Default values in constructors in Python - Step-by-Step Execution

Concept Flow - Default values in constructors

Start object creation

↓

Call constructor with args?

↓

Use given

↓

Set attributes

↓

Object ready

When creating an object, the constructor uses given values or defaults if none are provided.

Execution Sample

Python

class Car:
    def __init__(self, color='red', wheels=4):
        self.color = color
        self.wheels = wheels

car1 = Car()
car2 = Car('blue', 6)

Creates two Car objects, one with default values and one with given values.

Execution Table

Step	Action	Parameters	Attributes Set	Output
1	Create car1	No parameters	color='red', wheels=4	car1.color='red', car1.wheels=4
2	Create car2	color='blue', wheels=6	color='blue', wheels=6	car2.color='blue', car2.wheels=6
3	End	-	-	Objects created with correct attributes

💡 All objects created; constructor used defaults when no arguments given.

Variable Tracker

Variable	Start	After car1	After car2	Final
car1.color	undefined	red	red	red
car1.wheels	undefined	4	4	4
car2.color	undefined	undefined	blue	blue
car2.wheels	undefined	undefined	6	6

Key Moments - 2 Insights

Why does car1 get 'red' and 4 wheels without passing arguments?

Because the constructor has default values for color and wheels, used when no arguments are given (see execution_table step 1).

What happens if we pass only one argument to the constructor?

The first parameter gets the passed value, and the second uses its default. This is shown by how car2 overrides both, but if only one was passed, the other would stay default.

Visual Quiz - 3 Questions

Look at the execution_table, what is car1.color after creation?

A undefined

B 'red'

C 'blue'

D None

💡 Hint

Check execution_table row 1 under Attributes Set for car1.

At which step does the constructor use given parameters instead of defaults?

A Step 2

B Step 1

C Step 3

D Never

💡 Hint

Look at execution_table rows 1 and 2 to see when parameters are passed.

If we create car3 = Car('green'), what will car3.wheels be?

A green

B undefined

C 4

D Error

💡 Hint

Default values apply to parameters not passed, see concept_flow and variable_tracker.

Concept Snapshot

class ClassName:
    def __init__(self, param=default):
        self.attr = param

- Constructor uses given args or defaults
- Missing args get default values
- Allows flexible object creation

Full Transcript

This example shows how Python constructors can have default values. When creating car1 without arguments, the constructor uses 'red' for color and 4 for wheels. When creating car2 with 'blue' and 6, those values override defaults. Variables track these changes step-by-step. Beginners often wonder why defaults apply or what happens if some arguments are missing. The execution table clarifies these points by showing each step's parameters and attribute settings.