OOP-02 · Attributes and __init__

Attributes and __init__

Yesterday you made an object, then filled it in by hand — four lines each, and no warning if you forgot one. Today the class itself insists. One method, called automatically, gives every object its data at the moment it is created.

⏱ 1 hour🧩 Concept lesson📚 After OOP-01💻 Thonny, IDLE or VS Code

Learning Goals

3 min

By the end of this lesson you can:

Write an __init__ method so objects arrive complete.
Say what self is, and why it is the first parameter.
Explain the difference between self.name and name.
Give an attribute a default value, and calculate one from the others.

Warm-Up · The Half-Filled Object

5 min

Yesterday's style, with one line missed. Which line of output appears, and where does it stop?

class Student:
    """One pupil in the coding club."""


mia = Student()
mia.name = "Mia"
mia.score = 78

ana = Student()
ana.name = "Ana"

print(mia.name, mia.score)
print(ana.name, ana.score)

Answer

Output

Mia 78
Traceback (most recent call last):
AttributeError: 'Student' object has no attribute 'score'

Ana was created successfully with no score at all. Python only complained when something tried to read it — which in a real program might be days later.

The class should not allow a half-built student

Four lines per object is tedious. Worse, the class does not say what a student is, so nothing checks that you finished. Today's fix makes the data compulsory: no name and score, no object.

New Concept · The Method That Runs Itself

12 min

When you join the library, you do not get a card and fill it in later. You fill in the form and then they hand you the card. __init__ is that form.

class Student:
    """One pupil in the coding club."""

    def __init__(self, name, score):
        self.name = name
        self.score = score

It is an ordinary def — indented inside the class — with three special things:

The name. __init__ has two underscores on each side. Python calls it for you every time an object is made. Nobody types mia.__init__(...).
self. The first parameter is the brand-new object itself. Python passes it in automatically — you never write it at the call.
The assignments. self.name = name means "store the value that came in as an attribute on this object".

Now creating a student is one line, and the brackets that were empty yesterday finally have a job.

mia = Student("Mia", 78)
ana = Student("Ana", 65)

print(mia.name, mia.score)
print(ana.name, ana.score)

Output

Mia 78
Ana 65

Miss an argument and Python stops you immediately — TypeError: Student.__init__() missing 1 required positional argument: 'score'. A half-built student is no longer possible.

self.name and name are not the same thing

The one on the right is the parameter — a local, alive only while __init__ runs, exactly as you learned in FUN-05. The one on the left is an attribute on the object, which lives as long as the object does. They are usually spelled the same because that reads best, but the self. is what makes it stick.

Defaults work here too (FUN-04), and they are how you say "every new student starts with three lives".

class Student:
    """One pupil in the coding club."""

    def __init__(self, name, score=0, lives=3):
        self.name = name
        self.score = score
        self.lives = lives


new_pupil = Student("Hugo")
print(new_pupil.name, new_pupil.score, new_pupil.lives)

Output

Hugo 0 3

Not every attribute comes from a parameter. Some you work out on the spot:

class OrderLine:
    """One line on a canteen order."""

    def __init__(self, item, price, qty=1):
        self.item = item
        self.price = price
        self.qty = qty
        self.total = price * qty


line = OrderLine("Pizza", 4.50, 2)
print(line.item, line.total)

Output

Pizza 9.0

Worked Example · The Club List, Third Version

12 min

The same three pupils as yesterday. Count the lines — and count the ways to get it wrong.

class Student:
    """One pupil in the coding club."""

    def __init__(self, name, form, score=0, lives=3):
        self.name = name
        self.form = form
        self.score = score
        self.lives = lives


mia = Student("Mia", "2 Ceria", 78)
wei_jie = Student("Leo", "2 Ceria", 91)
ana = Student("Ana", "1 Bestari", 65)
hugo = Student("Hugo", "1 Bestari")

print(mia.name, "of", mia.form, "has", mia.score, "points")
print(wei_jie.name, "of", wei_jie.form, "has", wei_jie.score, "points")
print(ana.name, "of", ana.form, "has", ana.score, "points")
print(hugo.name, "of", hugo.form, "has", hugo.score, "points")

# Mia wins a round
mia.score = mia.score + 10
print("After the round:", mia.name, mia.score)
print("Unchanged:", wei_jie.name, wei_jie.score)

Output

Mia of 2 Ceria has 78 points
Leo of 2 Ceria has 91 points
Ana of 1 Bestari has 65 points
Hugo of 1 Bestari has 0 points
After the round: Mia 88
Unchanged: Leo 91

Hugo joined without a score, and the default gave him 0 rather than a missing attribute. Mia's ten points went to Mia alone.

