[Python] Python type annotation for unique value representing 'not undefined'

There is an idiom in Python to do something like,

UNDEFINED = object()

def do_something(value=UNDEFINED):
if value is UNDEFINED:
do1()
else if value is None:
do2()
else:
do3()


The point being that None may be a valid value and UNDEFINED causes some other default behavior.

The mypy signature would then be:

def do_something(value=int|None|object):
...


Which seems kind of pointless since everything is an object.

Literal[UNDEFINED] isn't a valid, either.

The best I can come up with so far is:

class _Undefined:
pass

UNDEFINED = _Undefined()


and

def do_something(value=int|None|_Undefined):
...


It's close, but introduces corner cases of someone passing _Undefined() (making a new instance) and means I'd be better off using isinstance rather than value is UNDEFINED as the old idiom encourages.

Is there a new idiom emerging to do this with mypy and type annotations?

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