NotImplementedError

NotImplementedError is a built-in exception that signals a method or function hasn’t been implemented yet.

Typically, you’ll raise NotImplementedError in a abstract base class (ABC) or interface to ensure that subclasses provide their own implementations. This exception also helps remind developers of any code that still needs completion.

NotImplementedError Occurs When

A NotImplementedError occurs when you call a method that explicitly raises this exception to indicate its implementation doesn’t exist yet.

NotImplementedError Can Be Used When

  • Indicating that a method in a base class needs to be implemented in a subclass
  • Serving as a placeholder for future code development
  • Alerting developers to incomplete code sections during development

NotImplementedError Example

An example of when the exception appears:

Python 
>>> class Animal:
...     def speak(self):
...         raise NotImplementedError(
...             "Subclasses must implement this method"
...         )
...

>>> class Dog(Animal):
...     pass
...

>>> dog = Dog()
>>> dog.speak()
Traceback (most recent call last):
    ...
NotImplementedError: Subclasses must implement this method

NotImplementedError How to Fix It

To fix code that raises NotImplementedError, you’ll need to implement the missing method in the subclass:

Python 
>>> class Animal:
...     def speak(self):
...         raise NotImplementedError(
...             "Subclasses must implement this method"
...         )
...

>>> class Dog(Animal):
...     def speak(self):
...         print("Woof!")
...

>>> dog = Dog()
>>> dog.speak()
Woof!
Implementing an Interface in Python

Tutorial

Implementing an Interface in Python

In this tutorial, you'll explore how to use a Python interface. You'll come to understand why interfaces are so useful and learn how to implement formal and informal interfaces in Python. You'll also examine the differences between Python interfaces and those in other programming languages.

advanced python

For additional information on related topics, take a look at the following resources:

By Leodanis Pozo Ramos • Updated March 18, 2025 • Reviewed by Martin Breuss
Python’s Built-in Exceptions Share Feedback