Avoid clashing class_ slot in List parameter

[jlstevens]

Here is one way to address #455, namely to make sure the List parameter does not use the same class_ slot as ClassSelector (I called it list_class for now instead).

Generally, I'm not sure why parameters inherit previous slot values at all (when they have a value of None) tbh...

[jlstevens]

This is probably the real issue: #97

[jlstevens]

The tests are passing and this is a fairly simple workaround for #97. Even then, I would argue there is a difference between class_ in class selector and value_class for the values in collections (lists, sets, dicts etc) justifying a separate slot.

[philippjfr]

I'd agree it justifies a different slot but as you say this is really a workaround for a deeper issue. Also not sure about list_class as the name, I'd suggest item_type but don't like that much more. In the long run I'd like towards making Parameters composable rather than having item_type-like slots.

[jlstevens]

Happy to rename to item_type and I also agree that a compositional model would be better for typing. After the renaming would you be happy with this PR (due to the difference in the slot semantics) or would you say #97 needs to be addressed instead (which is bound to be a much bigger job)?

[philippjfr]

Maybe @jbednar has a better suggestion for the naming. I'd at least like to see class_ to be aliased properly so whatever new name we decide on is also accepted as a kwarg.

[jbednar]

#97 does need addressing, but I agree it's a much larger job than putting in a workaround here, so we should do both. Regarding this specific workaround, I agree that the type for the items in a collection is semantically different than the type for a selector, so having a separate slot is very reasonable. The name item_type conveys to me that it's the type of the item and not of the collection, so I'm happy with that.

[jlstevens]

@jbednar @philippjfr Happy with this workaround fix for now?

[jlstevens]

@philippjfr I see you've merged but what about Jim's point about pickling and __getstate__ needing updating?

[philippjfr]

Missed that :/