docs(v1): cover the scalar-leftover-coord aux conflict (isel drop=True)#858
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FBumann merged 1 commit intoJul 20, 2026
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Positional indexing leaves a scalar coordinate behind: `x.isel(time=0)` drops `time` as a dimension but keeps it as a scalar coord (the first label). So a cyclic/boundary constraint like `x.isel(time=0) == x.isel(time=-1)` — extremely common in storage/energy models — hits §11's aux-coord conflict under v1 (`time` = first vs last) and raises, where legacy silently dropped it. The aux-coord row already covered this in principle, but listed only `.drop_vars` / `.assign_coords` as fixes and didn't name the scalar-leftover cause. Broaden the migration-guide row and add the natural fix — drop the coord at the indexing site with `.isel(..., drop=True)` / `.sel(..., drop=True)` — and add the same note to convention.md §11. Verified on the branch: `x.isel(time=0) == x.isel(time=-1)` raises "Auxiliary coordinate 'time' has conflicting values ... left=0, right=3" under v1; `drop=True` on both sides builds cleanly. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Documents a v1 aux-coord conflict that's easy to hit and easy to miss. Part of the #717 migration-doc pass.
Same v1 rule as #791 (
x[0] + x[1]/ summing scalar selections), closed COMPLETED as intended (legacy warns about it). This PR documents the comparison form and thedrop=Truefix.Note
The following was generated by AI.
The case. Positional indexing leaves a scalar coordinate behind:
x.isel(time=0)dropstimeas a dimension but keeps it as a scalar coord (the first label). So a cyclic/boundary constraint — very common in storage/energy models —hits §11's auxiliary-coordinate conflict under v1 (
time= first vs last) and raises, where legacy silently dropped it. Verified on the branch:Why a doc change. The existing "Conflicting auxiliary coordinates" row technically covered it (the raise even says "Auxiliary coordinate"), but it listed only
.drop_vars/.assign_coordsand didn't name the scalar-leftover cause — so a user comparing two.iselendpoints wouldn't connect their error to that row, nor reach for the natural fix (drop=Trueat the indexing site).doc/migrating-to-v1.rst— broadened the aux-coord row (situation names the scalar-leftover case; fix adds.isel/.sel(..., drop=True))arithmetics-design/convention.md§11 — same noteIndependent of #855/#856 (different regions of both files); should merge cleanly alongside them.