mypy doesn't seem to know an instance attribute is checked by a decorator

[hamez0r]

Please provide more information to help us understand the issue:
mypy doesn't see an instance attribute is checked by a decorator, in an instance method.

• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.

from abc import ABC
from enum import Enum, unique
from functools import wraps
from typing import Optional, Callable


@unique
class Role(Enum):
    USER = 'user'
    ADMIN = 'admin'


class User(object):
    def __init__(self, email: str, password: str, role: Role):
        self.email = email
        self.password = password
        self.role = role


class InitiatedCommand(ABC):
    def __init__(self, initiator: Optional[User]):
        self.initiator = initiator
        self.require_initiator = True


class MissingInitiator(Exception):
    pass


def check_initiator(func: Callable) -> Callable:
    @wraps(func)
    def wrapper(self: InitiatedCommand, *args, **kwargs):
        if not self.initiator:
            if self.require_initiator:
                raise MissingInitiator
            return
        return func(self, *args, **kwargs)
    return wrapper


class NotAllowed(Exception):
    pass


class UpdateUser(InitiatedCommand):
    def __init__(self, user: User, initiator: Optional[User] = None):
        self.user = user
        super().__init__(initiator)

        self.email = None
        self.password = None
        self.role = None

    @check_initiator
    def update_role(self) -> None:
        if self.initiator.role != Role.ADMIN:
            raise NotAllowed

        self.user.role = self.role

    def execute(self):
        self.update_role()


def test_initiator_is_required() -> None:
    user = User(email='[email protected]', password='super-secret', role=Role.USER)

    update_user = UpdateUser(user)
    user.role = Role.ADMIN

    try:
        update_user.execute()
    except MissingInitiator:
        print('Yep, this is expected')


def test_initiator_disabled_on_purpose() -> None:
    user = User(email='[email protected]', password='super-secret', role=Role.USER)

    update_user = UpdateUser(user)
    update_user.require_initiator = False

    user.role = Role.ADMIN

    update_user.execute()
    print('User updated without initiator')
    assert user.role == Role.ADMIN

• What is the actual behavior/output?
  mypy output

root@8506a4af0701:/app# mypy tests/test_instance_decorator.py
tests/test_instance_decorator.py:56: error: Item "None" of "Optional[User]" has no attribute "role"
tests/test_instance_decorator.py:59: error: Incompatible types in assignment (expression has type "None", variable has type "Role")
Found 2 errors in 1 file (checked 1 source file)

pytest output:

root@8506a4af0701:/app# py.test tests/test_instance_decorator.py -s -vv
==================================================================== test session starts ====================================================================
platform linux -- Python 3.6.8, pytest-4.6.6, py-1.8.0, pluggy-0.13.0 -- /usr/bin/python3
cachedir: .pytest_cache
rootdir: /anmo-app
plugins: cov-2.8.1
collected 2 items

tests/test_instance_decorator.py::test_initiator_is_required Yep, this is expected
PASSED
tests/test_instance_decorator.py::test_initiator_disabled_on_purpose User updated without initiator
PASSED

• What is the behavior/output you expect?
  I would expect mypy realized I'm checking that instance attribute in the decorator.

• What are the versions of mypy and Python you are using?
  7.4.0

• What are the mypy flags you are using? (For example --strict-optional)
  None

• More context about what I'm trying to do
  I'm building a small app that will be wrapped in a REST API. There are Users that have roles (user and admin). The business logic says a user's role can be elevated only by an admin (so that's what's up with the initiator). However, I want to use this same command from CLI, where I can manually disable the initiator check.

[hamez0r]

I just realized I'm looking for a macro-like behavior, and I don't think mypy should tap into that.