Protocol with attribute hinted as another protocol can not be satisfied

[altendky]

Bug Report

I seem to be unable to satisfy a Protocol with an attribute that is hinted as another Protocol. The inner element does satisfy the inner protocol as shown by the call to g(). The mypy docs do describe recursive protocols as being supported which while a different thing than this seems to suggest that expecting protocols in general to be usable as hints in other protocols is reasonable.

To Reproduce

https://mypy-play.net/?mypy=latest&python=3.10&gist=62ec93fabc3a77038d6b35840a01863e

from dataclasses import dataclass
from typing import Protocol

class PInner(Protocol):
    x: int

class POuter(Protocol):
    inner: PInner

def f(x: POuter) -> None:
    pass

def g(x: PInner) -> None:
    pass

@dataclass
class Inner:
    x: int

@dataclass
class Outer:
    inner: Inner

inner = Inner(x=3)
outer = Outer(inner=inner)

g(x=outer.inner)

f(x=outer)

Expected Behavior

I expect the code to pass mypy.

Actual Behavior

main.py:29: error: Argument "x" to "f" has incompatible type "Outer"; expected "POuter"
main.py:29: note: Following member(s) of "Outer" have conflicts:
main.py:29: note:     inner: expected "PInner", got "Inner"
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 0.961
• Mypy command-line flags: mypy-play.net default
• Mypy configuration options from mypy.ini (and other config files): mypy-play.net default
• Python version used: 3.10
• Operating system and version: mypy-play.net default

[pranavrajpal]

I think the error is correct. inner in POuter is a mutable attribute, which means that f is allowed to replace x.inner with an object of a different class that still implements the same protocol. Changing POuter to use a readonly property for inner fixes the error.

[altendky]

It took me a few attempts before I understood your explanation, but it makes sense now and does address both the example and my real case. The real case had three layers of protocols actually so I had to apply the read-only property pattern twice.

Thanks for your time and the explanation.

[altendky]

For what it's worth, fixing this in my code really made my day. :] Thanks again.