Add unit tests for Cube.coord_dims

[01luyicheng]

Description

Adds specific unit tests for Cube.coord_dims, addressing #4552.

Cube.coord_dims previously had no direct unit tests in lib/iris/tests/unit/cube/test_Cube.py (the existing Coord.cube_dims / CellMeasure.cube_dims / AncillaryVariable.cube_dims tests only mock-verify that those methods delegate to Cube.coord_dims, without exercising coord_dims itself).

This adds a Test_coord_dims class covering the seven scenarios listed in the issue, each exercising a distinct branch of the method:

Scenario (from #4552)	Test
coord on the cube	test_dim_coord
coord not on the cube (but matches one that is)	test_equivalent_coord_not_instance
coord not on the cube (and doesn't match one that is)	test_no_match_raises
string	test_string_name
coord on multiple dims	test_aux_coord_multiple_dims
coord is aux factory	test_aux_factory
coord is aux_coord	test_aux_coord_single_dim

Closes #4552.

Checklist

• Included a What's New entry — added in changelog/ (follow-up commit)
• Incorporated type hints in any changed code — N/A (test-only; existing tests in this file do not use type hints)
• Checked if tests need updating — this PR adds the tests
• Checked if benchmarks need updating — N/A
• Checked if documentation needs updating — N/A
• Checked if dependencies need updating — N/A
• Confirmed that the GitHub 'checks' on this PR are passing — pending CI

Testing

All 7 new tests pass, and the full test_Cube.py suite shows no regressions:

lib/iris/tests/unit/cube/test_Cube.py::Test_coord_dims ... 7 passed
lib/iris/tests/unit/cube/test_Cube.py ... 497 passed, 34 skipped

ruff check and ruff format --check both pass with no warnings.

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[CLAassistant]

All committers have signed the CLA.

[stephenworsley]

From @SciTools/peloton: Hi @01luyicheng, thanks for the PR, we'll get this reviewed soon, in the mean time, we just need you to sign the CLA (linked here #7198 (comment)).

[01luyicheng]

Hi, the CLA is signed now - looking forward to the review. Thanks!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.18%. Comparing base (c2ffc9b) to head (29e393d).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7198   +/-   ##
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  Coverage   90.18%   90.18%           
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  Files          91       91           
  Lines       25102    25102           
  Branches     4706     4706           
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  Hits        22638    22638           
  Misses       1685     1685           
  Partials      779      779           

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[ESadek-MO]

Hi @01luyicheng, thanks so much for tackling this!

I've got a couple of comments; the only problems I've found thus far are readability concerns, the code itself all looks great!

[ESadek-MO]

Since this is only used in one test, it possibly makes sense to move this to the test itself.

Normally, I'd avoid making tests unreadable, but I think in this case it's a clear enough set-up that it should be okay. Alternatively, I could see an argument for making this its own fixture.

[01luyicheng]

Hi @ESadek-MO,

Thanks so much for the review and the kind words! I've just pushed an update (commit 8d101a2) that addresses both points:

1. Hybrid-height set-up moved: The delta/sigma/orography/factory construction was only used by test_aux_factory, so I've moved it into that test method rather than keeping it in the shared _setup (and dropped the now-unused self.factory assignment).
2. _setup restructured: Reorganized into clearly separated, blank-line-delimited blocks — cube construction → DimCoords (and adds) → AuxCoords (and adds) → assignment to class variables — matching the order you suggested.

No test logic or assertions were changed. Let me know if you'd prefer the hybrid-height set-up extracted into its own fixture instead of inlined into the test; happy to switch it over.

Thanks again for your time!

[01luyicheng]

Hi @ESadek-MO, just a gentle follow-up on the changes I pushed in 8d101a2 — the _setup is now broken into blank-line-delimited blocks (cube → DimCoords → AuxCoords → class-variable assignment) and the hybrid-height delta/sigma/orography/factory construction has been moved into test_aux_factory, since it's the only test using it. No assertions or test logic were changed, and Codecov is green.

I'm still happy to extract the hybrid-height set-up into a fixture if you'd prefer that over inlining it in the test — just say the word and I'll switch it over. No rush at all on my end; thanks again for the review!

[ESadek-MO]

Hi @01luyicheng, apologies it took a while to get back to you, resource was a little bit tight over the last couple of days.

Please could you put all the hybrid height set-up into a fixture? On discussing with the rest of the team, we think that having a fixture is the best practice due to PyTest's ability to isolate the failures. Apologies to change my mind there!

As an aside, can I confirm that this is AI assisted development? It has a certain amount of AI hallmarks, not least the large number of comments. That isn't a problem, we just like to be sure.