Abstract base classes overview in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of abstract base class instantiation

What is the output of this Python code?

Python

from abc import ABC, abstractmethod

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

car = Vehicle()

A Vehicle object created successfully

B TypeError: Can't instantiate abstract class Vehicle with abstract methods start

C None

D AttributeError: 'Vehicle' object has no attribute 'start'

💡 Hint

Abstract base classes cannot be instantiated if they have abstract methods.

The Vehicle class has an abstract method 'start'. Python prevents creating instances of such classes directly, raising a TypeError.

❓ Predict Output

intermediate

Output of subclass implementing abstract method

What is the output of this code?

Python

from abc import ABC, abstractmethod

class Shape(ABC):
    @abstractmethod
    def area(self):
        pass

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

sq = Square(4)
print(sq.area())

A AttributeError: 'Square' object has no attribute 'area'

B TypeError: Can't instantiate abstract class Square with abstract methods area

C None

D 16

💡 Hint

The subclass Square implements the abstract method area.

Since Square provides a concrete implementation of area, it can be instantiated and calling area returns 4*4=16.

🧠 Conceptual

advanced

Purpose of abstract base classes

Which option best describes the main purpose of abstract base classes in Python?

A To provide a template for subclasses by enforcing implementation of certain methods

B To allow creating instances of base classes with default behavior

C To speed up program execution by precompiling methods

D To automatically generate user interfaces for classes

💡 Hint

Think about why you would want to force subclasses to implement some methods.

Abstract base classes define methods that must be implemented by subclasses, ensuring a consistent interface.

❓ Predict Output

advanced

Output when abstract method is not implemented

What happens when you run this code?

Python

from abc import ABC, abstractmethod

class Animal(ABC):
    @abstractmethod
    def sound(self):
        pass

class Dog(Animal):
    pass

d = Dog()

A TypeError: Can't instantiate abstract class Dog with abstract methods sound

B Dog object created successfully

C AttributeError: 'Dog' object has no attribute 'sound'

D None

💡 Hint

If a subclass does not implement all abstract methods, it cannot be instantiated.

Dog inherits from Animal but does not implement the abstract method sound, so instantiation raises a TypeError.

🧠 Conceptual

expert

Effect of @abstractmethod decorator

What is the effect of the @abstractmethod decorator in Python's abc module?

A It automatically implements the method with default behavior

B It makes the method private and inaccessible outside the class

C It marks a method that must be overridden in any subclass before instantiation

D It converts the method into a static method

💡 Hint

Consider what happens if a subclass does not provide its own version of the method.

The @abstractmethod decorator forces subclasses to provide their own implementation of the method to be instantiable.