Incorrect example of Protocol type check in documentation

[e1turin]

On the page "Protocols — typing documentation" in section "type[] and class objects vs protocols" is given wrong example: instead of class instances are given class references. Even more, error and success cases are mismatched:

from typing import Any, Protocol

class ProtoA(Protocol):
    def meth(self, x: int) -> int: ...
class ProtoB(Protocol):
    def meth(self, obj: Any, x: int) -> int: ...

class C:
    def meth(self, x: int) -> int: ...

a: ProtoA = C  # Type check error, signatures don't match!
b: ProtoB = C  # OK

Should be this:

a: ProtoA = C()  # OK
b: ProtoB = C()  # Error

[e1turin]

I suggest such a change: #2177

[JelleZijlstra]

No, this is correct. Class objects can match protocols.

[carljm]

Arguably this example is incorrect as written, but for the subtler reason that the obj parameter in ProtoB.meth should be positional-only, otherwise its name should matter for assignability.

[e1turin]

Well, now I understood what was going on. But are you really saying that this is correct behavior?:

a: ProtoA = C  # Type check error, signatures don't match!

[carljm]

But are you really saying that this is correct behavior?:

a: ProtoA = C # Type check error, signatures don't match!

Yes. C.meth has the signature (self: C, x: int) -> int. ProtoA.meth has the signature (x: int) -> int. The former signature is not assignable to the latter.

The protocol is interpreted as an instance type, so the method is bound; the self argument "disappears". But C.meth accessed off the class is an unbound method, so it still has its self argument. The idea of the example is that that self ends up mapping to the obj: Any argument in ProtoB.meth.

[e1turin]

Oh, I see now. Thank you!