Python cmp function, type annotated, and correctly handled nan's

cmp.py

from typing import Optional, TYPE_CHECKING

if TYPE_CHECKING:
    from _typeshed import SupportsLessThan


def cmp(a: 'SupportsLessThan', b: 'SupportsLessThan') -> Optional[int]:
    """Return 1 if a > b, 0 if a == b, -1 if a < b, otherwise None.

    >>> cmp(42, 69), cmp(666, 666), cmp(69, 42), cmp(0, float('nan'))
    (1, 0, -1, None)
    >>> cmp('42', '69'), cmp([666], [666]), cmp({0, 1}, {1}), cmp({0}, {1})
    (1, 0, -1, None)
    """
    return (a < b) - (a > b) or +(a != b) and None


if __name__ == "__main__":
    import doctest
    doctest.testmod()