3.1 Classes and objects: the basics

Big ideas

• A class is a blueprint; an object (instance) is one thing built from it: p = Point(3, 4). Each object has its own attributes (data) and shares the class’s methods (functions).
• __init__(self, ...) runs when an object is created and sets its attributes with self.x = x. self is the object the method is working on; Python passes it automatically.
• Methods are called with a dot: p.move(1, 2). Inside, self.x reads or changes that object’s state, so calling a method can change the object.
• __str__(self) returns the text print(p) shows. Other special methods: __eq__ for ==, __lt__ for sorting.
• Objects can be stored in lists and dictionaries and passed to functions. A list of objects plus a loop is the heart of most programs.

Code you should know

class BankAccount:
    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:
            return False
        self.balance -= amount
        return True

    def __str__(self):
        return f"{self.owner}: ${self.balance:.2f}"

acct = BankAccount("Nik", 50)
acct.deposit(25)
print(acct.withdraw(100), acct.withdraw(30))
print(acct)
print(acct.balance, acct.owner)

False True
Nik: $45.00
45 Nik

class Point:
    def __init__(self, x, y):
        self.x = x
        self.y = y

    def distance_to(self, other):
        return ((self.x - other.x) ** 2 + (self.y - other.y) ** 2) ** 0.5

    def __str__(self):
        return f"({self.x}, {self.y})"

points = [Point(0, 0), Point(3, 4), Point(6, 8)]
for p in points:
    print(p, p.distance_to(points[0]))
print(points[1].x + points[2].y)

(0, 0) 0.0
(3, 4) 5.0
(6, 8) 10.0
11

Worked example

class Counter:
    def __init__(self):
        self.n = 0
    def tick(self):
        self.n += 1
        return self.n

a = Counter()
b = Counter()
a.tick(); a.tick()
print(a.tick(), b.tick(), a.n + b.n)

Common mistakes

• Forgetting self in the method definition or writing x = x instead of self.x = x in __init__.
• Calling Point.move(1, 2) on the class instead of on an object.
• Returning a string from __str__ with print inside it, or forgetting the return.
• Sharing a mutable default such as a list between objects.