fix: extend TypeTracerArray with __eq__, __ne__, and __array_ufunc__.

[jpivarski]

dask-contrib/dask-awkward#133 (comment) happened because TypeTracerArray didn't have an __eq__ method. It was easy to add, along with __ne__.

In addressing this issue, I first added an __array_ufunc__ (NEP-13) because I thought that's what the issue was and I was surprised that it didn't have one. I'm still surprised it doesn't have one. We could implement all of the __eq__, __ne__, __gt__, __ge__, __lt__, __le__ methods in one swoop by making TypeTracerArray inherit from ak._connect.numpy.NDArrayOperatorsMixin.

What do you think, @agoose77? Actually, I've pretty much talked myself into it. The next commit will do that. It's git, everything's reversible, of course!

[agoose77]

This is definitely needed - the question for me is whether I'll be done with #1849 before this is 'needed' (it's already needed, but that's hard to define!)

Ths likelihood is that I'll be replacing this code in the near future. If you think it's needed now, merge! (I'll review anyway)

[jpivarski]

"Why not make TypeTracerArray inherit from NDArrayOperatorsMixin?"

1. It would cause a circular import unless NDArrayOperatorsMixin is moved somewhere more central, such as ak._util.

2. awkward/src/awkward/_typetracer.py

   Line 352 in 9ab248c

   i = where.index(Ellipsis)

   starts failing because somewhere, len of a TypeTracerArray is now being called.
3. Replacing that where.index with a loop over where to look for the Ellipsis results in yet more errors.

At step 3, I stopped looking into it. The refactor made the TypeTracerArray code a little nicer (dropped 6 very short methods), but not a lot, and it seems to be getting into some deep waters that I don't have time for right now. I.e. the cost-benefit is not good enough to keep following this line.

[jpivarski]

Yes, it's needed in the short term, and it's fine if it gets refactored. The ak.where function triggers it, here:

awkward/src/awkward/operations/ak_where.py

Line 111 in 9ab248c

tags = ak.index.Index8((npcondition == 0).view(np.int8))

((npcondition == 0) needs to return a TypeTracerArray, not a Python bool.) So I'll do a more useful thing and add a test of type-tracer through ak.where, so that when you refactor, it will not get lost.

[jpivarski]

This is ready to merge (I see your informal assent, above). The test failures are unrelated, but they need to be fixed.

As soon as tests pass, we'll merge.

[codecov]

Codecov Report

Merging #2021 (59aed28) into main (9ab248c) will decrease coverage by 0.01%.
The diff coverage is 57.14%.

Additional details and impacted files

Impacted Files	Coverage Δ
src/awkward/_typetracer.py	74.15% <50.00%> (-0.53%)	⬇️
src/awkward/operations/ak_to_numpy.py	100.00% <100.00%> (ø)