Default values in constructors in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of constructor with default values

What is the output of this Python code when creating an object without passing any arguments?

Python

class Car:
    def __init__(self, color='red', speed=0):
        self.color = color
        self.speed = speed

car = Car()
print(car.color, car.speed)

A red 0

B None 0

C red None

D TypeError

💡 Hint

Look at the default values set in the constructor parameters.

The constructor sets default values for color as 'red' and speed as 0. Since no arguments are passed, these defaults are used.

❓ Predict Output

intermediate

Constructor default values with partial arguments

What will be printed when creating an object with one argument, leaving the other to default?

Python

class Book:
    def __init__(self, title='Unknown', pages=100):
        self.title = title
        self.pages = pages

book = Book('Python 101')
print(book.title, book.pages)

A Python 101 100

B Unknown 100

C Python 101 Unknown

D TypeError

💡 Hint

Only the first parameter is given, the second uses its default.

The title is set to 'Python 101' from the argument, pages uses the default 100.

❓ Predict Output

advanced

Effect of mutable default argument in constructor

What is the output of this code after creating two objects and modifying one object's list?

Python

class Basket:
    def __init__(self, items=[]):
        self.items = items

basket1 = Basket()
basket2 = Basket()
basket1.items.append('apple')
print(basket2.items)

A []

B TypeError

C None

D ['apple']

💡 Hint

Think about what happens when a mutable default argument is shared.

The default list is shared between instances, so modifying basket1's items affects basket2's items.

❓ Predict Output

advanced

Correct way to avoid mutable default argument issue

What will be printed after creating two objects and modifying one object's list with this constructor?

Python

class Basket:
    def __init__(self, items=None):
        if items is None:
            items = []
        self.items = items

basket1 = Basket()
basket2 = Basket()
basket1.items.append('apple')
print(basket2.items)

A ['apple']

B None

C []

D TypeError

💡 Hint

Check how the list is created inside the constructor.

Each object gets its own list because a new list is created inside the constructor if no argument is given.

🧠 Conceptual

expert

Why avoid mutable default arguments in constructors?

Why is it recommended to avoid using mutable objects like lists or dictionaries as default values in constructor parameters?

A Because mutable default arguments cause syntax errors in Python.

B Because mutable default arguments are shared across all instances, causing unexpected behavior.

C Because mutable default arguments cannot be modified inside the constructor.

D Because mutable default arguments increase memory usage unnecessarily.

💡 Hint

Think about what happens when you change a mutable default argument in one object.

Mutable default arguments are created once when the function is defined, so all instances share the same object, leading to shared state bugs.