feat: Form.expected_from_buffers for names/dtypes ak.from_buffers needs.

[jpivarski]

Requested by @danielballan in bluesky/tiled#549 (comment).

[codecov]

Codecov Report

Merging #2660 (954140d) into main (0accfad) will increase coverage by 0.04%.
The diff coverage is 92.18%.

Additional details and impacted files

Files Changed	Coverage Δ
src/awkward/forms/unionform.py	86.25% <66.66%> (-1.83%)	⬇️
src/awkward/forms/form.py	81.68% <68.42%> (-0.92%)	⬇️
src/awkward/contents/emptyarray.py	75.36% <100.00%> (ø)
src/awkward/forms/bitmaskedform.py	83.78% <100.00%> (+1.25%)	⬆️
src/awkward/forms/bytemaskedform.py	85.71% <100.00%> (+1.17%)	⬆️
src/awkward/forms/emptyform.py	86.86% <100.00%> (+0.84%)	⬆️
src/awkward/forms/indexedform.py	80.00% <100.00%> (+1.49%)	⬆️
src/awkward/forms/indexedoptionform.py	89.62% <100.00%> (+0.84%)	⬆️
src/awkward/forms/listform.py	78.94% <100.00%> (+1.80%)	⬆️
src/awkward/forms/listoffsetform.py	94.28% <100.00%> (+0.40%)	⬆️
... and 5 more

[agoose77]

This looks pretty good!

I'm not yet sure whether we want this logic to be implemented by the Form classes, though. My thinking is thus;

• We want to separate the concerns of Form and Content.
• Content handles to_buffers, and a top-level routine handles from_buffers

As such, we should probably have a top-level routine that handles expected_from_buffers.

I'm also thinking about whether Form.expected_from_buffers should instead be accessed via ak.expected_from_buffers. We do have an array-aware method on Form: Form.length_zero_array, but that perhaps should be the exception rather than the rule.

I can make these changes — at this stage it's just musings.

[jpivarski]

This kind of functionality should be level-2: public but not widely advertised. I don't think it should be a high-level ak.* function, for instance.

It could be a free function in the ak.forms namespace, like ak.forms.from_dict and ak.forms.from_json. Nothing in the name expected_from_buffers says that it's about Forms, rather than layouts, so

ak.forms.expected_from_buffers(some_form)

would accomplish that as well as

some_form.expected_from_buffers

I'm not actually in favor of the following, but just to consider all options:

ak.from_buffers.expected(some_form)

[agoose77]

I don't think it should be a high-level ak.* function, for instance.

There're probably too many constraints here to make everything work. I've changed my mind; although from_buffers is written in the ak_ namespace, there's nothing to suggest that we can't encode a canonical form representation in ak.forms, which is what this does. I'm +1, although I'd therefore like to move the functions that are imported from ak_from_buffers to ak.forms itself (my todo).

[agoose77]

@jpivarski could you OK (nOK) my counter edits?

[jpivarski]

@jpivarski could you OK (nOK) my counter edits?

Okay!