Use Collections metaclasses to do BoardGroups

[trickeydan]

No description provided.

[kierdavis]

Let's wait for a decision on #37 before implementing this, as the outcome will affect which metaclass we use.

[kierdavis]

The outcome of #37 was that BoardGroups should be ordered (by serial number), so I think Sequence is the right metaclass to use here.

[kierdavis]

Actually, looking at the current implementation its more like an ordered Mapping.

[kierdavis]

Implementing collections.abc.Mapping would require us to change BoardGroup.__iter__ to iterate over keys (serial numbers) rather than values (boards). Maybe this isn't worth the effort. What do you think @trickeydan?

[trickeydan]

I think it would be really unintuitive if iteration was over serial numbers rather than boards.

I also agree that collections.abc.Mapping is most appropriate.

I think this might be a wontfix 😥

[kierdavis]

We lose out on a few automatically-provided methods, most of which are probably unimportant. The only useful-looking one is __contains__ which already has its own issue open (#241). I'm happy to call this a wontfix.

[Skybound1]

I can't think of a class of the top of my head which acts like a dictionary, but iterates over values... so if __contains__ is the only useful one, I'd agree not fixing and just implementing that is the easiest path to take.