`NotImplementedError`

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

NotImplementedError is typically raised in a user-defined base class or interface to signal that subclasses must provide their own implementations. In a true abstract base class (ABC) that uses @abstractmethod, Python already prevents instantiation with a TypeError, so NotImplementedError is the runtime signal for base classes that don’t rely on abc. This exception also helps flag code that’s intentionally incomplete during development.

NotImplementedError isn’t interchangeable with the built-in NotImplemented constant: NotImplemented is the value that binary special methods like .__eq__() and .__add__() return to signal that an operation isn’t supported for the other operand’s type, while NotImplementedError is an exception that gets raised.

`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!