Ever needed a global object that act as None but not quite? Like for example
keyword argument for function, where None make sense, so you need a default
value.

One solution is to create as singleton object:
mysingleton = object()

Though it becomes difficult to track the singleton across libraries, and teach
users where to import this from. It's also relatively annoying use this
singleton across library.

Introducing undefined:
>>> import undefined
>>> from undefined import Undefined
>>> undefined is Undefined
True