Specify fixed number of elements in the tuple

[ghost]

Greetings! Currently I am moving cryptographic library from Python2/3 to mypy. I have got so-called S-box-es: tuple consisting of 8 tuples with 16 integers inside. Like this:

sbox = (
    (4, 2, 15, 5, 9, 1, 0, 8, 14, 3, 11, 12, 13, 7, 10, 6),
    (12, 9, 15, 14, 8, 1, 3, 10, 2, 7, 4, 13, 6, 0, 11, 5),
    (13, 8, 14, 12, 7, 3, 9, 10, 1, 5, 2, 4, 6, 15, 0, 11),
    (14, 9, 11, 2, 5, 15, 7, 1, 0, 13, 12, 6, 10, 4, 3, 8),
    (3, 14, 5, 9, 6, 8, 0, 13, 10, 11, 7, 12, 2, 1, 15, 4),
    (8, 15, 6, 11, 1, 9, 12, 5, 13, 3, 7, 10, 0, 14, 2, 4),
    (9, 11, 12, 0, 3, 6, 7, 5, 4, 8, 14, 15, 1, 10, 2, 13),
    (12, 6, 5, 2, 11, 0, 9, 13, 3, 14, 7, 10, 15, 4, 1, 8),
)

And I have got function requiring exactly that structure as an argument. Currently I wrote the following SboxType:

SboxType = Tuple[
    Tuple[int, int, int, int, int, int, int, int, int, int, int, int, int, int, int, int],
    Tuple[int, int, int, int, int, int, int, int, int, int, int, int, int, int, int, int],
    Tuple[int, int, int, int, int, int, int, int, int, int, int, int, int, int, int, int],
    Tuple[int, int, int, int, int, int, int, int, int, int, int, int, int, int, int, int],
    Tuple[int, int, int, int, int, int, int, int, int, int, int, int, int, int, int, int],
    Tuple[int, int, int, int, int, int, int, int, int, int, int, int, int, int, int, int],
    Tuple[int, int, int, int, int, int, int, int, int, int, int, int, int, int, int, int],
    Tuple[int, int, int, int, int, int, int, int, int, int, int, int, int, int, int, int],
]

def encrypt(sbox: SboxType, data: bytes) -> bytes: ...

Possibly I am misunderstanding something. Is there any better and more beauty way on writing types for that kind of structures?

Similar problem arises when I want to specify bytes string (type: bytes) exactly 8 bytes long.

[JukkaL]

There's no better way to specify a constant-length tuple. I'd recommend considering Tuple[int, ...] (literal ...) for a variable-length tuple, though, as the extra type checking you might get from having a constant-length tuple might not be worth the awkward type.

There is no way to specify the length of a bytes object (or a list, for that matter).

[ghost]

I see. Thanks for clarification!

[aldanor]

@stargrave A bit late to this issue, but you can always do this:

>>> Tuple[(Tuple[(int,) * 8],) * 4]
typing.Tuple[
    typing.Tuple[int, int, int, int, int, int, int, int], 
    typing.Tuple[int, int, int, int, int, int, int, int], 
    typing.Tuple[int, int, int, int, int, int, int, int], 
    typing.Tuple[int, int, int, int, int, int, int, int]
]

[gvanrossum]

That doesn't actually work with mypy though.

…

--Guido (mobile)

[aldanor]

Ah, apologies then, it works with typing but didn't realise mypy would choke on that.