ModuleNotFoundError: No module named 'pytest_cov'

TL;DR

Run python -m pip install pytest-cov. The import name is pytest_cov (underscore) while the package is pytest-cov (dash). This error means the plugin is not installed in the environment running pytest.

ModuleNotFoundError: No module named 'pytest_cov'

Fix it

1. Install the plugin. Run python -m pip install pytest-cov.

• Expected: Successfully installed pytest-cov-....

1. Verify it loads. Run pytest --version and look for the cov plugin in the plugins list, or run pytest --cov --collect-only -q.

• Expected: no ModuleNotFoundError; coverage options are accepted.

When this applies

• You use --cov flags but never installed the plugin in this environment.
• CI installs pytest but the coverage step assumes pytest-cov is present.

When it does not apply

• If --cov is accepted but coverage import errors mention coverage.results or coverage.report, that is a version mismatch, not a missing install.

Tool compatibility

pytest-cov 4.x/5.x/6.x with pytest 7/8, Python 3.8+.

Why it happens

--cov is not built into pytest; it comes from the pytest-cov plugin. Without the plugin installed, anything importing pytest_cov fails.

Edge cases

• Remember the dash/underscore split: pip install pytest-cov, import pytest_cov.