fixing-diff-cover-failures

name	fixing-diff-cover-failures
description	Use when make test-diff-coverage fails, CI reports diff coverage below 100%, or a GitHub PR is blocked on uncovered changed lines.

Fixing diff-cover failures

Overview

GitHub merge requires 100% test coverage on the PR diff (branch vs base). Overall project coverage (e.g. 85%) is separate and does not unblock merge.

Loop until green

git fetch origin
make test-diff-coverage
# Non-main base: make test-diff-coverage COMPARE_BRANCH=origin/<base>

1. Read uncovered lines printed by diff-cover (same signal as CI).
2. Add or extend tests so every changed line executes in tests (branches, errors, edge cases).
3. Re-run make test (full suite), then make test-diff-coverage.
4. Repeat until both pass.

Use python3 -m pytest --cov=pulp_tool --cov-report=term-missing only to debug specific files; make test-diff-coverage is the authoritative gate.

Red flags

• Declaring PR-ready after make test alone (without diff-cover)
• Relying on "nearby" coverage from unrelated tests
• Stopping after one green run before a later fix broke coverage
• Using # pragma: no cover to avoid writing tests (only if truly unreachable and justified)

Common rationalizations

Excuse	Reality
"Overall coverage is high"	Merge gate is 100% on the diff only
"That line is hard to test"	Add a focused unit test or refactor for testability
"CI might pass anyway"	diff-cover locally matches CI; fix before push

References

• Merge policy: CONTRIBUTING.md
• Full check script: git fetch origin && make pre-commit-ci (or ./scripts/check-all.sh for Python lint + tests + diff-cover)