Remove length check in ValenceDict

Is your feature request related to a problem? Please describe.

ValenceDict has a number of restrictions that require the length of the key to be no more than 4 atoms. Is there a real reason for this?

Describe the solution you’d like

Remove length check here:

https://github.com/openforcefield/openff-toolkit/blob/c32527bbdd98e5141c0f7a98c968bf0e514217b0/openff/toolkit/topology/topology.py#L94-L103

Refactor this to just take the forward or backward permutation. Also, it’s much more idiomatic python to make this a classmethod instead of a staticmethod that uses __class__.

permutations = OrderedDict({refkey: 0, refkey[::-1]:1})

https://github.com/openforcefield/openff-toolkit/blob/c32527bbdd98e5141c0f7a98c968bf0e514217b0/openff/toolkit/topology/topology.py#L105-L165

Describe alternatives you’ve considered

Additional context

[j-wags]

Gotcha, I see some points on both sides but ultimately I agree with @lilyminium on all proposed changes here.

This is sorta related to a legacy of rough edges around vsites, which came to be because, when @trevorgokey and I were working on it, vsites were supposed to take like a month, and then at the 6 month point, when crazy edge cases kept popping up, and we had unilaterally made changes to the SMIRNOFF spec, and @trevorgokey had rewritten everything several times, we finally said “this is good enough”, and this got out with some rough edges. So thanks to both of you for the great discussion, I think @lilyminium’s correct-er on every point (except I’m giving up on trying to understand the serialization thing, so I have no opinion there)

I’ve approved the PR that will close this issue.

[lilyminium]

Also, it’s much more idiomatic python to make this a classmethod instead of a staticmethod that uses __class__.

ValenceDict and ImproperDict both have staticmethods that use __class__, so I felt my example was more representative. Again, I have nothing against staticmethods for the reasons you’ve mentioned – they’re a useful construct and exist for a reason. I just think that when you require the class in the method, you should switch to a classmethod to communicate the code more clearly.

https://github.com/openforcefield/openff-toolkit/blob/c32527bbdd98e5141c0f7a98c968bf0e514217b0/openff/toolkit/topology/topology.py#L129