Tuple-like Sequence

There are situations where a variable can be either a list or a tuple (or more generally, a sequence), but it is used/unpacked in a tuple-like way. That is, the length is fixed and types are possibly heterogeneous. A practical example of this is mpi4py's buffer specifications.

I don't believe it's possible to express a fixed-length heterogeneous sequence without using Tuple and forcing a specific type/inheritance. It'd be useful to have an abstract type that expresses tuple semantics without the inheritance bounds. So the analogy is roughly: List is to Sequence as Tuple is to FixedSequence (or whatever name we choose).

def foo(x: FixedSequence[int, str]) -> int:
    return x[0]

foo((0, ''))
foo([0, ''])

Currently I think it's necessary to do Sequence[Union[int, str]] and this loses both the fixed length and the ordering of the elements in the sequence.

Esamina l’astrazione FixedSequence richiesta e confrontane la semantica con Tuple e Sequence. L’attività sarà completata quando verranno preservate la lunghezza fissa e l’ordinamento eterogeneo degli elementi e saranno supportati sia i valori di tupla sia quelli di lista, come mostrato negli esempi dell’issue.