Attributes of every kind

A constructor doesn't have to take every attribute as an argument. Some attributes have the same starting value in every new object: a new player has no points yet and no badges. The constructor sets those itself:

class Player:
    def __init__(self, name):
        self.name = name
        self.points = 0
        self.badges = []


ada = Player("Ada")
ben = Player("Ben")

ada.points += 15
ada.badges.append("first win")
ben.points += 5

print(ada.name, ada.points, ada.badges)
print(ben.name, ben.points, ben.badges)

Ada 15 ['first win']
Ben 5 []

A few things to notice here:

• You can change an attribute at any time, from outside the class too, like any variable: ada.points += 15.
• An attribute can hold any kind of value: a number, a string, a list, a dictionary, a date.
• Every object gets a list of its own. Each time the constructor runs, [] makes a new empty list, so appending to Ada's badges doesn't touch Ben's.

A date as an attribute

An attribute can hold an object such as a date, and then you can use that object's own attributes and methods through it. Read loan.borrowed.month from left to right: the loan's borrowed date, and that date's month.

from datetime import date


class Loan:
    def __init__(self, title, borrowed):
        self.title = title
        self.borrowed = borrowed
        self.returned = False


loan = Loan("Dune", date(2026, 9, 28))
print(loan.title, loan.borrowed, loan.returned)
print(loan.borrowed.month)

loan.returned = True
print(loan.returned)

Dune 2026-09-28 False
9
True

Values worked out in the constructor

The constructor can also calculate an attribute from its arguments, and a parameter can have a default value, as in any function:

class Room:
    def __init__(self, name, width, length, floor=1):
        self.name = name
        self.width = width
        self.length = length
        self.area = width * length
        self.floor = floor


kitchen = Room("kitchen", 3, 4)
attic = Room("attic", 5, 6, 3)

print(kitchen.name, kitchen.area, kitchen.floor)
print(attic.name, attic.area, attic.floor)

kitchen 12 1
attic 30 3

Be careful with calculated attributes: the area is worked out once, when the object is made. If the program later changes kitchen.width, kitchen.area keeps its old value. The next module shows a better way for values like this.