Purpose of constructors in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Python code using a constructor?

Consider this Python class with a constructor. What will be printed when the code runs?

Python

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

my_dog = Dog("Buddy")
print(my_dog.name)

A Buddy

B Dog object

C None

D Error: missing argument

💡 Hint

The constructor sets the name attribute when the object is created.

The __init__ method is the constructor. It sets the name attribute to 'Buddy'. So printing my_dog.name outputs 'Buddy'.

🧠 Conceptual

intermediate

What is the main purpose of a constructor in Python?

Choose the best description of what a constructor does in a Python class.

A It initializes new objects with default or given values.

B It deletes objects when they are no longer needed.

C It defines methods that can be called on objects.

D It creates new classes from existing ones.

💡 Hint

Think about what happens when you create a new object from a class.

A constructor (__init__) runs automatically when a new object is created. It sets up initial values for the object.

❓ Predict Output

advanced

What will this code print regarding constructor behavior?

Look at this Python code with a constructor and a method. What is the output?

Python

class Car:
    def __init__(self, brand):
        self.brand = brand

    def show_brand(self):
        print(f"Brand: {self.brand}")

car1 = Car("Toyota")
car1.show_brand()

A Error: show_brand missing argument

B Brand: car1

C Brand: None

D Brand: Toyota

💡 Hint

The constructor sets the brand attribute, which the method prints.

The constructor sets self.brand to 'Toyota'. The show_brand method prints it. So output is 'Brand: Toyota'.

🧠 Conceptual

advanced

Which statement about constructors is true?

Select the true statement about constructors in Python classes.

A Constructors can have any name you choose.

B Constructors run automatically when an object is created.

C Constructors must return a value.

D Constructors can only have one parameter.

💡 Hint

Think about when the __init__ method runs.

The constructor __init__ runs automatically when a new object is created. Its name must be __init__, it does not return a value, and it can have multiple parameters.

❓ Predict Output

expert

What is the output of this Python code with multiple constructors?

Python does not support multiple constructors by default. What will happen when this code runs?

Python

class Book:
    def __init__(self, title):
        self.title = title

    def __init__(self, title, author):
        self.title = title
        self.author = author

b = Book("1984")
print(b.title)

A None

B 1984

C Error: missing 1 required positional argument: 'author'

D Error: multiple constructors not allowed

💡 Hint

The second __init__ replaces the first one.

In Python, the second __init__ method overwrites the first. So Book expects two arguments. Calling Book("1984") with one argument causes a TypeError.

Practice

1. What is the main purpose of a constructor in a Python class?

easy

A. To print information about the class

B. To delete objects when they are no longer needed

C. To initialize new objects with starting values

D. To create new functions inside the class

Solution

1. Step 1: Understand what a constructor does

   A constructor is a special method that runs when a new object is created.

2. Step 2: Identify the purpose of initialization

   It sets up the object with initial values so it is ready to use.

3. Final Answer:

   To initialize new objects with starting values -> Option C

4. Quick Check:

   Constructor = initialize objects [OK]

Hint: Constructors set starting values when creating objects [OK]

Common Mistakes:

• Confusing constructors with methods that delete objects
• Thinking constructors print information automatically
• Believing constructors create new functions

2. Which of the following is the correct way to define a constructor in a Python class?

easy

A. def __init__(self):

B. def constructor(self):

C. def init(self):

D. def __start__(self):

Solution

1. Step 1: Recall Python constructor syntax

   Python uses a special method named __init__ to define constructors.

2. Step 2: Match the exact method name

   The method must be named exactly __init__ with double underscores before and after.

3. Final Answer:

   def __init__(self): -> Option A

4. Quick Check:

   Constructor method = __init__ [OK]

Hint: Constructor method is always named __init__ in Python [OK]

Common Mistakes:

• Using 'constructor' or 'init' without underscores
• Using wrong method names like __start__
• Forgetting double underscores before and after init

3. What will be the output of this code?

class Dog:
    def __init__(self, name):
        self.name = name
    def bark(self):
        return f"{self.name} says Woof!"

my_dog = Dog("Buddy")
print(my_dog.bark())

medium

A. Buddy says Woof!

B. Woof!

C. my_dog says Woof!

D. Error: missing argument

Solution

1. Step 1: Understand the constructor usage

   The constructor sets self.name to "Buddy" when my_dog is created.

2. Step 2: Check the bark method output

   bark returns a string with self.name followed by "says Woof!" so it returns "Buddy says Woof!".

3. Final Answer:

   Buddy says Woof! -> Option A

4. Quick Check:

   Constructor sets name, bark uses it [OK]

Hint: Constructor sets name; bark prints name with Woof [OK]

Common Mistakes:

• Ignoring the name argument in constructor
• Expecting bark to print only 'Woof!'
• Thinking my_dog is printed instead of its name

4. Identify the error in this class definition:

class Car:
    def __init__(self, model):
        model = model

my_car = Car("Tesla")
print(my_car.model)

medium

A. The print statement should be print(model)

B. The constructor name is incorrect

C. The class is missing a return statement

D. The constructor does not assign model to self.model

Solution

1. Step 1: Check constructor assignment

   The constructor assigns model to local variable model, not to self.model.

2. Step 2: Understand attribute access

   Without self.model, the object has no model attribute, causing an error on print.

3. Final Answer:

   The constructor does not assign model to self.model -> Option D

4. Quick Check:

   Use self.model = model to store attribute [OK]

Hint: Always assign to self.attribute inside __init__ [OK]

Common Mistakes:

• Assigning to local variable instead of self.attribute
• Thinking constructor name is wrong
• Expecting print(model) to work outside class

5. You want to create a class Book that stores title and author. Which constructor correctly initializes these attributes and allows creating a Book object with both values?

hard

A. def __init__(self, author):
self.title = title
self.author = author

B. def __init__(self, title, author):
self.title = title
self.author = author

C. def __init__(self, title):
self.title = title
self.author = author

D. def __init__(self):
title = ''
author = ''

Solution

1. Step 1: Check parameters needed

   Both title and author must be passed to the constructor to initialize attributes.

2. Step 2: Verify attribute assignment

   Constructor must assign both self.title and self.author from parameters.

3. Final Answer:

   def __init__(self, title, author): self.title = title self.author = author -> Option B

4. Quick Check:

   Constructor with all attributes assigned = def __init__(self, title, author): self.title = title self.author = author [OK]

Hint: Constructor needs all attributes as parameters and assigns them [OK]

Common Mistakes:

• Missing parameters for all attributes
• Assigning attributes without parameters
• Using local variables instead of self attributes