Wrong type of variable in the class with __slots__

[belegnar]

I believe it's a bug, but PR template suggests posting here first

What's wrong with self.filters.pattern here? self.filters is immutable, but even with

class Impl(Abstract):
    __slots__ = ()
    
    filters: RegExFilter = RegExFilter("(.+)")

pyright considers it mutable

class Filter:
    __slots__ = ()

    def do_filter(self):
        print("Main")

class RegExFilter(Filter):
    __slots__ = ("pattern",)

    def __init__(self, pattern: str):
        self.pattern = pattern

class Abstract(abc.ABC):
    __slots__ = ()

    filters: Filter

class Impl(Abstract):
    __slots__ = ()
    
    filters = RegExFilter("(.+)")

    def apply_filter(self):
        print(self.filters.pattern)

https://pyright-play.net/?code=JYWwDg9gTgLgBAQwEYGMBQaUBsEGddwBiwWMAplAFxpy1wD69uWEMujcAvHABQCUGOnAAmZAGYiI9MSXJQeuMljF9qQoWCjAAdjB4AiALIId%2BgZhz44AJTIBzAKIAPYqQo9Xc1TTqNmrdnouXn0wBBg5bX0AGnMhUQlGHWAYRgUlMWi4MIiKbUo4XBgob3U6RWUAOhzI4Jq8jGw8AgBBJCKoBBQ9ZBRKloAhAGFSuhk3KFwCzwpGywIASXAsHjaOrphR2nG5Am5bRxdZd30eSoBqPjNBOgTEMDAsAE9pY-kKlTUy7K1ddKqdhRcNVwpEBEA

[erictraut]

The fact that you're using __slots__ here has nothing to do with the type violation you're seeing. You can completely remove __slots__ from your example, and you'll see the same behavior.

You have declared that the variable filters in the class Abstract must be of type Filter. In the class Impl, you are assigning a value to this variable that indeed conforms to the type Filter (since RegExFilter is a subclass of Filter), so that is not a type violation. However, you're later relying on the fact that Abstract.filters is a RegExFilter rather than a Filter. That's a type violation, and pyright is correct to highlight it. There is nothing that prevents Impl.filters from being overwritten by another Filter instance — one that is not an instance of RegExFilter.

You could make your program type correct by adding an assert:

    def apply_filter(self):
        assert isinstance(self.filters, RegExFilter)
        print(self.filters.pattern)

[belegnar]

__slots__ makes sense here because with slots, Impl.filters is read-only and can't be anything other than a RegExFilter

[erictraut]

class ImplSub(Impl): pass

i = ImplSub()
i.filters = Filter()
i.apply_filter() # Crash

[belegnar]

Indeed. Thx