coverage "Combine" Finds No Data / "No source for code" in Parallel CI

Splitting tests across parallel or matrix jobs produces multiple .coverage data files that must be combined. Combine fails when the per-job files were not collected, were written without parallel mode, or recorded paths that do not line up across runners.

What this error means

coverage combine reports "No data to combine" or the combined report shows 0% / "No source for code", even though each job measured coverage. The per-job data exists but was not gathered, named, or path-mapped correctly.

coverage output

$ coverage combine
No data to combine
# or after combine:
No source for code: '/runner/work/job2/src/app.py'.

Common causes

Data files not produced in parallel mode

Without parallel = true (or coverage run -p), each job overwrites a single .coverage instead of writing uniquely named .coverage.<id> files to combine.

Per-job files not collected to one place

Matrix jobs run in isolation; their data files must be uploaded as artifacts and downloaded into one directory before combine can see them.

Path mismatch across runners

Different absolute working directories per job make the recorded source paths disagree, so the combined report cannot map back to source without [paths] aliasing.

How to fix it

Enable parallel mode and map paths

Write uniquely named data files per process, then alias the per-runner paths back to one source root.

pyproject.toml

[tool.coverage.run]
parallel = true
source = ["myapp"]

[tool.coverage.paths]
source = ["src/myapp", "*/site-packages/myapp", "/*/work/*/src/myapp"]

Gather per-job data then combine and report

Terminal

# each matrix job: upload its .coverage.<id> as an artifact
# a final job downloads them all into ./cov, then:
coverage combine ./cov
coverage report --fail-under=85

How to prevent it

Set parallel = true and map paths with [tool.coverage.paths].
Upload/download per-job data files before combining.
Run combine and the threshold check in one final aggregation job.