Python NotImplemented Constant

This page explains Python NotImplemented constant, its typical use cases, and its difference from NotImplementedError.

Typical use cases

The built-in NotImplemented constant is used to indicate that an operation or method is not implemented for a given object.

It is typically used with comparison methods such as __eq__(), __lt__(), __gt__(), when comparison between the two objects does not make sense or is not implemented.

In abstract base classes, a method can be defined but left unimplemented by returning the NotImplemented constant. This allows individual subclasses to provide their own implementation. If a subclass does not have an implementation, a TypeError will be raised when the method is called.

NotImplementedType

The NotImplenented constant has its own special data type NotImplementedType.

>>> type(NotImplemented)
<class 'NotImplementedType'>

NotImplemented vs. NotImplementedError

NotImplemented (constant) is different from NotImplementedError (exception).

>>> a = NotImplemented
>>> a
NotImplemented
>>> type(a)
<class 'NotImplementedType'>
>>> 
>>> b = NotImplementedError
>>> b
<class 'NotImplementedError'>
>>> type(b)
<class 'type'>

While NotImplementedError can be raised, NotImplemented is a constant – not exception, so trying to raise it will raise a TypeError.

>>> raise NotImplementedError
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NotImplementedError
>>> raise NotImplemented
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: exceptions must derive from BaseException

Official documentation

NotImplemented (constant)
Official documentation at docs.python.org

NotImplementedError (exception)
https://docs.python.org/3/library/exceptions.html#NotImplementedError