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.

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