Raising NotImplemented

The constant NotImplemented is not an Exception, but is often confused for NotImplementedError. If it is used as an exception, such as in raise NotImplemented or raise NotImplemented("message"), a TypeError will be raised rather than the expected NotImplemented. 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 raise NotImplementedError.

Recommendation

If a NotImplementedError is intended to be raised, replace the use of NotImplemented with that. If NotImplemented is intended to be returned rather than raised, replace the raise with return NotImplemented

Example

In the following example, the method wrong will incorrectly raise a TypeError when called. The method right will raise a NotImplementedError.

class Abstract(object):

    def wrong(self):
        # Will raise a TypeError
        raise NotImplemented()

    def right(self):
        raise NotImplementedError()

References

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