__len__ of fixed length tuples?

[anabelle2001]

I'm a bit confused how pyright handles __len__ for fixed-length tuples. Here's a toy example:

def foo(x: tuple[int,int,int] | tuple[int,int]):
    if len(x) == 3:
        a,b,c = x
        l: Literal[3] = len(x)  # Unexpectedly Errors
    elif len(x) == 2:
        a,b,c = x  # Correctly Errors
    else:
        assert_never(x)  # pyright correctly rules out this line

(edit: removed typescript example)

My first thought was that a finite-length tuple should have an overriden type for __len__, such that tuple[int,int,int].__len__() -> Literal[3]. Is this a quirk of pyright's implementation, or is there a reason why pyright doesn't narrow x.__len__?

[erictraut]

Pyright doesn't generally have special-case handling for len function calls. It's treated like any other function call, and it's evaluated according to the typeshed definition which indicates that it returns a value of type int.

The one place where pyright does special-case len calls is in the context of a type guard (i.e. within an if or elif condition). In that case, it can narrow the type of the argument expression passed to the len call.