`NotImplementedError`

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

Typically, you’ll raise NotImplementedError in an abstract base class (ABC) or interface to ensure that subclasses provide their own implementations. This exception also helps flag code that’s intentionally incomplete during development.

`NotImplementedError` Occurs When

Calling a method that explicitly raises this exception to indicate that 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 sections of code during development

`NotImplementedError` Example

An example of when the exception appears:

>>> 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, implement the missing method in the subclass:

>>> 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!

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: