Make concat mismatch warning for scalar coords more accurate

[wjbenfold]

🚀 Pull Request

Description

Added a "reason" keyword argument to _coordinate_differences as currently it attaches the reason, and whilst it can diagnose some of the problem, it doesn't know if it's just been given metadata or also values to check, whereas the calling method does.

Fixes #4096

Question for reviewer:

• Is this the best place to implement this fix? Should it be shallower or deeper instead?

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Consult Iris pull request check list

[wjbenfold]

@rcomer I've added you as reviewer as you raised the issue, but can get someone else on it instead if you'd prefer

[rcomer]

Hi @wjbenfold, I’m happy to have a look at this, though I don’t think I’m qualified to answer this question:

Is this the best place to implement this fix? Should it be shallower or deeper instead?

[jamesp]

Seems reasonable to me. How many places is this function called in the codebase? Is it just the two you point out in #4096?
Do we need an additional test that picks up the default case?

[wjbenfold]

How many places is this function called in the codebase?

It's called repeatedly in the method where it's called on scalar coordinates, but nowhere else I can see:

iris/lib/iris/_concatenate.py

Lines 525 to 554 in 2d53161

	# Check dim coordinates.
	if self.dim_metadata != other.dim_metadata:
	differences = self._coordinate_differences(other, "dim_metadata")
	msgs.append(
	msg_template.format("Dimension coordinates", *differences)
	)
	# Check aux coordinates.
	if self.aux_metadata != other.aux_metadata:
	differences = self._coordinate_differences(other, "aux_metadata")
	msgs.append(
	msg_template.format("Auxiliary coordinates", *differences)
	)
	# Check cell measures.
	if self.cm_metadata != other.cm_metadata:
	differences = self._coordinate_differences(other, "cm_metadata")
	msgs.append(msg_template.format("Cell measures", *differences))
	# Check ancillary variables.
	if self.av_metadata != other.av_metadata:
	differences = self._coordinate_differences(other, "av_metadata")
	msgs.append(
	msg_template.format("Ancillary variables", *differences)
	)
	# Check scalar coordinates.
	if self.scalar_coords != other.scalar_coords:
	differences = self._coordinate_differences(
	other, "scalar_coords", reason="values or metadata"
	)
	msgs.append(
	msg_template.format("Scalar coordinates", *differences)
	)

Do we need an additional test that picks up the default case?

This is already covered by the other tests in that file, e.g.

iris/lib/iris/tests/unit/concatenate/test_concatenate.py

Lines 137 to 143 in 2d53161

	def test_aux_coords_metadata_difference_message(self):
	cube_1 = self.cube
	cube_2 = cube_1.copy()
	cube_2.coord("foo").units = "m"
	exc_regexp = "Auxiliary coordinates metadata differ: .* != .*"
	with self.assertRaisesRegex(ConcatenateError, exc_regexp):
	_ = concatenate([cube_1, cube_2], True)

[rcomer]

Thanks for doing this @wjbenfold and @jamesp 🎉