Classes and objects in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of method call with class and instance variables

What is the output of this Python code?

Python

class Car:
    wheels = 4
    def __init__(self, color):
        self.color = color

car1 = Car('red')
car2 = Car('blue')
car1.wheels = 3
print(car1.wheels, car2.wheels)

A 3 4

B 4 3

C 3 3

D 4 4

💡 Hint

Remember that instance variables override class variables for that instance only.

car1 has an instance variable wheels set to 3, so car1.wheels is 3. car2 does not have an instance variable wheels, so it uses the class variable wheels which is 4.

❓ Predict Output

intermediate

Output of method modifying instance variable

What will be printed after running this code?

Python

class Counter:
    def __init__(self):
        self.count = 0
    def increment(self):
        self.count += 1

c = Counter()
c.increment()
c.increment()
print(c.count)

A Error

B 1

C 0

D 2

💡 Hint

Each call to increment adds 1 to count.

The increment method adds 1 to the count attribute each time it is called. After two calls, count is 2.

❓ Predict Output

advanced

Output of class method and static method

What is the output of this code?

Python

class Example:
    @classmethod
    def class_method(cls):
        return 'class method called'

    @staticmethod
    def static_method():
        return 'static method called'

print(Example.class_method())
print(Example.static_method())

A static method called\nclass method called

B class method called\nstatic method called

C Error at class_method call

D Error at static_method call

💡 Hint

Class methods receive the class as first argument, static methods do not receive any implicit argument.

Both methods are called on the class. class_method returns 'class method called', static_method returns 'static method called'.

❓ Predict Output

advanced

Output of inheritance and method overriding

What will this code print?

Python

class Animal:
    def speak(self):
        return 'Animal sound'

class Dog(Animal):
    def speak(self):
        return 'Woof'

class Cat(Animal):
    pass

print(Dog().speak())
print(Cat().speak())

A Animal sound\nAnimal sound

B Error

C Woof\nAnimal sound

D Woof\nWoof

💡 Hint

Dog overrides speak, Cat uses the inherited method.

Dog's speak method returns 'Woof'. Cat does not override speak, so it uses Animal's speak returning 'Animal sound'.

🧠 Conceptual

expert

What error does this code raise?

What error will this code raise when run?

Python

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

p = Person()
print(p.name)

A TypeError: __init__() missing 1 required positional argument: 'name'

B AttributeError: 'Person' object has no attribute 'name'

C NameError: name 'Person' is not defined

D No error, prints None

💡 Hint

Check how the constructor is called and what arguments it requires.

The constructor __init__ requires a 'name' argument, but none was given when creating p, so Python raises a TypeError.

Practice

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

easy

A. To execute code immediately

B. To store data in variables only

C. To create a blueprint for objects

D. To perform mathematical calculations

Solution

1. Step 1: Understand what a class represents

   A class is like a blueprint or template that defines how objects are created and what they can do.

2. Step 2: Identify the role of a class

   Classes organize code by grouping data and functions that belong together, allowing creation of many objects from the same blueprint.

3. Final Answer:

   To create a blueprint for objects -> Option C

4. Quick Check:

   Class = blueprint for objects [OK]

Hint: Classes define blueprints; objects are instances [OK]

Common Mistakes:

• Thinking classes run code immediately
• Confusing classes with simple variables
• Believing classes only store data

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

easy

A. class Car():

B. def Car():

C. function Car():

D. Car class:

Solution

1. Step 1: Recall Python class syntax

   In Python, classes are defined using the keyword class followed by the class name and parentheses.

2. Step 2: Check each option

   class Car(): uses class Car(): which is correct syntax. Others use wrong keywords or formats.

3. Final Answer:

   class Car(): -> Option A

4. Quick Check:

   Class definition starts with 'class' keyword [OK]

Hint: Use 'class ClassName():' to define a class [OK]

Common Mistakes:

• Using 'def' instead of 'class'
• Using 'function' keyword (not Python)
• Missing 'class' keyword

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. Error: missing self parameter

B. Buddy says Woof!

C. Dog says Woof!

D. Woof!

Solution

1. Step 1: Understand the __init__ method

   The __init__ method sets the name attribute to "Buddy" when my_dog is created.

2. Step 2: Analyze the bark method call

   The bark method returns a string using the dog's name, so it returns "Buddy says Woof!".

3. Final Answer:

   Buddy says Woof! -> Option B

4. Quick Check:

   Method uses self.name = Buddy [OK]

Hint: Methods use self to access object data [OK]

Common Mistakes:

• Forgetting to pass 'self' in methods
• Confusing class name with object name
• Expecting method to print instead of return

4. Find the error in this class definition:

class Person():
    def __init__(self, name):
        name = name

p = Person("Alice")
print(p.name)

medium

A. Class name should be lowercase

B. Missing self in __init__ parameters

C. print(p.name) should be print(name)

D. Should assign to self.name, not name

Solution

1. Step 1: Check attribute assignment in __init__

   The code assigns name = name, which only changes the local variable, not the object's attribute.

2. Step 2: Correct assignment to object attribute

   It should be self.name = name to store the value in the object for later access.

3. Final Answer:

   Should assign to self.name, not name -> Option D

4. Quick Check:

   Use self.attribute = value to save data [OK]

Hint: Assign attributes with self.attribute = value [OK]

Common Mistakes:

• Assigning to local variable instead of self.attribute
• Forgetting self in method parameters
• Trying to print undefined variables

5. You want to create a class BankAccount that stores an account holder's name and balance. It should have a method deposit(amount) that adds money to the balance only if the amount is positive. Which code correctly implements this?

hard

A. class BankAccount():
def __init__(self, name, balance=0):
self.name = name
self.balance = balance
def deposit(self, amount):
if amount > 0:
self.balance += amount

B. class BankAccount():
def __init__(self, name):
self.name = name
balance = 0
def deposit(self, amount):
self.balance = self.balance + amount

C. class BankAccount():
def __init__(self, name, balance=0):
self.name = name
self.balance = balance
def deposit(self, amount):
self.balance += amount

D. class BankAccount():
def __init__(self, name):
self.name = name
self.balance = 0
def deposit(self, amount):
if amount >= 0:
self.balance = amount

Solution

1. Step 1: Check __init__ method for attributes

   class BankAccount(): def __init__(self, name, balance=0): self.name = name self.balance = balance def deposit(self, amount): if amount > 0: self.balance += amount correctly sets self.name and self.balance with a default balance of 0.

2. Step 2: Verify deposit method logic

   class BankAccount(): def __init__(self, name, balance=0): self.name = name self.balance = balance def deposit(self, amount): if amount > 0: self.balance += amount adds amount to self.balance only if amount > 0, which matches the requirement.

3. Final Answer:

   Correctly implements the class with proper attribute initialization and deposit validation -> Option A

4. Quick Check:

   Check attribute setup and positive amount condition [OK]

Hint: Check attribute setup and validate input in methods [OK]

Common Mistakes:

• Not using self.balance to store balance
• Adding amount without checking if positive
• Overwriting balance instead of adding
• Missing default balance initialization