OOP principles overview in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this code using inheritance?

Consider this Python code that uses inheritance. What will it print when run?

Python

class Animal:
    def sound(self):
        return "Some sound"

class Dog(Animal):
    def sound(self):
        return "Bark"

pet = Dog()
print(pet.sound())

A TypeError

B Some sound

C None

D Bark

💡 Hint

Think about which method is called when the child class overrides a method from the parent.

The Dog class overrides the sound method from Animal. So calling pet.sound() uses Dog's version, printing Bark.

❓ Predict Output

intermediate

What does this code print demonstrating encapsulation?

Look at this Python class using encapsulation. What will be printed?

Python

class BankAccount:
    def __init__(self, balance):
        self.__balance = balance
    def get_balance(self):
        return self.__balance

account = BankAccount(100)
print(account.__balance)

A None

B 100

C AttributeError

D 0

💡 Hint

Check how private variables are accessed outside the class.

The variable __balance is private and cannot be accessed directly outside the class, causing AttributeError.

🧠 Conceptual

advanced

Which principle does this code best demonstrate?

This code shows a class with a method that behaves differently depending on the object. Which OOP principle is this?

Python

class Shape:
    def area(self):
        return 0

class Square(Shape):
    def __init__(self, side):
        self.side = side
    def area(self):
        return self.side * self.side

class Circle(Shape):
    def __init__(self, radius):
        self.radius = radius
    def area(self):
        return 3.14 * self.radius * self.radius

A Polymorphism

B Inheritance

C Encapsulation

D Abstraction

💡 Hint

Think about how the same method name works differently for different classes.

Polymorphism allows methods with the same name to behave differently depending on the object calling them.

❓ Predict Output

advanced

What is the output of this code demonstrating abstraction?

What will this code print when run?

Python

from abc import ABC, abstractmethod

class Vehicle(ABC):
    @abstractmethod
    def start(self):
        pass

class Car(Vehicle):
    def start(self):
        return "Car started"

my_car = Car()
print(my_car.start())

A Car started

B TypeError

C AttributeError

D Vehicle started

💡 Hint

Look at how abstract methods are implemented in child classes.

The abstract method start is implemented in Car, so calling my_car.start() returns Car started.

❓ Predict Output

expert

What is the value of x after running this code with multiple inheritance?

Consider this code with multiple inheritance. What is the value of x after execution?

Python

class A:
    def __init__(self):
        self.x = 1

class B(A):
    def __init__(self):
        super().__init__()
        self.x = 2

class C(A):
    def __init__(self):
        super().__init__()
        self.x = 3

class D(B, C):
    def __init__(self):
        super().__init__()

obj = D()
x = obj.x

A 1

B 2

C 3

D TypeError

💡 Hint

Check the method resolution order (MRO) for class D.

Class D inherits from B then C. super().__init__() in D calls B's __init__, which sets x=2. So x is 2.

Practice

1. Which of the following is NOT one of the four main principles of Object-Oriented Programming (OOP)?

easy

A. Compilation

B. Inheritance

C. Polymorphism

D. Encapsulation

Solution

1. Step 1: Recall the four main OOP principles

   The four main principles are Encapsulation, Inheritance, Polymorphism, and Abstraction.

2. Step 2: Identify the option not in the list

   Compilation is a process related to converting code, not an OOP principle.

3. Final Answer:

   Compilation -> Option A

4. Quick Check:

   OOP principles exclude Compilation [OK]

Hint: Remember OOP principles: E, I, P, A [OK]

Common Mistakes:

• Confusing compilation with OOP concepts
• Mixing up abstraction with compilation
• Thinking all programming terms are OOP principles

2. Which Python keyword is used to create a new class?

easy

A. def

B. func

C. object

D. class

Solution

1. Step 1: Recall Python syntax for defining classes

   In Python, the keyword class is used to define a new class.

2. Step 2: Check other options

   def defines functions, object is a base class, and func is not a Python keyword.

3. Final Answer:

   class -> Option D

4. Quick Check:

   Use 'class' to define classes [OK]

Hint: Classes start with 'class' keyword in Python [OK]

Common Mistakes:

• Using def instead of class for classes
• Confusing object with class keyword
• Trying to use func which is invalid

3. What will be the output of this code?

class Animal:
    def speak(self):
        return "Sound"

class Dog(Animal):
    def speak(self):
        return "Bark"

pet = Dog()
print(pet.speak())

medium

A. Bark

B. Sound

C. None

D. Error

Solution

1. Step 1: Understand inheritance and method overriding

   Dog class inherits from Animal and overrides the speak method to return "Bark".

2. Step 2: Check which speak method is called

   pet is an instance of Dog, so pet.speak() calls Dog's speak method, returning "Bark".

3. Final Answer:

   Bark -> Option A

4. Quick Check:

   Overridden method returns 'Bark' [OK]

Hint: Child class method overrides parent method [OK]

Common Mistakes:

• Expecting parent class method output
• Confusing method overriding with overloading
• Thinking print outputs None

4. Find the error in this code snippet:

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

    def display(self):
        print(Model)

medium

A. Constructor name is wrong

B. Missing self in display method

C. Model should be self.model in print

D. Class name should be lowercase

Solution

1. Step 1: Check the print statement inside display method

   The print statement uses Model which is undefined; it should use self.model to access the instance variable.

2. Step 2: Verify other parts

   The constructor name __init__ is correct, and method has self parameter. Class name capitalization is fine.

3. Final Answer:

   Model should be self.model in print -> Option C

4. Quick Check:

   Use self to access instance variables [OK]

Hint: Use self.variable to access instance data [OK]

Common Mistakes:

• Forgetting self in method parameters
• Using variable name without self prefix
• Thinking constructor name is incorrect

5. You want to create a class that hides its internal data and only allows access through methods. Which OOP principle does this demonstrate?

hard

A. Inheritance

B. Encapsulation

C. Polymorphism

D. Abstraction

Solution

1. Step 1: Understand the principle of hiding data

   Hiding internal data and controlling access through methods is called Encapsulation.

2. Step 2: Differentiate from other principles

   Inheritance is about reusing code, Polymorphism is about using methods in different ways, Abstraction is about hiding complexity but not necessarily data.

3. Final Answer:

   Encapsulation -> Option B

4. Quick Check:

   Data hiding = Encapsulation [OK]

Hint: Data hiding means Encapsulation [OK]

Common Mistakes:

• Confusing encapsulation with abstraction
• Mixing inheritance with data hiding
• Thinking polymorphism hides data