Hand the patch back when an operation has nothing to do

[d-chambers]

Description

patch.convert_units(distance="m") on a patch whose distance is already in metres built a whole new patch to hand back the same numbers. So did dropping a coordinate the patch does not have, dropna on data with no nulls, and real() on real data. This makes those operations return the patch they were given.

The rule is deliberately narrow: the test has to cost less than the copy it saves, so it is made from metadata — units, coordinate names, a mask the function had already computed — and never from comparing two data arrays. A function which cannot answer cheaply still returns a new patch. transpose, select, squeeze, sort_coords and snap_coords already worked this way; this extends it to the operations which could tell but did not.

What now short circuits

operation	when
convert_units / set_units	the coords and data already carry exactly those units
simplify_units	the units are already base units
drop_coords	none of the named coordinates are on the patch
drop_private_coords	there are none
dropna	the mask it computed found nothing nullish
fillna	the same
real / conj	the dtype says there is no imaginary part

The unit answers come from a new dascore.units.units_match, which is stricter than == on the quantities on purpose: 1 m == 100 cm is True for pint, but converting between them relabels the coordinate, so they are not a match. dascore/io/index/planned.py keeps the looser == deliberately — it is asking whether the values are already at the right scale, not what the units are spelled — and now says so in a comment.

How it is arranged

Each layer answers "did I change anything" by identity, so the layers compose:

• BaseCoord.convert_units is now a guarded template over a new abstract _convert_units; BaseCoord.set_units guards in place.
• CoordManager.set_units / convert_units / simplify_units collect only the coords which actually changed and return self when there are none.
• The patch functions return patch when the coords came back unchanged and the data units were not asked to move.

Notes

• _convert_units is concrete, not abstract. Making it abstract would have left a BaseCoord subclass which implemented the previously abstract public convert_units unable to instantiate — three reviewers raised it independently. It raises NotImplementedError instead, so such a subclass still builds, and test_every_coord_class_implements_the_hook does the job the abstract check would have done for DASCore's own classes.
• BaseCoord._get_fingerprintable_coord calls _convert_units directly, bypassing the guard — and so also bypassing a subclass which overrode the public convert_units rather than the hook. It normalizes for fingerprinting, and the conversion's numeric form is half of what makes an integer range in metres and the same range in centimetres fingerprint alike; handing the first one back unchanged would keep its integer dtype and break that.
• An integer coordinate already in base units keeps its dtype. simplify_units reached its answer by converting unconditionally, and the conversion recomputed the range as floats; a coordinate which needs no conversion is now left alone, so get_coord(start=0, stop=10, step=1, units="m").simplify_units() stays int64 where it used to become float64.
• A short circuit records no history entry, and chunk compares history, so one call mapped over an uneven spool can need conflict="drop" to merge. Pre-existing rather than new: sort_coords, untouched here, does the same on dev, and dascore/io/index/planned.py already cites the hazard as its reason for calling raw_function.
• A no-op which does not fire is only a missed optimization, never a wrong answer, so nothing needs to depend on the identity.

Changelog

• changed: patch functions which are asked to do nothing — convert_units to the units already set, drop_coords for coordinates which are not there, dropna with no nulls, real on real data, and others — now return the patch they were given instead of building an identical copy.
• changed: simplify_units leaves a coordinate which is already in base units alone, so an integer coordinate keeps its dtype instead of being recomputed as floats.
• added: dascore.units.units_match reports whether two unit specifications name the same units.

Checklist

I have:

• filled in the Changelog section above (see docs/contributing/general_guidelines.qmd).

I have (if applicable):

• referenced the GitHub issue this PR closes.
• documented the new feature with docstrings and/or appropriate doc page. (Docstrings only — the tutorial section was dropped as requested; the behavior is safe and needs no user-facing note.)
• included tests. See testing guidelines.
• added the "ready_for_review" tag once the PR is ready to be reviewed.

Summary by CodeRabbit

• Performance Improvements

  • No-op coordinate and unit operations now avoid unnecessary reconstruction.
  • Data-processing operations preserve the original result when no changes are needed.
  • Partial coordinate updates rebuild only the coordinates that changed.

