Ever needed a global object that act as None but not quite ?

Like for example key-word argument for function, where None make sens, 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

It work (for now) mostly like a singleton object

Though it's neither truthy not falsy

>>> if undefined: print(True)
raise NotImplementedError

Because it is a module you can use it lowercase:

import undefined

Because it looks more like a keyword (None, True, False), you can use it upper case:

import undefined as Undefined

or

from undefined import Undefined

I tend to be torn between lowercase, for simplicity, and Uppercase.

undefined is likely slower, and as it is a regular Python object there are a few on purpose (or not difference).

>>> None = 3 
SyntaxError: can't assign to keyword

>>> undefined = 3
>>> undefned
3

>>> undefined.value = 42
>>> undefined.value
42

(might be able to fix that with __get_attr__

If you test for boolean value of undefind if will raise. That is to say: the following will fail:

value = undefined
if value:
   pass # will raise before reaching here.

You have to check for identity:

value = undefined
other = 1
if value is undefined:
    pass # will execute

for info, undefined is not True,False, not undefined with respect to identity

>>> undefined is True
False

>>> undefined is False
False

>>>: undefined is None
False

str(undefined) raises. repr(undefined) is the unicode string 'Undefined'