[nawaaaaaAaar]

Summary

Fixes #570.

Reject MultiIndex level names containing whitespace before serializing the CF compress attribute. Its level names are space-separated, so encoding a level such as "lat lon" currently succeeds but decoding then looks for nonexistent "lat" and "lon" coordinates.

The error names the offending level and tells the caller to rename it. Ordinary Unicode names remain supported; no arbitrary restriction to ASCII or Python identifiers is introduced.

Reproduction

Reproduced on unchanged upstream ee13cb860717299ca8f66d35461e5e50910f3d47: encode a MultiIndex with a "lat lon" level and decode it. Encoding succeeds and decoding raises KeyError: 'lat'. The initial regression matrix failed in all 15 cases before the production change.

Verification

• python -m pytest cf_xarray/tests/test_coding.py -q: 35 passed on current head.
• New coverage: spaces, tabs, newlines, leading/trailing whitespace, Unicode whitespace, all three index-selection forms, unchanged input after rejection, and a valid Unicode-name round trip.
• ruff check cf_xarray/coding.py cf_xarray/tests/test_coding.py: passed.
• ruff format --check cf_xarray/coding.py cf_xarray/tests/test_coding.py: passed.
• git diff --check: passed.

This deliberately validates an unrepresentable input instead of silently renaming user coordinates or changing CF's attribute grammar.

Coverage follow-up

The current head also tests that unrelated spaced variable names remain allowed and that unselected MultiIndexes are not validated, and documents the restriction in the coding guide.

A local coverage run confirms all newly added executable statements are covered. The only uncovered lines in coding.py are pre-existing lines 50, 113, 116, 133, 134, 138, and 139. This run passed all 35 coding tests.

Codecov currently reports 75% aggregate patch coverage while its unittests flag reports 100% patch coverage, and warns that the report is 132 commits behind main. The aggregate patch gate remains unresolved; no threshold or configuration has been weakened to bypass it.

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[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 87.12%. Comparing base (a9cebee) to head (43d3a2c).
⚠️ Report is 132 commits behind head on main.

Files with missing lines	Patch %	Lines
cf_xarray/coding.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #676      +/-   ##
==========================================
+ Coverage   85.78%   87.12%   +1.33%     
==========================================
  Files          13       16       +3     
  Lines        2364     3355     +991     
  Branches      183      313     +130     
==========================================
+ Hits         2028     2923     +895     
- Misses        303      386      +83     
- Partials       33       46      +13     

Flag	Coverage Δ
mypy	46.02% <0.00%> (+7.49%)	⬆️
unittests	92.84% <100.00%> (-1.15%)	⬇️

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[nawaaaaaAaar]

Follow-up coverage check: the current Codecov comment reports 75% combined patch coverage, but its unittests flag reports 100% patch coverage and mypy reports 0%; CI/typing uploads are still pending. A fresh local coding-suite coverage run passes all 35 tests and covers all new executable validation lines (55-57). I have not weakened coverage configuration or changed production behavior to work around the aggregate report. The two added scope controls and coding-guide update are in 43d3a2c.