Safer treatment of @runtime protocols in isinstance()

[ilevkivskyi]

Currently, isinstance with protocols uses the same logic as with nominal classes, i.e. only erases the type variables, but sometimes this may be not safe (as discussed in PEP 544), for example:

@runtime
class P(Protocol):
    attr: int

class C:
    attr = 'Hi!'

if isinstance(C(), P):
    # this branch is considered unreachable by mypy, but will be executed

The idea is to perform a "deeper" erasure (setting all attribute types to Any) when considering isinstance() with protocols, thus mimicking the runtime behaviour.

[ilevkivskyi]

This is a follow-up for #3132

[gvanrossum]

I'm curious how far this should go. With the @runtime decorator IIUC an object with a method that has the same name as the protocol but a different signature may be considered an instance of that protocol. Are you proposing to erase this too? (I guess it's fine, but your example and wording aren't very clear about it -- "attributes" is sometimes assumed not to include methods.)

[ilevkivskyi]

I'm curious how far this should go.

TBH I am not sure myself, this is why "discussion needed" label :-) One possible option is to erase types of all attributes (both methods and variables) completely. But maybe this is too radical, and we can for example only erase types of instance variables, but keep the fact that methods are callable (so that they will become Callable[..., Any] after erasure).