# pytest-cov vs coverage.py

> pytest-cov is a pytest plugin that runs coverage.py, so the numbers match. The real difference is pytest-xdist: plain coverage run reported 0% of lines.

Web page: https://supercov.com/compare/pytest-cov-vs-coverage · Published 2026-10-12
All Supercov pages for agents: https://supercov.com/llms.txt · Full text: https://supercov.com/llms-full.txt

## pytest-cov or coverage.py: which should you run?

They do not compete: pytest-cov is a pytest plugin that runs coverage.py for
you, so the numbers are the same. Use pytest-cov when you run tests in
parallel with pytest-xdist, or want coverage as a pytest flag. Run coverage.py
directly when you want one dependency fewer, or your runner is not pytest.

## The two commands

```sh
pip install coverage
coverage run --branch --source=src -m pytest
coverage report -m
```

```sh
pip install pytest-cov
pytest --cov=src --cov-branch --cov-report=term-missing
```

Both read the same configuration: `[tool.coverage]` tables in `pyproject.toml`,
or `.coveragerc`. Exclusions, `fail_under` and contexts apply either way.

## Same numbers, one real difference

One module and three tests, on coverage.py 7.16.2, pytest-cov 7.1.0, pytest
9.1.1 and Python 3.13.1:

| Command | Lines | Branches |
| --- | --- | --- |
| `coverage run -m pytest` | 6/6 | 2/2 |
| `pytest --cov` | 6/6 | 2/2 |
| `coverage run -m pytest -n 2` | 0/6 | none |
| `pytest -n 2 --cov` | 6/6 | 2/2 |

With `-n 2`, pytest-xdist runs the tests in worker processes. Plain
`coverage run` measured only the process that started them and warned "No data
was collected". pytest-cov starts coverage in every worker and combines the
results.

coverage.py can do it alone with two settings and one more command. This also
gave 6/6:

```ini
[run]
parallel = True
patch = subprocess
```

```sh
coverage run -m pytest -n 2
coverage combine
coverage report
```

## What pytest-cov adds, and what it dropped

Its [documentation](https://pytest-cov.readthedocs.io/en/latest/readme.html)
lists four things over `coverage run -m pytest`: xdist support, combining and
reporting in one step, `--cov-context=test` to record which test ran each
line, and the same `sys.path` as a plain pytest run.

Since [7.0.0](https://pytest-cov.readthedocs.io/en/latest/changelog.html) it no
longer measures subprocesses your tests start. Use coverage.py's
`patch = subprocess` for that. It needs coverage.py 7.10.6 or newer.

## What neither shows

Both report 100% on this module. Two things were never tested:

- `if signed_in and not expired` ran both ways, but no test has a signed-in
  session that has expired.
- `return user["name"] if user else fallback` never returned `fallback`.
  coverage.py [measures a branch](https://coverage.readthedocs.io/en/7.16.2/branch.html)
  as a line that can jump to more than one next line, and this line has one.

## What Supercov adds

```sh
npx supercov -- pytest
npx supercov runs latest gaps
```

Supercov runs your own pytest command in your virtual environment, xdist
workers included. On the same module it reported 6/6 lines, 5/6 branches and
1/3 MC/DC conditions, and named both gaps. It records coverage per test.

For other Python tools, see
[Python coverage: coverage.py, SlipCover, and Supercov](https://supercov.com/compare/python-code-coverage.md).


Example evidence: [fixture source, commands and recorded results (JSON)](https://supercov.com/downloads/tool-comparisons-2026-10-12.json). Measured October 12, 2026 with Supercov 4.3.0. These small examples are not a performance benchmark or a general conformance test.