Purpose of constructors in Python - Step-by-Step Execution

Concept Flow - Purpose of constructors

Create object

↓

Call constructor __init__

↓

Initialize attributes

↓

Object ready to use

When you create an object, Python automatically calls the constructor __init__ to set up initial values.

Execution Sample

Python

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

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

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

Execution Table

Step	Action	Evaluation	Result
1	Create Dog object with name 'Buddy'	Call Dog.__init__(self, 'Buddy')	Object created
2	Constructor finishes	Object initialized	my_dog.name = 'Buddy'
3	Print my_dog.name	Access attribute	Output: Buddy

💡 Constructor __init__ completes, object is ready with initialized attributes

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
self.name	undefined	'Buddy'	'Buddy'	'Buddy'
my_dog	undefined	object created	object initialized	object with name 'Buddy'

Key Moments - 2 Insights

Why do we need the __init__ method?

The __init__ method sets up the object with initial values when it is created, as shown in step 1 of the execution_table.

Is __init__ called automatically or do we call it ourselves?

__init__ is called automatically by Python when we create the object, as seen in step 1 where Dog('Buddy') calls __init__.

Visual Quiz - 3 Questions

Look at the execution_table, what value does self.name have after step 1?

A undefined

B None

C 'Buddy'

D Empty string

💡 Hint

Check the variable_tracker row for self.name after Step 1

At which step is the object fully initialized and ready to use?

A Step 2

B Step 1

C Step 3

D Before Step 1

💡 Hint

Look at the execution_table row where constructor finishes and object is initialized

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

A Constructor is not called

B self.name is set to 'Max' instead of 'Buddy'

C my_dog.name remains 'Buddy'

D Output is 'Buddy'

💡 Hint

The constructor sets self.name to the argument passed, see step 1 in execution_table

Concept Snapshot

Constructor (__init__) is a special method called automatically when an object is created.
It initializes the object's attributes with starting values.
You define __init__ inside a class with self and parameters.
When you create an object, __init__ runs to set it up.
This makes objects ready to use right after creation.

Full Transcript

When you create an object in Python, the constructor method __init__ is called automatically. This method sets up the object's initial state by assigning values to its attributes. For example, when we create a Dog object with a name, __init__ sets the name attribute. This happens before you use the object. The execution table shows step by step how the object is created, __init__ runs, and the attribute is set. This is why constructors are important: they prepare the object for use immediately after creation.