__init__ method behavior in Python - Step-by-Step Execution

Concept Flow - __init__ method behavior

Create object instance

↓

Call __init__ method

↓

Initialize attributes

↓

Return new object

↓

Object ready to use

When you create a new object, Python calls __init__ to set up initial values inside the object.

Execution Sample

Python

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

d = Dog('Buddy')
print(d.name)

This code creates a Dog object with a name and prints the name.

Execution Table

Step	Action	Evaluation	Result
1	Create Dog object with 'Buddy'	Calls Dog.__init__(self, 'Buddy')	New Dog object created
2	Inside __init__, assign self.name = 'Buddy'	self.name set to 'Buddy'	Object attribute name initialized
3	Return from __init__	No return value (returns None)	Object fully initialized
4	Print d.name	Access attribute d.name	Output: Buddy

💡 Object created and initialized; program ends after printing name

Variable Tracker

Variable	Start	After Step 2	Final
self.name	undefined	'Buddy'	'Buddy'
d	undefined	Dog object created	Dog object with name='Buddy'

Key Moments - 2 Insights

Why do we use self.name inside __init__ instead of just name?

self.name stores the value inside the object so it can be used later; 'name' alone is just a temporary input. See execution_table step 2 where self.name is assigned.

Does __init__ return the new object?

__init__ does not return anything; Python creates the object first and then calls __init__ to initialize it. See execution_table step 3.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of self.name after step 2?

A None

B 'Buddy'

C undefined

D 'Dog'

💡 Hint

Check variable_tracker row for self.name after step 2

At which step does the __init__ method finish running?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

See execution_table where __init__ returns with no value

If we create Dog('Max') instead, what changes in the variable_tracker?

A d becomes None

B self.name stays 'Buddy'

C self.name becomes 'Max' instead of 'Buddy'

D No change at all

💡 Hint

Look at how self.name is assigned in step 2 and variable_tracker

Concept Snapshot

__init__ is a special method called when creating an object.
It sets up initial attributes using self.
self.name = value stores data inside the object.
__init__ does not return the object; Python does that.
Use __init__ to prepare your object for use.

Full Transcript

When you create a new object in Python, the __init__ method runs automatically. It sets up the object's attributes using the self keyword. For example, in the Dog class, __init__ takes a name and saves it as self.name. This means the object remembers its name. The __init__ method itself does not return the object; Python creates the object first and then calls __init__ to initialize it. After __init__ finishes, the object is ready to use. This process is shown step-by-step in the execution table and variable tracker.

Practice

1.

What is the main purpose of the __init__ method in a Python class?

easy

A. To create a new class

B. To delete an object from memory

C. To print information about the class

D. To set up initial values for the object's attributes

Solution

1. Step 1: Understand the role of __init__

   The __init__ method runs automatically when an object is created to set up initial values.

2. Step 2: Compare options with this role

   Only To set up initial values for the object's attributes describes setting initial attribute values, which matches __init__'s purpose.

3. Final Answer:

   To set up initial values for the object's attributes -> Option D

4. Quick Check:

   __init__ sets initial values = A [OK]

Hint: Remember: __init__ sets starting info for objects [OK]

Common Mistakes:

• Thinking __init__ deletes objects
• Confusing __init__ with printing methods
• Believing __init__ creates classes

2.

Which of the following is the correct way to define an __init__ method inside a Python class?

class Car:
    ?

easy

A. def __init__(self, model):

B. def init(self, model):

C. def __start__(self, model):

D. def __init__(model):

Solution

1. Step 1: Check method name and parameters

   The method must be named exactly __init__ and include self as the first parameter.

2. Step 2: Compare options

   Only def __init__(self, model): uses the correct name and includes self properly.

3. Final Answer:

   def __init__(self, model): -> Option A

4. Quick Check:

   Correct __init__ syntax = D [OK]

Hint: Always include self as first parameter in __init__ [OK]

Common Mistakes:

• Omitting self parameter
• Misspelling __init__ method name
• Using wrong method names like init or __start__

3.

What will be the output of this code?

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

my_dog = Dog("Buddy")
print(my_dog.name)

medium

A. Buddy

B. name

C. Dog

D. Error

Solution

1. Step 1: Understand object creation and attribute assignment

   The __init__ method sets self.name to "Buddy" when my_dog is created.

2. Step 2: Check the print statement

   Printing my_dog.name outputs the string "Buddy" stored in the attribute.

3. Final Answer:

   Buddy -> Option A

4. Quick Check:

   Object attribute value = Buddy [OK]

Hint: Print object.attribute to see stored value from __init__ [OK]

Common Mistakes:

• Printing class name instead of attribute
• Expecting attribute name instead of value
• Assuming code causes error

4.

Find the error in this class definition:

class Person:
    def __init__(name):
        self.name = name

p = Person("Alice")

medium

A. Wrong class name

B. Missing self parameter in __init__

C. Missing parentheses when creating object

D. No error

Solution

1. Step 1: Check __init__ method parameters

   The __init__ method must have self as the first parameter, but it is missing here.

2. Step 2: Identify impact of missing self

   Without self, self.name causes an error because self is undefined.

3. Final Answer:

   Missing self parameter in __init__ -> Option B

4. Quick Check:

   __init__ needs self first [OK]

Hint: Always put self as first __init__ parameter [OK]

Common Mistakes:

• Forgetting self in method parameters
• Thinking class name must change
• Missing parentheses when creating object

5.

Consider this class:

class Book:
    def __init__(self, title, author="Unknown"):
        self.title = title
        self.author = author

b1 = Book("Python 101")
b2 = Book("Learn AI", "Dr. Smith")
print(b1.author, b2.author)

What will be printed?

hard

A. Dr. Smith Unknown

B. Python 101 Learn AI

C. Unknown Dr. Smith

D. Error due to missing author argument

Solution

1. Step 1: Understand default parameter in __init__

   The author parameter has a default value "Unknown", so it is optional when creating an object.

2. Step 2: Analyze object creations

   b1 is created with only title, so author is "Unknown". b2 provides both title and author "Dr. Smith".

3. Step 3: Check print output

   Printing b1.author and b2.author prints "Unknown Dr. Smith".

4. Final Answer:

   Unknown Dr. Smith -> Option C

5. Quick Check:

   Default parameters fill missing values = A [OK]

Hint: Default values fill missing arguments in __init__ [OK]

Common Mistakes:

• Expecting error when argument is missing
• Mixing up title and author values
• Assuming both arguments are always required