Read the two moments

class Student: and its def describe — nothing runs. Student("Mia", "2 Ceria", 78) creates a blank object, hands it to __init__ as self, runs the four assignments, and gives the finished object back to be stored in mia. That is the whole life cycle, and it happens once per pupil.

Try breaking it deliberately: call Student("Nia") with no form. The error arrives at the moment of creation, in the line you wrote — not three screens later.

Try It Yourself

13 min

01 🟢 The drink, upgraded

Rewrite yesterday's Drink class with an __init__ that takes name, price, and iced defaulting to False. Make three drinks in three lines and print them.

Hint

class Drink:
    """One drink on the stall menu."""

    def __init__(self, name, price, iced=False):
        self.name = name
        self.price = price
        self.iced = iced


cocoa_ais = Drink("Cocoa", 3.00, True)

02 🟡 A stall item that does its own arithmetic

Write a class StallItem whose __init__ takes name, price and qty (default 1), and sets two more attributes without being asked:

self.total — price times quantity.
self.label — a string such as "2 x Hot dog", built with an f-string.

Make three items and print label and total for each. Then change one item's qty after creation and print its total again. It will be wrong — write one sentence explaining why. (Lesson 3 gives you the tool to fix it.)

Mini-Challenge 🔥 · Debug: The Constructor That Never Runs

8 min

Ella's player class crashes on the very first object. There are three mistakes in six lines. Find them all, then say what error each one causes.

class Player:
    """One player in the game."""

    def _init_(name, hp):
        self.name = name
        hp = hp
        self.lives = 3


hero = Player("Felix", 100)
print(hero.name, "has", hero.hp, "HP")

Answer

Single underscores. _init_ is just an ordinary method with an odd name. Python never calls it, so Player("Felix", 100) raises TypeError: Player() takes no arguments. It must be __init__ — two each side.
No self. The first parameter of every method is the object. Without it, name would receive the object and self would not exist at all.
hp = hp stores nothing. It assigns the parameter to itself and throws it away when the method ends. It must be self.hp = hp, which is why hero.hp would fail even after the first two fixes.

The working version:

class Player:
    """One player in the game."""

    def __init__(self, name, hp):
        self.name = name
        self.hp = hp
        self.lives = 3


hero = Player("Felix", 100)
print(hero.name, "has", hero.hp, "HP")

Output

Felix has 100 HP

Recap

3 min

__init__ is a method Python runs for you the moment an object is created. Its first parameter, self, is that new object, and self.something = value is how data gets attached for good. Parameters may have defaults, and an attribute can be calculated rather than passed in. The class now states what one of its objects is — so a half-built object stops being possible.

Vocabulary Card

__init__: The constructor: the method Python calls automatically when an object is made.
self: The object the method is working on. Always the first parameter, never passed by you.
dunder: Short for "double underscore" — a name like __init__ that Python treats specially.
instance attribute: A value stored on one object with self., separate from every other object's.

Homework

4 min

Take the class you wrote for OOP-01 homework and give it a proper __init__.

At least four attributes.
At least one with a default value, used both ways somewhere.
At least one calculated in __init__ rather than passed in.
Create three objects in three lines, and print a tidy line for each.

Hint: good calculated attributes — a total from price and quantity, a label from two other attributes, a starting value that is always the same.

