Pytrees: defaultdict key order is significant?

[cherrywoods]

Hi,
I have a bit of a niche question: When looking through the base Pytree registry definitions in jax/_src/tree_util.py, I stumbled across the definition for defaultdicts in this line: https://github.com/google/jax/blob/9beb4f14741679eef8f0cf8af22c04b4509e7448/jax/_src/tree_util.py#L794

For regular dicts, the keys are sorted so that dictionaries with different insertion order still correspond to the same pytree. However, for defaultdicts the keys are not sorted. I understand why the keys are not sorted for OrderedDict, but would have expected them to get sorted for defaultdict.
I am just curious here whether I'm missing a detail of Python or, otherwise, what the rationale is for not sorting the keys of defaultdicts.

[yashk2810]

Good point, I think we should sort it. @hawkinsp WDYT?

[yashk2810]

@jakevdp too

[jakevdp]

Yes, we should sort keys on lowering here.