankAccount:
    def __init__(self, owner, balance=0):
        self.owner = owner
        self.balance = balance

    def deposit(self, amount):
        self.balance += amount

    def __str__(self):
        return f"{self.owner}: ${self.balance}"

acct1 = BankAccount("Asha", 120)
acct2 = BankAccount("Ben")
acct1.deposit(30)
acct2.deposit(15)

print(acct1)
print(acct2)
print("Ben's balance:", acct2.balance)

Expected outputAsha: $150
Ben: $15
Ben's balance: 15

7Python Classes/Objects: how the example works

Hover or tap a step to highlight its lines in the example above.

1. Line 1class BankAccount: starts the blueprint. Defining a class does not create any accounts yet.
2. Line 2-4__init__ runs every time a new account is created. It stores the owner and the starting balance as attributes on self; balance=0 is a default value.
3. Line 6-7deposit is a method. self.balance += amount changes the balance of whichever object the method is called on.
4. Line 9-10__str__ returns the text that print() should show for an account.
5. Line 12-13Calling the class creates two separate objects. Python passes each new object to __init__ as self. Ben's account uses the default balance of 0.
6. Line 14-15In acct1.deposit(30), self is acct1, so only Asha's balance changes (to 150). Ben's account gets 15.
7. Line 17-18print() calls each object's __str__ method to get the text to display.
8. Line 19You can read an attribute directly with a dot: acct2.balance is the number 15.

8Python Classes/Objects: common mistakes

class Counter:
    def __init__(self):
        self.count = 0

    def increase():
        self.count += 1

clicks = Counter()
clicks.increase()

class Counter:
    def __init__(self):
        self.count = 0

    def increase(self):
        self.count += 1

clicks = Counter()
clicks.increase()
print(clicks.count)

class Player:
    def __init__(self, name):
        name = name
        score = 0

mia = Player("Mia")
print(mia.name)

class Player:
    def __init__(self, name):
        self.name = name
        self.score = 0

mia = Player("Mia")
print(mia.name)

class Team:
    members = []

    def add(self, name):
        self.members.append(name)

red = Team()
blue = Team()
red.add("Asha")
print(blue.members)

class Team:
    def __init__(self):
        self.members = []

    def add(self, name):
        self.members.append(name)

red = Team()
blue = Team()
red.add("Asha")
print(blue.members)

9Python Classes/Objects: exercises

Write your answer, then press Check. The checks run in your browser.

class Rectangle:
    # write __init__(self, width, height) that stores both values on self
    # write area(self) that returns width * height
    pass

room = Rectangle(4, 5)
print(room.area())

class Rectangle:
    def __init__(self, width, height):
        self.width = width
        self.height = height

    def area(self):
        return self.width * self.height

room = Rectangle(4, 5)
print(room.area())

class Counter:
    def __init__(self, start=0):
        self.value = start

    def click(self):
        self.value += 1
        return self.value

a = Counter()
b = Counter(10)
a.click()
a.click()
print(b.click())
print(a.value, b.value)

11
2 11

2 more exercises → 4 more examples

Further reading

• Classes
• dataclasses
• Data model

10Next lesson

Related:
Python Class Attributes vs Instance AttributesPython Encapsulation: Public, Protected and PrivatePython Dunder (Magic) Methods

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