Submitting a pull request

Creating a Pull Request

When you are ready, go ahead and click on “New Pull Request” on the pull request page. On the next page, you need to select your fork and branch to merge into the PennyLane master branch.

On the page for creating a new pull request, you need to specify the name of the PR and write an introductory description. We have a PR template with a checklist that can help you determine what to say. Please be as detailed as possible, and assume the reader does not have extensive background knowledge of the issue at hand.

If the PR is in response to an Issue, please link the issue with the relevant keywords. One simple way is adding a related sentence to the PR description, e.g. “Closes #ABCD” where ABCD is the number of the PR.

You can make a “Work in Progress” PR with [WIP] in the title or with the “WIP” label. That way, we can see your work and give feedback early.

Once you create the PR, you can also view the status of the Continuous Integration (CI) checks for things like tests, code quality, and documentation rendering.

Continuous Integration Checks

Once a PR is opened on the repository, GitHub runs a series of checks on the branch. All tests need to pass (green check) for the PR to get approved and merged.

  • Formatting check: We use black to autoformat our code to PEP8 standards. You should run black locally on changed files, as the CI check determines whether black would change anything. Black installs with pip. We run black on both the source folder pennylane and the tests folder. We format with a line length of 100. From the PennyLane’s root folder, you can run:

    black -l 100 pennylane tests
  • Code factor: Code factor checks several common code quality characteristics.

To make sure that the formatting and the quality of the new piece of code is up to PEP8 dedicated tools (black and pylint) should be run locally.

The PennyLane repository provides top-level files (.pre-commit-config.yaml and a pyproject.toml) for configuring pre-commit to run black and pylint as a git pre-commit hook. Once configured, issuing git commit will run the tools automatically. If any of the checks fail, committing fails too. A failed black check will reformat the required files. Running the pre-commit hook mechanisms can be disabled for a commit by passing the -n/--no-verify option.

The pre-commit package can be installed e.g., via pip:

pip install pre-commit

Then, it can be installed for a specific repository by running

pre-commit install

in the folder where the .pre-commit-config.yaml file exists (the top-level folder for PennyLane).


The .pre-commit-config.yaml file pins black to a specific version. As new versions of black are released, developers will have to run pre-commit autoupdate to update the package versions used by pre-commit.

  • Tests: Github Actions runs the core tests and device tests for a series of different Python versions and interface installations.

  • Code Coverage: codecov determines if all lines are executed during the automated testing and tells you which ones are not. We want all lines to be covered, but that is insufficient to determine if the testing is complete enough. That takes thought and thinking through the test design to ensure that all edge cases are being tested.

  • Documentation: The “Documentation check” determines if there are any issues with the build, but you also need to inspect the relevant pages in the website build visually. You can access the temporary website by clicking on Details for docs/, as shown in the following image.


Using the “Search” toolbar on the top left of the generated website can help with navigating to new or updated pages. New functions or classes that are available for users should appear as search results.

Note that including skip ci (or a similar string) will mark a git commit such that CI checks are being skipped.

Ready for review?

A review-ready pull request (PR) includes the following:

  • All new code is clearly commented and documented. See our documentation guidelines for more details.

  • All changes must include tests. If you fix a bug, write a test that would fail before the bugfix. See Tests for more information.

  • Code conforms to PEP8 standards. As mentioned in the section above, all code needs to be formatted via Black with a line length of 100.

  • Write a changelog entry for all Pull Requests. The changelog entry should include a link back to the PR. More significant user-facing changes should include a code example. In addition to the changelog entry itself, add your name to the alphabetical contributors’ list at the bottom of each release’s section. The changelog is located at doc/releases/

Please note that our review process can take some time and require a bit of back and forth. We try to enforce a high level of detail in reviews to reduce future bugs and prevent technical debt from accruing. You can speed the review process by keeping pull requests small and fully explaining decisions in the pull request introduction.