Make SemanticParent explicitly a MutableMapping[str, Semantic]

[liamhuber]

Right now we can for node in some_composite: and iterate over the child node instances, and quickly look at some_composite.child_labels to see all the keys. It would be much clearer to just explicitly follow the mutable mapping pattern and use for node in some_composite.values() and some_composite.keys().

I haven't thought deeply about the side effects this might have, but naively I expect it to work fine. It is a bunch of leg work to go through and modify all the existing invokations of for-loops, since they'd suddenly be looping over keys rather than values, but while nit-picky these changes should be straightforward.

[XzzX]

I 💯 agree. I just did not dare to mention it.

[liamhuber]

No, of course it should be so; saying so is fine as long as you don't expect me to act on it with any particular timeline 😝

collections.abc compliance was not on my radar back when I was writing this, but I agree it's a much nicer way to do things. I'm sure there are other places we can exploit it, but this is the one that has been bugging me most. I only got around to raising an issue for it when it came up online Sam's PR.