Fix coverage double-counting (src/ vs installed package)

[Lasse Borgholt (borgholt)]

Problem

After #21, reported coverage dropped from ~93% to ~50%. This is a measurement artifact, not a real loss of test coverage.

The CI coverage table lists every module twice: the executed copy under …/site-packages/error_align/* (real coverage) and an untouched copy under src/error_align/* reported at 0%. The phantom src/ copy has the same statement count as the real package with all statements missed, so it doubles the denominator while hits stay flat — mechanically halving the percentage (645 hit / 645 → 634/1290 ≈ 49%).

Root cause

Two coverage sources were tracked at once:

• --cov=src (from pytest.ini)
• --cov=error_align (from the test workflow)

poetry run pip install . installs the package non-editably (this is required — it compiles the C++ extension, so it is not redundant). Imports then resolve to site-packages, while --cov=src keeps measuring the on-disk src/ tree at 0%. While an editable install dominated, both resolved to the same paths and coverage deduped them; the 0.1.0b9 → 0.1.0b10 version bump made pip install . actually lay down the non-editable copy (previously a no-op), exposing the double count.

Fix

• pytest.ini: measure a single source — --cov=error_align instead of --cov=src.
• pyproject.toml: add [tool.coverage.paths] mapping src/error_align ↔ */site-packages/error_align (plus [tool.coverage.run] source) so the two locations merge into one logical path regardless of editable vs non-editable install.

The C++-compiling pip install . step is left intact.

Verification

Local run now shows a single copy per module, TOTAL 94% (non-baseline), 19 passed — no src/ + site-packages duplication.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.31%. Comparing base (9c5a1d4) to head (59d5334).

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #22       +/-   ##
===========================================
+ Coverage   50.47%   93.31%   +42.83%     
===========================================
  Files          14        9        -5     
  Lines        1369      658      -711     
  Branches      229      106      -123     
===========================================
- Hits          691      614       -77     
+ Misses        651       17      -634     
  Partials       27       27               

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[Copilot]

Pull request overview

Updates the project’s pytest/coverage configuration to prevent coverage double-counting when tests execute the installed error_align package (in site-packages) while also tracking the src/ tree.

Changes:

• Switch pytest coverage collection from --cov=src to --cov=error_align to avoid measuring an unexecuted src/ copy at 0%.
• Add Coverage.py path-mapping in pyproject.toml to unify src/error_align and */site-packages/error_align into a single logical source location.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
pytest.ini	Updates default pytest addopts to measure only error_align (not src).
pyproject.toml	Adds Coverage.py path mapping (and run source) intended to merge editable/non-editable install paths.

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