Assertion and checking functions

I'll just leave this here for potential future consideration as it came up on gitter's typing-dev channel. Typescript has something called assertion functions, which enables the type checker to understand that a function does not return if a certain condition is false. I think something similar, but more generalized, would be interesting, especially for unit test functions like assert_is_instance(), assert_true() etc.

def assert_positive(x: object) -> AssertsInstance["x", int]:
    assert isinstance(x, int) and x > 0

Related, checking functions could work like this:

def is_positive(x: object) -> ChecksInstance["x", int]:
    return isinstance(x, int) and x > 0

def my_func(x: Union[int, str]) -> None:
    if is_positive(x):
        # x must be an int
        ...
    else:
        # x can be an int or str
        ...

Commencez par la documentation liée de TypeScript 3.7 sur les assertion-functions et par les exemples assert_positive et is_positive de l’issue. Comme aucun fichier du dépôt ni aucun test ne sont nommés, définissez d’abord comment les annotations généralisées d’assertion et de vérification doivent exprimer le rétrécissement de type, puis documentez le comportement convenu et la couverture de tests requise.