Undefined

Code examples

Use as default to help with None:

from undefined import Undefined, undefined

mapping: dict[str, object | Undefined] = {}

if value := mapping.get(key, undefined) is undefined:
    assert_type(value, Undefined)
else:
    assert_type(value, object) # 'value' May be None

Or use as pre-defined immutable function argument.

def foo(nullable: object | None | Undefined = undefined):
    if nullable is undefined:
        nullable = ...

Long read

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:

>>> from undefined import undefined, Undefined
>>> undefined is Undefined
True

behavior

undefined - is for runtime 'is' operation

Undefined - is for type hinting

It work mostly like a singleton object

Though it's neither truthy not falsy

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

Why not None, difference with None

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

Unlike None, you can assign to it

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

>>> undefined = 3
>>> undefned
3

Unlike None, undefined is neither true not false.

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

String form

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