Yeah, that gives me the heebie jeebies. Fair enough if you want to use magic comments for an entirely third-party system like MyPy, but using them for a built-in language feature feels wrong.

Besides, how necessary is it? Presumably those constrained types can be instantiated:

    x = Dict[str, str]()

The constrained type's __new__ would be annotated appropriately, and return an ordinary dict. It's a bit less "pure" in that this is no longer strictly an annotation, but I'd much prefer that bit of impurity than magic comments.

Agreed. It seems like a simple typo e.g.

    x = {}  # tyep: Dict[str, str]

is a recipe for spending 6 hours wondering why you've got integers in your string-only dict.

(Assuming the interpreter ignores anything not matching `# type:`

Truly terrifying: I had to read your comment four times before I could find the typo.

Yeah, something like

    x: dict<str, str> = {}

would be nicer, I think.