@property hides multiple type errors

[Dreamsorcerer]

Bug Report

This code defines self._body as Optional[bytes] and does not produce any type errors: https://github.com/aio-libs/aiohttp/blob/f1eb09b3756f4ed378a25f8a634f734952d2da35/aiohttp/web_response.py#L589
If I instead define the type on the class, like:

class Response(StreamResponse):
    _body: Optional[bytes]

Then I get a type error: Incompatible types in assignment (expression has type "Optional[bytes]", base class "StreamResponse" defined the type as "None") [assignment]

To Reproduce

See next comment.

Your Environment

• Mypy version used: 0.930 (but, those type errors must have been there for months already)
• Mypy configuration options from mypy.ini: https://github.com/aio-libs/aiohttp/blob/master/.mypy.ini

[Dreamsorcerer]

OK, seems to be a problem with @property, managed to create a minimal reproducer. This produces no type errors, but there are actually 2 issues in this code:

from typing import Optional

class Foo:
    def __init__(self) -> None:
        self._a = None  # Foo._a is defined as None

class Bar(Foo):
    def __init__(self, a: Optional[bytes]) -> None:
        self.a = a  # setter says type should be bytes, but we are assigning Optional[bytes].

    @property
    def a(self) -> Optional[bytes]:
        return self._a

    @a.setter
    def a(self, thing: bytes) -> None:
        self._a: Optional[bytes] = thing  # We are defining _a as Optional[bytes], but superclass says it should be None.

[Dreamsorcerer]

It also causes incorrect type errors. If I try to assign to a property, it will give a type error if the type does not match the return type, when it should be checking the parameter type in the setter:

    @property
    def last_modified(self) -> Optional[datetime.datetime]: ...

    @last_modified.setter
    def last_modified(self, value: Optional[Union[int, float, datetime.datetime, str]]) -> None: ...

resp.last_modified = filepath.stat().st_mtime

error: Incompatible types in assignment (expression has type "float", variable has type "Optional[datetime]")  [assignment]

[hauntsaninja]

mypy doesn't do setters that take different types particularly well, see #3004 There's been some movement on that issue recently, so hopefully gets fixed soon

[AlexWaygood]

This produces no type errors, but there are actually 2 issues in this code

On 0.941, mypy now does report one of these errors (but still not the other one):

from typing import Optional

class Foo:
   def __init__(self) -> None:
       self._a = None  # Foo._a is defined as None

class Bar(Foo):
   def __init__(self, a: Optional[bytes]) -> None:
       self.a = a  # setter says type should be bytes, but we are assigning Optional[bytes].

   @property
   def a(self) -> Optional[bytes]:
       return self._a

   @a.setter
   def a(self, thing: bytes) -> None:
       self._a: Optional[bytes] = thing  # error: Incompatible types in assignment (expression has type "Optional[bytes]", base class "Foo" defined the type as "None")