Check that attributes are defined

[Goldziher]

Hi, i would like to ask for a small feature- a check that tests that attributes declared in slots are actually defined on the class.

[ariebovenberg]

What would this look like? Something like this?

class Foo:
    __slots__ = ('a', 'b', 'c', 'd')

    def __init__(self):
        self.a = ...
        self.b = ...
        self.c = ...
        # ERROR: slot 'd' is not used

[Goldziher]

What would this look like? Something like this?

class Foo:
    __slots__ = ('a', 'b', 'c', 'd')

    def __init__(self):
        self.a = ...
        self.b = ...
        self.c = ...
        # ERROR: slot 'd' is not used

Exactly

[ariebovenberg]

I can definitely see this being useful, although I can't think of a way to do this without some kind of static analysis.
That makes this a complex feature and triggers design decisions about slotscheck: should static analysis be added? Should it be in a separate flake8 plugin?

The decision to make it an import-time checker does give major advantages (#6).

For now, I'll have to put this on hold -- if there is more interest that will bump priority

[ariebovenberg]

Giving this some more thought:

1. some basic static analysis wouldn't be too hard, although I can imagine it to be quite costly for performance (each class imported would also need to be statically analyzed)
2. There should probably be an easy way to disable this on a per-class basis, since I can imagine rare (but not implausible) cases where a slot is not used in the class definition itself, but only set from outside.
3. There should probably also be a mechanism to allow 'unused' slots if they are used in a subclass. Or perhaps slotscheck should enforce these to be defined in subclasses only... 🤔