Matthew Rahtz <matthew.ra...@gmail.com> added the comment:
Apologies for the slow reply - coming back to this now that the docs and pickling issues are mostly sorted. [Serhiy] > > Alias = C[T, *Ts] > > Alias2 = Alias[*tuple[int, ...]] > > # Alias2 should be C[int, *tuple[int, ...]] > > tuple[int, ...] includes also an empty tuple, and in this case there is no > value for T. This was my initial intuition too, but Pradeep pointed out to me in https://github.com/python/cpython/pull/31021#discussion_r815853784 that for tuple[int, ...], Python has chosen the opposite mindset: instead of assuming the worst-case scenario, we assume the best-case scenario. Thus, the following type-checks correctly with mypy (https://mypy-play.net/?mypy=latest&python=3.10&gist=b9ca66fb7d172f939951a741388836a6): def return_first(tup: tuple[int, ...]) -> int: return tup[0] tup: tuple[()] = () return_first(tup) > > We actually deliberately chose not to unpack concrete tuple types - see the > > description of https://github.com/python/cpython/pull/30398, under the > > heading 'Starred tuple types'. (If you see another way around it, though, > > let me know.) > > You assumed that *tuple[str, bool] in def foo(*args: *tuple[str, bool]) > should give foo.__annotations__['args'] = tuple[str, bool], but it should > rather give (str, bool). No confusion with tuple[str, bool]. Fair point, we could *technically* distinguish between tuple[str, bool] and (str, bool). But if I was a naive user and I saw `foo.__annotations__['args'] == (str, bool)`, I don't think it'd be immediately obvious to me that the type of `args` was `*tuple[str, bool]`. Also though, there's a second reason mentioned in https://github.com/python/cpython/pull/30398 why `(str, bool)` wouldn't be the best choice. We decided that the runtime behaviour of `*args: *something` should be that we essentially do `(*something,)[0]`. If we made `tuple[int, str]` unpack to `(int, str)`, then we'd end up with `__annotations__['args'] == (int,)`. > And one of PEP 646 options is to implement star-syntax only in subscription, > not in var-parameter type annotations. As in, we would allow `Generic[*Ts]`, but not `*args: *Ts`? That'd be a *major* change to the PEP - not an option I'm willing to consider at this stage in the process. > > I'm also not sure about this one; disallowing unpacked TypeVarTuples in > > argument lists to generic aliases completely (if I've understood right?) > > No, it will only be disallowed in substitution of a VarType. Tuple[T][*Ts] -- > error. Tuple[*Ts][*Ts2] -- ok. Ah, gotcha. My mistake. [Guido] I ran out of time this evening :) Will reply properly soon. ---------- _______________________________________ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue47006> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
