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Code coverage

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The “Code coverage” value of a codebase indicates how much of the production/development code is covered by the running unit tests. Maintainers try their best to keep this percentage high, and this process is often automated using tools such as GitHub Actions and Codecov. Hence, code coverage becomes a good metric (not always) to check if a particular codebase is well tested and reliable.

Tools and libraries used to calculate, read, and visualize coverage reports:

  • pytest-cov: allows developers to calculate and visualize the coverage value
  • Codecov: integrates with remote repositories, allowing developers to see and compare coverage value with each CI run
  • GitHub Actions: allows users to automatically upload coverage reports to Codecov

Are there any alternatives?

coverage.py is a popular Python library used to calculate coverage values, but it is usually paired with python’s unittest. On the other hand, pytest-cov is built to integrate with pytest with minimal extra configurations. Further, Coveralls is an alternative coverage platform, but we recommend using Codecov because of its ease of use and integration with GitHub Actions.

Should increasing the coverage value be my top priority?

A low coverage percentage will definitely motivate you to add more tests, but adding weak tests just for coverage’s sake is not a good idea. The tests should test your codebase thoroughly and should not be unreliable.

Calculating code coverage locally

pytest allows users to pass the --cov option to automatically invoke pytest-cov, which then generates a .coverage file with the calculated coverage value. The value of --cov option should be set to the name of your package. For example, run the following command to run tests to generate a .coverage file for the vector package -

python -m pytest -ra --cov=vector --cov-branch tests/

--cov option will also print a minimal coverage report in the terminal itself! See the docs for more options. The --cov-branch option will enable branch coverage.

Calculating code coverage in your workflows

Your workflows should also run tests with the --cov option, which must be set to your package name. For example -

- name: Test package
  run: python -m pytest -ra --cov=vector tests/

This will automatically invoke pytest-cov, and generate a .coverage file, which can then be uploaded to Codecov using the codecov/codecov-action action.

Configuring Codecov and uploading coverage reports

Interestingly, Codecov does not require any initial configurations for your project, given that you have already signed up for the same using your GitHub account. Codecov requires you to push or upload your coverage report, after which it automatically generates a Codecov project for you.

All Scikit-HEP packages using Codecov, with detailed coverage reports and graphs, are available at https://app.codecov.io/gh/scikit-hep. After your workflows push your first coverage report to Codecov your repository should also appear here.

Codecov maintains the codecov/codecov-action GitHub Action to make uploading coverage reports easy for users. A minimal working example for uploading coverage reports through your workflow, which should be more than enough for a simple testing suite, can be written as follows:

- name: Upload coverage report
  uses: codecov/codecov-action@v3.1.0

The lines above should be added after the step that runs your tests with the --cov option. See the docs for all the optional options.

Using codecov.yml

One can also configure Codecov and coverage reports passed to Codecov using codecov.yml. codecov.yml should be placed inside the .github folder, along with your workflows folder. Additionally, Codecov allows you to create and edit this YAML file directly through your Codecov project’s settings!

A recommended configuration for Codecov:

codecov:
  notify:
    after_n_builds: x

where x is the number of uploaded reports Codecov should wait to receive before sending statuses. This would ensure that the Codecov checks don’t fail before all the coverage reports are uploaded. See the docs for all the options.