Sample · `rak_buku.py`

# rak_buku.py - the same shelf, now with a constructor

class Book:
    """One book on my shelf."""

    def __init__(self, title, author, price, read=False):
        self.title = title
        self.author = author
        self.price = price
        self.read = read
        self.label = f"{title} by {author}"


sejarah = Book("Sejarah Tingkatan 2", "Kementerian Pendidikan", 12.90, True)
komik = Book("Lat: Village Boy", "Lat", 24.00, True)
python_book = Book("Python for Kids", "Jason Briggs", 79.90)

print(sejarah.label, "- $", sejarah.price, "- read:", sejarah.read)
print(komik.label, "- $", komik.price, "- read:", komik.read)
print(python_book.label, "- $", python_book.price, "- read:", python_book.read)

Two books pass read=True, one takes the default. label is calculated, not asked for. Your attributes will be different — what matters is that every object is complete the moment it exists.

Attributes and __init__

⏱ 1 hour🧩 Concept lesson📚 After OOP-01💻 Thonny, IDLE or VS Code

class Student: """One pupil in the coding club.""" def __init__(self, name, score=0, lives=3): self.name = name self.score = score self.lives = lives new_pupil = Student("Hugo") print(new_pupil.name, new_pupil.score, new_pupil.lives)

class OrderLine: """One line on a canteen order.""" def __init__(self, item, price, qty=1): self.item = item self.price = price self.qty = qty self.total = price * qty line = OrderLine("Pizza", 4.50, 2) print(line.item, line.total)

class Student: """One pupil in the coding club.""" def __init__(self, name, form, score=0, lives=3): self.name = name self.form = form self.score = score self.lives = lives mia = Student("Mia", "2 Ceria", 78) wei_jie = Student("Leo", "2 Ceria", 91) ana = Student("Ana", "1 Bestari", 65) hugo = Student("Hugo", "1 Bestari") print(mia.name, "of", mia.form, "has", mia.score, "points") print(wei_jie.name, "of", wei_jie.form, "has", wei_jie.score, "points") print(ana.name, "of", ana.form, "has", ana.score, "points") print(hugo.name, "of", hugo.form, "has", hugo.score, "points") # Mia wins a round mia.score = mia.score + 10 print("After the round:", mia.name, mia.score) print("Unchanged:", wei_jie.name, wei_jie.score)

class Drink: """One drink on the stall menu.""" def __init__(self, name, price, iced=False): self.name = name self.price = price self.iced = iced cocoa_ais = Drink("Cocoa", 3.00, True)

class Player: """One player in the game.""" def _init_(name, hp): self.name = name hp = hp self.lives = 3 hero = Player("Felix", 100) print(hero.name, "has", hero.hp, "HP")

class Player: """One player in the game.""" def __init__(self, name, hp): self.name = name self.hp = hp self.lives = 3 hero = Player("Felix", 100) print(hero.name, "has", hero.hp, "HP")

Attributes and `init`

Learning Goals

Warm-Up · The Half-Filled Object

New Concept · The Method That Runs Itself

Worked Example · The Club List, Third Version

Try It Yourself

Mini-Challenge 🔥 · Debug: The Constructor That Never Runs

Recap

Vocabulary Card

Homework

Sample · `rak_buku.py`

Attributes and `init`

Learning Goals

Warm-Up · The Half-Filled Object

New Concept · The Method That Runs Itself

Worked Example · The Club List, Third Version

Try It Yourself

Mini-Challenge 🔥 · Debug: The Constructor That Never Runs

Recap

Vocabulary Card

Homework

Sample · `rak_buku.py`

Learning Goals

Warm-Up · The Half-Filled Object

New Concept · The Method That Runs Itself

Worked Example · The Club List, Third Version

Try It Yourself

Mini-Challenge 🔥 · Debug: The Constructor That Never Runs

Recap

Vocabulary Card

Homework

Sample · rak_buku.py

Learning Goals

Warm-Up · The Half-Filled Object

New Concept · The Method That Runs Itself

Worked Example · The Club List, Third Version

Try It Yourself

Mini-Challenge 🔥 · Debug: The Constructor That Never Runs

Recap

Vocabulary Card

Homework

Sample · rak_buku.py

Sample · `rak_buku.py`

Sample · `rak_buku.py`