Abstract base classes overview in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an Abstract Base Class (ABC) in Python?

An Abstract Base Class is a class that cannot be instantiated directly and is designed to be a blueprint for other classes. It defines methods that must be created within any child classes.

beginner

How do you declare an abstract method in an Abstract Base Class?

You use the @abstractmethod decorator from the abc module to mark a method as abstract. This means subclasses must override this method.

beginner

Why can't you create an instance of an Abstract Base Class?

Because it contains abstract methods that have no implementation, Python prevents creating objects from it to ensure subclasses provide the required method implementations.

beginner

Which module in Python provides support for Abstract Base Classes?

The abc module provides the tools to create Abstract Base Classes and abstract methods.

intermediate

What happens if a subclass does not implement all abstract methods of its Abstract Base Class?

The subclass cannot be instantiated and will raise a TypeError until all abstract methods are implemented.

What decorator is used to declare an abstract method in Python?

A @abstractmethod

B @staticmethod

C @classmethod

D @property

The @abstractmethod decorator marks a method as abstract, requiring subclasses to implement it.

Can you create an instance of an Abstract Base Class directly?

A Only if subclassed

B Yes, always

C Only if it has no abstract methods

D No, never

Abstract Base Classes cannot be instantiated directly because they have abstract methods without implementation.

Which Python module must you import to use Abstract Base Classes?

A base

B abstract

C abc

D collections

The abc module provides the Abstract Base Class functionality.

What error occurs if a subclass does not implement all abstract methods?

A SyntaxError

B TypeError

C AttributeError

D ValueError

Python raises a TypeError when trying to instantiate a subclass missing implementations of abstract methods.

Why use Abstract Base Classes?

A To enforce a common interface for subclasses

B To speed up program execution

C To store data persistently

D To create variables

Abstract Base Classes ensure subclasses follow a specific interface by requiring method implementations.

Explain what an Abstract Base Class is and why it is useful in Python.

Think about how ABCs act like blueprints for other classes.

Describe the steps to create and use an Abstract Base Class in Python.

Focus on the code structure and rules for subclassing.

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