Description
Python version: 3.5.2
mypy version: 0.630
Bug of Feature request: Feature Request
When defining a class with __slots__ mypy complains that the instance has no attribute, even if it was defined in __slots__.
Consider the following code:
class Foo:
__slots__ = ['a']
instance = Foo()
instance.a = 10 # <-- False positive ("Foo" has no attribute "a")
instance.b = 20 # <-- True positive
In this case I would expect mypy to accept assigning something to a (using type Any), but still complain about assigning to b.
The docs about __slots__ state that any iterable can be assigned, and contains the following section:
Any non-string iterable may be assigned to slots. Mappings may also be used; however, in the future, special meaning may be assigned to the values corresponding to each key.
So it may also be interesting to allow typing of the entries in __slots__, for example:
class Bar:
__slots__ = {'a': int}
instance2 = Bar()
instance2.a = 10
instance2.a = 'foo' # <-- I would expect an error here
The docs about __slots__ don't say anything about ordering of the items. So this should be safe even in older version of Python where dicts did not keep ordering yet.
I also came across #1211 and #1325 which both mention __slots__ but are different enough that this should be a separate issue I think.