Constructor parameters in Python - Step-by-Step Execution

Concept Flow - Constructor parameters

Start object creation

↓

Call __init__ with parameters

↓

Assign parameters to attributes

↓

Object ready with attributes set

↓

Use object

When creating an object, parameters are passed to the constructor (__init__) to set up initial values.

Execution Sample

Python

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

my_dog = Dog("Buddy", 3)
print(my_dog.name, my_dog.age)

This code creates a Dog object with name and age set by constructor parameters, then prints them.

Execution Table

Step	Action	Parameters Passed	Attribute 'name'	Attribute 'age'	Output
1	Call Dog constructor	name='Buddy', age=3	Not set	Not set
2	Inside __init__: assign self.name = name	name='Buddy', age=3	Buddy	Not set
3	Inside __init__: assign self.age = age	name='Buddy', age=3	Buddy	3
4	Constructor returns, object created	name='Buddy', age=3	Buddy	3
5	Print my_dog.name and my_dog.age		Buddy	3	Buddy 3

💡 Constructor finishes, object has attributes set, program prints values.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
self.name	Not set	Buddy	Buddy	Buddy
self.age	Not set	Not set	3	3
name (parameter)	N/A	Buddy	Buddy	N/A
age (parameter)	N/A	3	3	N/A

Key Moments - 2 Insights

Why do we assign parameters to self.name and self.age inside __init__?

Because parameters are temporary values passed in; assigning them to self.name and self.age stores them as object attributes for later use, as shown in steps 2 and 3 of the execution_table.

What happens if we forget to assign a parameter to self.attribute?

The object won't have that attribute set, so trying to access it later will cause an error or show 'Not set', as seen before step 2 in the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of self.age after step 3?

A 3

B Not set

C Buddy

D None

💡 Hint

Check the 'Attribute age' column in row for step 3 in execution_table.

At which step does the constructor finish creating the object?

A Step 2

B Step 5

C Step 4

D Step 3

💡 Hint

Look for the row mentioning 'Constructor returns' in execution_table.

If we change the parameter name to 'Max', what will be printed at step 5?

A Buddy 3

B Max 3

C None 3

D 3 Max

💡 Hint

Refer to variable_tracker for self.name and the print output in execution_table step 5.

Concept Snapshot

Constructor parameters are inputs given when creating an object.
Inside __init__, these parameters are assigned to self attributes.
This sets up the object's initial state.
Without assignment, attributes won't exist.
Use self.attribute = parameter to store values.

Full Transcript

When you create an object in Python, you often want to give it some starting information. This is done by passing parameters to the constructor method called __init__. Inside __init__, these parameters are assigned to the object's attributes using self.attribute = parameter. This way, the object remembers these values. For example, if you create a Dog with a name and age, you pass those values to __init__, and it saves them as self.name and self.age. Later, you can use these attributes to get information about the object. If you forget to assign a parameter to self.attribute, the object won't have that attribute, and trying to use it will cause an error. The execution table shows each step: calling the constructor, assigning attributes, finishing creation, and printing the values. The variable tracker shows how self.name and self.age change from not set to their final values. This helps you understand how constructor parameters work step-by-step.