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.

Practice

1. What is the main purpose of using default values in a Python class constructor (__init__ method)?

easy

A. To prevent the class from being instantiated

B. To make the constructor run faster

C. To allow creating objects without providing all arguments

D. To force the user to always provide all arguments

Solution

1. Step 1: Understand default values in constructors

   Default values let you set a value for a parameter if no argument is given when creating an object.

2. Step 2: Identify the effect on object creation

   This means you can create an object without giving all arguments, and the defaults fill in the missing ones.

3. Final Answer:

   To allow creating objects without providing all arguments -> Option C

4. Quick Check:

   Default values = optional arguments [OK]

Hint: Defaults let you skip arguments when creating objects [OK]

Common Mistakes:

• Thinking defaults speed up the constructor
• Believing defaults prevent object creation
• Assuming defaults force all arguments

2. Which of the following is the correct way to set a default value for the parameter age in a Python class constructor?

easy

A. def __init__(self, age:30):

B. def __init__(self, age): age=30

C. def __init__(self, age): age == 30

D. def __init__(self, age=30):

Solution

1. Step 1: Recall Python syntax for default parameters

   Default values are set by assigning a value in the parameter list, like age=30.

2. Step 2: Check each option

   def __init__(self, age=30): uses correct syntax. def __init__(self, age): age=30 tries to assign inside the method header, which is invalid. def __init__(self, age): age == 30 uses comparison operator instead of assignment. def __init__(self, age:30): uses incorrect type hint syntax.

3. Final Answer:

   def __init__(self, age=30): -> Option D

4. Quick Check:

   Default parameter = param=value [OK]

Hint: Default values go in the parameter list with = [OK]

Common Mistakes:

• Assigning default inside the method body
• Using == instead of = for defaults
• Confusing type hints with default values

3. What will be the output of this code?

class Person:
    def __init__(self, name, age=25):
        self.name = name
        self.age = age

p = Person('Alice')
print(p.name, p.age)

medium

A. Alice 25

B. Alice None

C. Alice 0

D. Error: missing argument for age

Solution

1. Step 1: Analyze the constructor parameters

   The constructor has age=25 as a default, so if age is not given, it uses 25.

2. Step 2: Check object creation and print

   We create p = Person('Alice') without age, so age is 25. Printing p.name and p.age shows 'Alice 25'.

3. Final Answer:

   Alice 25 -> Option A

4. Quick Check:

   Missing argument uses default [OK]

Hint: Missing argument uses default value [OK]

Common Mistakes:

• Expecting error when argument is missing
• Assuming default is None if not given
• Confusing default with zero or empty string

4. Find the error in this class constructor:

class Car:
    def __init__(self, model='Sedan', year):
        self.model = model
        self.year = year

medium

A. Missing return statement in __init__

B. Default parameter must come after non-default parameters

C. self is missing in parameters

D. Cannot assign to self attributes in constructor

Solution

1. Step 1: Check parameter order in constructor

   In Python, parameters with default values must come after parameters without defaults.

2. Step 2: Identify the error in parameter order

   Here, model='Sedan' is a default parameter before year which has no default. This causes a syntax error.

3. Final Answer:

   Default parameter must come after non-default parameters -> Option B

4. Quick Check:

   Default params last in list [OK]

Hint: Put all default parameters after non-default ones [OK]

Common Mistakes:

• Placing default parameters before required ones
• Thinking __init__ needs return
• Forgetting self parameter

5. You want to create a class Book where the author defaults to 'Unknown' and pages defaults to 100 if not provided. Which constructor is correct?

hard

A. def __init__(self, author='Unknown', pages=100): self.author = author; self.pages = pages

B. def __init__(self, author, pages=100='Unknown'):

C. def __init__(self, author='Unknown', pages):

D. def __init__(self, author='Unknown', pages=100):

Solution

1. Step 1: Check parameter defaults and order

   Both author and pages have default values, so order is flexible. Options B and C have syntax errors.

2. Step 2: Verify constructor body assigns attributes

   def __init__(self, author='Unknown', pages=100): self.author = author; self.pages = pages correctly sets defaults and assigns self.author and self.pages inside the constructor body.

3. Final Answer:

   def __init__(self, author='Unknown', pages=100): self.author = author; self.pages = pages -> Option A

4. Quick Check:

   Defaults set in params, assign inside method [OK]

Hint: Set defaults in params, assign inside __init__ [OK]

Common Mistakes:

• Incorrect default assignment syntax
• Not assigning parameters to self
• Mixing default values and assignments