How to Fix NotImplementedError

What is NotImplementedError?

The NotImplementedError is a built-in exception in Python that is typically raised when an abstract method or required functionality in a base class or interface has not been implemented by a subclass. It serves as a reminder for developers to provide the necessary method implementations in subclasses, especially when designing frameworks or reusable code.

Common Causes

• Defining a method in a base class that should be implemented by subclasses, but leaving it unimplemented.
• Accidentally calling a method that only raises NotImplementedError as a placeholder.
• Forgetting to override an abstract method in a subclass.

How to Fix

1. Identify the method or operation that raises NotImplementedError.
2. Provide a concrete implementation for that method in the subclass or relevant class.

Wrong Code

# Wrong code that causes error
class Animal:
    def speak(self):
        raise NotImplementedError('Subclasses must implement this method')

class Dog(Animal):
    pass

d = Dog()
d.speak()  # Raises NotImplementedError

Correct Code

# Correct code
class Animal:
    def speak(self):
        raise NotImplementedError('Subclasses must implement this method')

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

d = Dog()
print(d.speak())  # Outputs: Woof!