RaisingNotImplemented

The constantNotImplemented is not anException, but is often confused forNotImplementedError. If it is used as an exception, such as inraise NotImplemented orraise NotImplemented("message"), aTypeError will be raised rather than the expectedNotImplemented. This may make debugging more difficult.

NotImplemented should only be used as a special return value for implementing special methods such as__lt__. Code that is not intended to be called should raiseNotImplementedError.

Recommendation

If aNotImplementedError is intended to be raised, replace the use ofNotImplemented with that. IfNotImplemented is intended to be returned rather than raised, replace theraise withreturn NotImplemented

Example

In the following example, the methodwrong will incorrectly raise aTypeError when called. The methodright will raise aNotImplementedError.

classAbstract(object):defwrong(self):# Will raise a TypeErrorraiseNotImplemented()defright(self):raiseNotImplementedError()

References

• Python Language Reference:The NotImplementedError exception.
• Python Language Reference:The NotImplemented constant.
• Python Language Reference:Emulating numeric types.