+1 for me on the usefulness of having a way to express "Missing Data". One common use-case I encounter is in function signatures when one wants arguments that are meant as overrides: class Foo: def __init__(self, bar=42, baz=None): self.bar = bar self.baz = baz def do_something(bar_override=None, baz_override=None) ... It works if None is not a valid value for bar or baz, but if it is, you'd have to do something like this class Missing: pass MISSING = object() class Foo: ... def do_something(bar_override=MISSING, baz_override=MISSING) ... Which quickly becomes a problematic to type-hint: Union[Optional[int], Missing] would be the correct way (mypy seems to agree), but is technically wrong. Missing is a sentinel value, not a valid value for this argument. If MISSING were an instance of a subclass of None, then Optional[int] should (in theory) work and users of my method would be None the wiser (pun firmly intended :p). PS: BTW, Missing/MISSING is how the dataclasses module does it