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OOP – init Method

__init__ is called automatically when an object is created and sets up its initial attributes

What it does: __init__ runs automatically when Dog('Rex') is called. self.name stores the value. Once set in __init__, the attribute is accessible on the object as d.name. Dog('Rex') creates the object and immediately calls __init__ with 'Rex'. Python uses self (not this) — it's the reference to the object being created. Dunder (double-underscore) methods always have two underscores on each side. Parameters after self become the arguments you pass when creating the object. Without __init__ you can still create objects — they just won't have initial attributes. __init__ must take 'self' as its first parameter. Without it, Python passes the instance and causes a TypeError.

Common mistakes: Missing self as the first parameter. Need DOUBLE underscores: __init__ not _init_.

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Learning

Duo

What does __init__ do?

Initializes a new object automaticallyDeletes the object

Matches

❌ class Dog: def __init__(name): self.name = name

↓

Error: First param must be 'self'

Applying

Meanings

Term

__init__

Meaning

Constructor method that runs automatically when an object is created.

Practice

class Dog:

Testing

Gaps

def (self, name):

Check Answer

Spots

1class Dog:

2 def __init__(name):

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Quick Questions

1. What does __init__ do?

A. Initializes a new object automatically ✓

B. Deletes the object

__init__ sets up the object's initial state — it's called automatically on creation

2. When does __init__ run?

A. When an object is deleted

B. When an object is created ✓

Dog('Rex') creates the object and immediately calls __init__ with 'Rex'

3. What must be the first parameter of __init__?

A. self ✓

B. this

Python uses self (not this) — it's the reference to the object being created

4. class Dog:\n def __init__(self, name):\n self.name = name\nDog('Rex') what is self.name?

A. 'name'

B. 'Rex' ✓

self.name = name stores the argument — so self.name holds 'Rex', not the string 'name'

5. Do you pass 'self' when creating an object?

A. No Python passes it automatically ✓

B. Yes

Python injects self automatically — Dog('Rex') calls __init__(dog_instance, 'Rex')

Match the Pairs

class Dog: def __init__(name): self.name = name → Error: First param must be 'self'

class Dog: def __init__(self, name): self.name = name Dog() → Error: Missing required arg 'name'

class Dog: def __init__(self, name): self.name = name → Valid: __init__ with parameter

d = Dog('Rex') print(d.name) → Ok: Access attribute after creation

class Dog: def __init__(self,n): self.name=n d=Dog('Rex') print(d.name) → Output: Rex

class Point: def __init__(self,x,y): self.x=x self.y=y p=Point(3,4) print(p.x) → Output: 3

Key Term

__init__

Constructor method that runs automatically when an object is created.~~self

Fill in the Blanks

1. class Dog: def __init__(self, name): self.name = name

2. class Dog: def __init__(self): self.name = name

3. class Dog: def __init__(self, name): self.name = name

4. dog = Dog('Rex') print(dog.name)

5. class Person: def __init__(self, name, age): self.age = age

Spot the Error

Question 1

A class Dog:

B def __init__(name):

C self.name = name

Line B — Missing self as the first parameter

Question 2

A class Dog:

B def _init_(self, name):

C self.name = name

Line B — Need DOUBLE underscores: __init__ not _init_

Question 3

A class Dog:

B def __init__(self, name):

C name = name

Line C — Must use self.name to save on the object. name alone is just a local variable.

Question 4

A class Dog:

B def __init__(self, name):

C self.name = name

E dog = Dog()

F print(dog.name)

Line E — __init__ expects a name argument Dog() without it causes TypeError

Question 5

A class Dog:

B def __init__(self, name):

C self.name = name

E dog = Dog('Rex', self)

Line E — Don't pass self Python does it automatically

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