Output empty `<label/>` instead of omitting it.

[lindsay-stevens]

• as per #439
• pyxform should always output a label node even if there is no label specified.
• pyxform had a warning for no label but this is removed since there is no requirement for a label.
• added tests to verify that a label is output and no warning raised
• fixed existing tests that either expected a warning or no label

Closes #439

Why is this the best possible solution? Were any other approaches considered?

As discussed in the issue ticket, the XForms spec requires a label node. It would not be feasible to change the spec, so this solution brings pyxform output in to line with the spec.

What are the regression risks?

It's possible that existing users expect there to be a warning if no label is supplied but a hint is supplied. So the regression risk would be in other apps that use pyxform. In this case the other app could add a check for empty labels.

Does this change require updates to documentation? If so, please file an issue here and include the link below.

The xlsform.org site says that the survey sheet label is mandatory. However, the update here is specific to pyxform behaviour. It would not be accurate to say that the label is not mandatory, because it is. It's just that for pyxform, in the absence of a label it will emit one in order to comply with the spec.

Before submitting this PR, please make sure you have:

• [x] included test cases for core behavior and edge cases in tests_v1
• [x] run nosetests and verified all tests pass
• [x] run black pyxform to format code
• [x] verified that any code or assets from external sources are properly credited in comments

[lognaturel]

From @MartijnR at https://github.com/XLSForm/pyxform/issues/439#issuecomment-867815645:

If ODK Validate does not get fooled by an empty label (and still requires either a label or hint text content)

The PR description doesn't address this that I can see -- will Validate still throw an error if there's an empty label and no hint? That would be the desired behavior.

[lognaturel]

I've come back to try this, and I do confirm that there is still a pyxform error thrown if neither label nor hint is present which is what we want. 👍

I went straight to tests to try to see if that was covered and didn't see it because the forms aren't inline. Also, I now see that test_row_without_label_or_calculation_throws_error already existed and remains which is great.

[lindsay-stevens]

will Validate still throw an error if there's an empty label and no hint?

For this case there is an existing test: in test_sheet_columns.InvalidSurveyColumnsTests.test_missing_label: https://github.com/XLSForm/pyxform/blob/6bb547b409f46f78bb54af398b6da7e869ed418b/pyxform/tests_v1/test_sheet_columns.py#L36

For reference, the two e2e spreadsheets look like this: hint_with_no_label_column: hint_with_no_label_value:

[codecov-commenter]

Codecov Report

Merging #543 (a0d3f7c) into master (cf7ba39) will decrease coverage by 0.03%. The diff coverage is 81.25%.

@@            Coverage Diff             @@
##           master     #543      +/-   ##
==========================================
- Coverage   83.88%   83.85%   -0.04%     
==========================================
  Files          25       25              
  Lines        3736     3747      +11     
  Branches      872      875       +3     
==========================================
+ Hits         3134     3142       +8     
- Misses        455      457       +2     
- Partials      147      148       +1     

Impacted Files	Coverage Δ
pyxform/survey_element.py	94.07% <76.92%> (-1.00%)	:arrow_down:
pyxform/xls2json.py	78.56% <100.00%> (+0.03%)	:arrow_up:

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[lindsay-stevens]

Hi @lognaturel, resolving this turned out to be not as simple as restoring the check code block. When it was restored, a couple of tests needed to be updated. In particular there is an old-style test with a large "warnings.xls" fixture, which is now refactored into markdown. This should make it clearer when the label warning will and will not be raised. From here, #542 could add in the new condition to exclude warnings for field-list groups, and add the new tests. More context / info in the 3e4a84e commit message.