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.

Practice

1. What is the main purpose of an abstract base class in Python?

easy

A. To store data without any methods

B. To create objects directly from the base class

C. To automatically run code without subclassing

D. To define methods that must be implemented by subclasses

Solution

1. Step 1: Understand abstract base class role

   An abstract base class sets a template for other classes by defining methods that subclasses must implement.

2. Step 2: Analyze options

   Only 'To define methods that must be implemented by subclasses' correctly states this purpose. The other options describe incorrect uses.

3. Final Answer:

   To define methods that must be implemented by subclasses -> Option D

4. Quick Check:

   Abstract base class = method template [OK]

Hint: Abstract base classes require method implementation [OK]

Common Mistakes:

• Thinking abstract classes can be instantiated
• Confusing abstract classes with regular classes
• Believing abstract classes store data only

2. Which of the following is the correct way to declare an abstract method in a Python abstract base class?

easy

A. @abstractmethod\ndef method(self): pass

B. @abstract\ndef method(self): pass

C. def abstract method(self): pass

D. def method(self): pass

Solution

1. Step 1: Recall abstract method syntax

   In Python, abstract methods are decorated with @abstractmethod from the abc module.

2. Step 2: Match options with correct syntax

   Only '@abstractmethod\ndef method(self): pass' uses @abstractmethod decorator correctly. 'def method(self): pass' misses the decorator, '@abstract\ndef method(self): pass' uses a wrong decorator name, and 'def abstract method(self): pass' uses invalid syntax.

3. Final Answer:

   @abstractmethod\ndef method(self): pass -> Option A

4. Quick Check:

   Use @abstractmethod decorator [OK]

Hint: Use @abstractmethod decorator for abstract methods [OK]

Common Mistakes:

• Omitting the @abstractmethod decorator
• Using wrong decorator names
• Writing invalid method definitions

3. What will be the output of this code?

from abc import ABC, abstractmethod

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

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

print(Dog().sound())

medium

A. Bark

B. TypeError

C. None

D. AttributeError

Solution

1. Step 1: Understand abstract base class behavior

   The abstract method sound must be implemented in subclass Dog. Here, Dog provides the implementation returning "Bark".

2. Step 2: Predict output of print statement

   Creating Dog() instance is allowed because all abstract methods are implemented. Calling sound() returns "Bark" which is printed.

3. Final Answer:

   Bark -> Option A

4. Quick Check:

   Implemented abstract method returns 'Bark' [OK]

Hint: Subclass must implement all abstract methods to instantiate [OK]

Common Mistakes:

• Expecting error despite full implementation
• Confusing abstract method with normal method
• Thinking abstract base class can be instantiated

4. Identify the error in this code snippet:

from abc import ABC, abstractmethod

class Vehicle(ABC):
    @abstractmethod
    def move(self):
        print("Moving")

class Car(Vehicle):
    pass

car = Car()

medium

A. No error, code runs fine

B. TypeError: Can't instantiate abstract class Car with abstract method move

C. SyntaxError due to missing method implementation

D. AttributeError: 'Car' object has no attribute 'move'

Solution

1. Step 1: Check abstract method implementation

   The abstract method move in Vehicle is decorated but has a body. However, Car does not implement move.

2. Step 2: Understand instantiation rules

   Since Car lacks implementation of abstract method move, Python raises a TypeError when trying to create Car().

3. Final Answer:

   TypeError: Can't instantiate abstract class Car with abstract method move -> Option B

4. Quick Check:

   Missing abstract method implementation causes TypeError [OK]

Hint: All abstract methods must be implemented before instantiation [OK]

Common Mistakes:

• Assuming abstract methods with body are implemented
• Trying to instantiate subclass without method override
• Confusing SyntaxError with TypeError here

5. You want to create an abstract base class Shape with an abstract method area. Then, create two subclasses Circle and Square that implement area. Which code correctly achieves this?

hard

A. from abc import ABC

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

class Circle(Shape):
def area(self):
return 3.14 * self.r ** 2

class Square(Shape):
def area(self):
return self.s * self.s

B. class Shape:
def area(self):
pass

class Circle(Shape):
def area(self):
return 3.14 * self.r ** 2

class Square(Shape):
def area(self):
return self.s * self.s

C. from abc import ABC, abstractmethod

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

class Circle(Shape):
def __init__(self, r):
self.r = r
def area(self):
return 3.14 * self.r ** 2

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

D. from abc import ABC, abstractmethod

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

class Circle(Shape):
def area(self):
return 3.14 * self.r ** 2

class Square(Shape):
def area(self):
return self.s * self.s

Solution

1. Step 1: Check abstract base class definition

   from abc import ABC, abstractmethod class Shape(ABC): @abstractmethod def area(self): pass class Circle(Shape): def __init__(self, r): self.r = r def area(self): return 3.14 * self.r ** 2 class Square(Shape): def __init__(self, s): self.s = s def area(self): return self.s * self.s correctly imports ABC and abstractmethod, defines Shape as abstract with @abstractmethod on area.

2. Step 2: Verify subclass implementations

   Both Circle and Square implement area properly, allowing instantiation.

3. Step 3: Analyze other options

   The other options miss the @abstractmethod decorator or do not inherit from ABC properly, so they are not true abstract base classes.

4. Final Answer:

   from abc import ABC, abstractmethod class Shape(ABC): @abstractmethod def area(self): pass class Circle(Shape): def __init__(self, r): self.r = r def area(self): return 3.14 * self.r ** 2 class Square(Shape): def __init__(self, s): self.s = s def area(self): return self.s * self.s -> Option C

5. Quick Check:

   Use ABC and @abstractmethod for abstract base classes [OK]

Hint: Use ABC and @abstractmethod to enforce method implementation [OK]

Common Mistakes:

• Not using @abstractmethod decorator
• Not inheriting from ABC
• Defining abstract methods without decorator