Repository navigation
false positive "Tuple index out of range" when indexing tuple[Any, *tuple[Any, ...]] #19858
Closed as not planned
Description
from typing import Any
def f0n(xs: tuple[Any, ...]) -> int:
return xs[42] # ✔️
def f1n(xs: tuple[int, *tuple[Any, ...]]) -> int:
return xs[42] # ❌ main.py:7: error: Tuple index out of range [misc]
main.py:7: note: Variadic tuple can have length 1
https://mypy-play.net/?mypy=latest&python=3.13&gist=9dc847117579110c1406d6b1cf3f2b35
this occurs with 1.17.1, 1.18.1, and master
possibly related to #19109
Activity
ilevkivskyi commentedon Sep 16, 2025
As you can see from the note this is intentional. For historical reasons plain
tuple[X, ...]is handled in unsafe manner. But types that can be only expressed with unpack syntax are handled strictly.ilevkivskyi commentedon Sep 16, 2025
Oh, I guess you want
Anyto be special-cased, if yes, then we already have an issue for this IIRC.ilevkivskyi commentedon Sep 16, 2025
#18665
jorenham commentedon Sep 16, 2025
Hm no that's not what I meant. You see, the issue here is that
tuple[Any, ...]is the "gradual tuple", which is not only "any-isch" in value, but also in size. That is,tuple[Any, ...]is assignable to every other tuple type and vice-versa. That's why mypy correctly (as can be seen in the example) accepts indexing it for any index.A restricted form of the gradual tuple is
tuple[int, *tuple[Any, ...]], which has the additional requirement that 1) the type at index 0 isintand 2) that the size must be at least 1 (not 0). Just like before, it should be allowed to arbitrarily index it, since it's gradual in size. So it's length is a a hypothetical gradual positiveintof at least1, i.e.Literal[1] | (PosInt & Any).But mypy does not behave that way here. Instead, it reports that indexing it beyond
1can be type-unsafe. Technically this is true. But by gradual construction, we have indicated that it should be treated as if it has any size.The issue you linked is certainly related, but I don't think its the same thing here. This is about the special-cased tuple type itself, not about user-defined variadically-gradual types.
Lenient handling of *tuple[Any, ...] (part 2) (#22006)