Fixed-size, multi-type sequence

This came up in python/typeshed#2287, but I have encountered the same problem myself. As far as I know, currently the only way to annotate a multi-type, fixed length sequence is using Tuple. But for input types this is often too strict:

def foo(x: Tuple[int, str]) -> None:
    a, b = x

def bar(x: Tuple[int, str]) -> None:
    a = x[0]
    b = x[1]

The implementations accept any iterable (first case) or sequence (second case) that yield two values:

class Iter:
    def __iter__(self):
        yield 1
        yield ""

foo((1, ""))
foo([1, ""])
foo(Iter())
bar((1, ""))
bar([1, ""])

It would be useful to have a way to annotate this. Of course type checkers would still need to reject generic cases like this:

def foo(x: TupleLike[str, str]): ...
x: List[str] = ...
foo(x)

But it could allow some things that are not possible at the moment, and opens the possibility for future improvements.

Inizia leggendo python/typeshed#2287 e gli esempi di questa issue, quindi esamina come typing rappresenta attualmente tuple, iterabili e sequenze. L’issue non indica file di implementazione o test; per considerarla completata sarebbe necessario concordare un design delle annotazioni che supporti gli input descritti, preservando al contempo utili restrizioni sul controllo dei tipi.