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.