pytest-cov Reports 0% / "module-not-imported" - No Source Measured

By Daniel Zoghalchali·Latchkey

pytest-cov ran but measured no real code, reporting 0% or warning that modules were never imported. The --cov target points at a path that is not the imported package, so coverage watches files that never load.

What this error means

Coverage prints 0% for your package, or CoverageWarning: Module X was never imported / No data was collected. Tests pass, but the report is empty because the measured path and the imported path differ (often a src-layout or installed-package mismatch).

CoverageWarning: Module myapp was never imported. (module-not-imported)
CoverageWarning: No data was collected. (no-data-collected)
TOTAL      0      0   0%

Diagnose it: what did pytest actually collect?

Most pytest failures that only happen in CI are collection or import-path problems rather than test failures. The runner has a different working directory, a different sys.path, and usually no editable install, so a test module that imports your package locally may not resolve at all.

# what would run, without running it
pytest --collect-only -q | tail -20

# where pytest thinks the root is (drives conftest and import mode)
pytest --collect-only 2>&1 | grep -i rootdir

# is the package importable at all from the runner cwd?
python -c "import yourpackage, sys; print(yourpackage.__file__)"
python -c "import sys; print(sys.path)"

Common causes

--cov points at the wrong path

Passing --cov=src or a directory that is not the importable package name means coverage instruments files that tests never import, so nothing is recorded.

Installed package measured instead of source (or vice versa)

With a src-layout and an installed/editable package, tests import from site-packages while --cov watches the source tree (or the reverse), so the imported module is not the measured one.

How to fix it

Point --cov at the import package name

Measure by the importable package name so coverage tracks the modules tests actually load.

pytest --cov=myapp --cov-report=term-missing
# not --cov=src (a directory) unless that is the import name

Configure source and path mapping for src-layout

Tell coverage the source root and map installed paths back to source so measurement and imports line up.

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

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

Make a zero-test run fail the build

# fail explicitly when nothing is collected
pytest --strict-markers -q
test "${PIPESTATUS[0]}" -ne 5 || { echo "pytest collected no tests"; exit 1; }

How to prevent it

• Use the import package name for --cov, not a directory.
• Configure [tool.coverage.paths] for src-layout/installed packages.
• Fail the build on coverage warnings to catch a zero-measurement run early.

Frequently asked questions

Related guides

References

• Python documentation
• pip documentation
• GitHub Actions documentation