GH-49826: [Python] Return NotImplemented from Scalar/Array arithmetic dunders for unsupported types

[alex-anast]

Rationale for this change

In pyarrow 24.0.0, Scalar and Array gained arithmetic dunder methods (#32007) that unconditionally dispatch to pyarrow.compute.call_function. When the other operand is an unrecognized type, _pack_compute_args raises TypeError instead of returning NotImplemented. This prevents Python from falling back to the other operand's reflected methods (__radd__, __rsub__, etc.), breaking downstream libraries that relied on this protocol.

What changes are included in this PR?

• Adds a _compute_binary_op helper in scalar.pxi that wraps call_function in a try/except TypeError and returns NotImplemented on failure. This follows the same pattern already used by Scalar.__eq__ and Array.__eq__.
• Updates all binary arithmetic and bitwise dunders on both Scalar (scalar.pxi) and Array (array.pxi) to use this helper.
• Adds parametrized tests in test_scalars.py and test_array.py verifying that reflected operators on custom types are correctly invoked.

Are these changes tested?

Yes. New parametrized tests (test_dunders_return_notimplemented_for_unknown_types) cover all 10 binary operators for both Scalar and Array. The bug was also manually reproduced against pyarrow 24.0.0 to confirm the tests exercise the right code path.

Are there any user-facing changes?

Yes. Scalar and Array arithmetic dunders now return NotImplemented instead of raising TypeError when the other operand is not a recognized Arrow/NumPy type. This restores the pre-24.0.0 behavior where Python would fall back to the other operand's reflected method.

AI-generated code disclosure

This PR was developed with assistance from an AI coding tool (Claude, Anthropic). All changes have been reviewed, understood, and verified.

• GitHub Issue: [Python] Scalar arithmetic dunders raise TypeError instead of returning NotImplemented #49826
  Closes [Python] Scalar arithmetic dunders raise TypeError instead of returning NotImplemented #49826

[github-actions]

⚠️ GitHub issue #49826 has been automatically assigned in GitHub to PR creator.

[github-actions]

⚠️ GitHub issue #49826 has been automatically assigned in GitHub to PR creator.

[AlenkaF]

Thank you for opening a PR.
There is already one PR open: #49833 so we will proceed with that one first. The existing comments apply for both though. Also, please be upfront about the AI use in your PRs, see #49833.

[github-actions]

⚠️ GitHub issue #49826 has been automatically assigned in GitHub to PR creator.

[github-actions]

⚠️ GitHub issue #49826 has been automatically assigned in GitHub to PR creator.

[alex-anast]

@AlenkaF apologies, hadn't seen the other PR. I think there is something sketchy going on there, though, if you look at the user's profile, with e.g. ~280 git commits in a day, it might be that these PRs are fully auto-generated.

This one covers Array and also has tests, though, as you mentioned. In the most recent commit, I have also tried to address the narrower-catch comment, here.

Let me know if I should keep this PR or not.

[AlenkaF]

@AlenkaF apologies, hadn't seen the other PR.

All good, no harm done ;)

I think there is something sketchy going on there, though, if you look at the user's profile, with e.g. ~280 git commits in a day, it might be that these PRs are fully auto-generated.

Yeah, lots of sketchy things going on in these days :) Happy to see a normal response on this PR!

This one covers Array and also has tests, though, as you mentioned. In the most recent commit, I have also tried to address the narrower-catch comment, here.

This is great, thanks! Will have a look tomorrow.

Let me know if I should keep this PR or not.

Yes, please do keep it. If there is no response on the other PR I plan to proceed with yours.

[AlenkaF]

Thanks for the updates! I have circled back to the starting point regarding the catch issue. Let me know what you think.

[AlenkaF]

Thank you for trying to address the narrower-catch issue here. The suggestion looks a bit more complicated than necessary, I think. So I tried to come up with possible examples to test the exceptions and came to the conclusion we always return an Arrow specific exception in the compute functions, see

arrow/python/pyarrow/error.pxi

Line 95 in 0600621

cdef object convert_status(const CStatus& status):

This specific exception tests should already be covered in the test_compute so I think the original, simpler idea should actually do the job. Though adding a simple check that confirms the compute function related errors do not become NotImplemented would also be nice to add as part of the tests in this PR.

@raulcd what do you think?

[raulcd]

I agree it feels overly complex, we seem to be converting the value to scalar and then converting again when we are doing the compute function. This will penalize performance. The except Exception also seems to broad. I am wondering if simplifying everything to just something like isn't enough?

  def _compute_binary_op(func_name, left, right):                                                                      
      try:        
          return _pc().call_function(func_name, [left, right])
      except TypeError:                                                                                                
          return NotImplemented

This could swallow some internal TypeError but from my understanding kernel dispatch failures would raise ArrowNotImplementedError, (we should validate that calling a function with an unsupported type). So it would raise TypeError for operand conversion failures which is the one we want to return NotImplemented instead.
We already do something really similar with __eq__:

arrow/python/pyarrow/scalar.pxi

Lines 165 to 169 in 4373fdf

	def __eq__(self, other):
	try:
	return self.equals(other)
	except TypeError:
	return NotImplemented

I haven't tested those above but those are my assumptions, if someone working on the problem can validate :)

[AlenkaF]

Could we move this helper to a shared file (so that it is clearer it is used for both scalars and arrays)?