• Bug Fixes

  • Unit comparisons now distinguish equivalent spellings, magnitude differences, and unset units correctly.
  • Complex-valued data continues to be handled correctly during conjugation.

• Tests

  • Expanded coverage for no-op operations, unit matching, coordinate updates, and missing-value handling.

[coderabbitai]

📝 Walkthrough

Walkthrough

The change adds identity-preserving paths for coordinate managers and patch operations. It introduces strict unit matching, updates coordinate conversion fallback behavior, and adds regression coverage for no-op and conversion cases.

Changes

Identity-preserving processing

Layer / File(s)	Summary
Unit matching and coordinate conversions dascore/units.py, dascore/core/coords.py, dascore/io/index/planned.py, dascore/core/coord_segmented.py, tests/test_units.py, tests/test_core/test_coords.py, tests/test_core/test_coord_segmented.py	Adds units_match, documents equivalent unit-label handling, and provides a concrete _convert_units fallback that raises NotImplementedError. Tests cover unit identity, equivalent spellings, and coordinate identity preservation.
Manager and patch unit operations dascore/core/coordmanager.py, dascore/proc/units.py, tests/test_core/test_coordmanager.py	Unit operations now preserve unchanged managers and patches, while partial conversions replace only changed coordinates.
General patch no-op processing dascore/proc/basic.py, dascore/proc/coords.py, tests/test_proc/test_noop.py	conj, real, dropna, fillna, drop_coords, and drop_private_coords return the original patch when no processing is required. Regression tests cover guard conditions and complex object arrays.

Possibly related PRs

• DASDAE/dascore#765: Adds related no-op identity-preserving paths for coordinate managers and patch operations.
• DASDAE/dascore#769: Contains overlapping no-op optimizations in coordinate manager and patch coordinate operations.
• DASDAE/dascore#921: Modifies conj and real behavior in dascore/proc/basic.py.

Suggested labels: proc, patch

Merge Risk: ⚪ Minimal · up to a2ca5

The PR makes unchanged operations return the existing patch or manager without altering results. No actionable merge-blocking risk remains; it is merge-ready after normal checks and review.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: no-op operations return the original patch.
Description check	✅ Passed	The description explains the problem, scope, implementation, edge cases, tests, documentation, and checklist status in sufficient detail.
Docstring Coverage	✅ Passed	Docstring coverage is 98.91% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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Actionable comments posted: 1

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Inline comments:
In `@dascore/core/coords.py`:
- Around line 391-393: Make BaseCoord._convert_units concrete instead of
abstract so direct subclasses that only implement convert_units remain
instantiable; retain the default NotImplementedError fallback for subclasses
that do not provide the internal conversion implementation.

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📒 Files selected for processing (15)

• dascore/core/coordmanager.py
• dascore/core/coords.py
• dascore/io/index/planned.py
• dascore/proc/basic.py
• dascore/proc/coords.py
• dascore/proc/units.py
• dascore/units.py
• docs/tutorial/processing.qmd
• tests/test_core/test_coord_segmented.py
• tests/test_core/test_coordmanager.py
• tests/test_core/test_coords.py
• tests/test_proc/test_basic.py
• tests/test_proc/test_proc_coords.py
• tests/test_proc/test_proc_units.py
• tests/test_units.py

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (8964b6c) to head (a2ca5da).
⚠️ Report is 1 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##               dev      #943    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files          195       197     +2     
  Lines        24990     25228   +238     
==========================================
+ Hits         24990     25228   +238     

Flag	Coverage Δ
network	44.21% <23.88%> (-0.03%)	⬇️
unittests	100.00% <100.00%> (ø)

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Inline comments:
In `@tests/test_core/test_coordmanager.py`:
- Around line 1425-1431: Extend test_one_of_two_coords_changing to cover
all-no-op replacements where convert_units, set_units, and simplify_units
receive units that already match. Assert each operation returns the original
coordinate manager cm by identity, while preserving the existing assertion that
the unchanged coordinate is not rebuilt.

Apply the same fix in `@tests/test_proc/test_noop.py` around lines 11 - 14.

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📒 Files selected for processing (5)

• tests/test_core/test_coord_segmented.py
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