3. Lifecycle of a Pull Request

3.1. Introduction

CPython uses a workflow based on pull requests. What this means is that you create a branch in Git, make your changes, push those changes to your fork on GitHub (origin), and then create a pull request against the official CPython repository (upstream).

3.2. Quick Guide

Clear communication is key to contributing to any project, especially an Open Source project like CPython.

Here is a quick overview of how you can contribute to CPython:

  1. Create an issue that describes your change [*]
  2. Create a new branch in Git
  3. Work on changes (e.g. fix a bug or add a new feature)
  4. Run tests and make patchcheck
  5. Commit and push changes to your GitHub fork
  6. Create Pull Request on GitHub to merge a branch from your fork
  7. Review and address comments on your Pull Request
  8. When your changes are merged, you can delete the PR branch
  9. Celebrate contributing to CPython! :)
[*]If an issue is trivial (e.g. typo fixes), or if an issue already exists, you can skip this step.

3.3. Step-by-step Guide

You should have already set up your system, got the source code, and built Python.

  • Create a new branch in your local clone:

    git checkout -b <branch-name> upstream/master
    
  • Make changes to the code, and use git status and git diff to see them.

    (Learn more about Making Good PRs)

  • Make sure the changes are fine and don’t cause any test failure:

    make patchcheck
    ./python -m test
    

    (Learn more about patchcheck and about Running & Writing Tests)

  • Once you are satisfied with the changes, add the files and commit them:

    git add <filenames>
    git commit -m '<message>'
    

    (Learn more about Making Good Commits)

  • Then push your work to your GitHub fork:

    git push origin <branch-name>
    
  • If someone else added new changesets and you get an error:

    git fetch upstream
    git rebase upstream/master
    git push --force origin <branch-name>
    
  • Finally go on https://github.com/<your-username>/cpython: you will see a box with the branch you just pushed and a green button that allows you to create a pull request against the official CPython repository.

  • When people start adding review comments, you can address them by switching to your branch, making more changes, committing them, and pushing them to automatically update your PR:

    git checkout <branch-name>
    # make changes and run tests
    git add <filenames>
    git commit -m '<message>'
    git push origin <branch-name>
    
  • After your PR has been accepted and merged, you can delete the branch:

    git branch -D <branch-name>  # delete local branch
    git push origin -d <branch-name>  # delete remote branch
    

Note

You can still upload a patch to bugs.python.org, but the GitHub pull request workflow is strongly preferred.

3.4. Making Good PRs

When creating a pull request for submission, there are several things that you should do to help ensure that your pull request is accepted.

First, make sure to follow Python’s style guidelines. For Python code you should follow PEP 8, and for C code you should follow PEP 7. If you have one or two discrepancies those can be fixed by the core developer who merges your pull request. But if you have systematic deviations from the style guides your pull request will be put on hold until you fix the formatting issues.

Second, be aware of backwards-compatibility considerations. While the core developer who eventually handles your pull request will make the final call on whether something is acceptable, thinking about backwards-compatibility early will help prevent having your pull request rejected on these grounds. Put yourself in the shoes of someone whose code will be broken by the change(s) introduced by the pull request. It is quite likely that any change made will break someone’s code, so you need to have a good reason to make a change as you will be forcing someone to update their code. (This obviously does not apply to new classes or functions; new arguments should be optional and have default values which maintain the existing behavior.) If in doubt, have a look at PEP 387 or discuss the issue with experienced developers.

Third, make sure you have proper tests to verify your pull request works as expected. Pull requests will not be accepted without the proper tests!

Fourth, make sure the entire test suite runs without failure because of your changes. It is not sufficient to only run whichever test seems impacted by your changes, because there might be interferences unknown to you between your changes and some other part of the interpreter.

Fifth, proper documentation additions/changes should be included.

3.5. patchcheck

patchcheck is a simple automated patch checklist that guides a developer through the common patch generation checks. To run patchcheck:

On UNIX (including Mac OS X):

make patchcheck

On Windows (after any successful build):

python.bat Tools/scripts/patchcheck.py

The automated patch checklist runs through:

  • Are there any whitespace problems in Python files? (using Tools/scripts/reindent.py)
  • Are there any whitespace problems in C files?
  • Are there any whitespace problems in the documentation? (using Tools/scripts/reindent-rst.py)
  • Has the documentation been updated?
  • Has the test suite been updated?
  • Has an entry under Misc/NEWS.d/next been added?
  • Has Misc/ACKS been updated?
  • Has configure been regenerated, if necessary?
  • Has pyconfig.h.in been regenerated, if necessary?

The automated patch check doesn’t actually answer all of these questions. Aside from the whitespace checks, the tool is a memory aid for the various elements that can go into making a complete patch.

3.6. Making Good Commits

Each feature or bugfix should be addressed by a single pull request, and for each pull request there may be several commits. In particular:

  • Do not fix more than one issue in the same commit (except, of course, if one code change fixes all of them).
  • Do not do cosmetic changes to unrelated code in the same commit as some feature/bugfix.

Commit messages should follow the following structure:

bpo-42: the spam module is now more spammy. (GH-NNNN)

The spam module sporadically came up short on spam. This change
raises the amount of spam in the module by making it more spammy.

The first line or sentence is meant to be a dense, to-the-point explanation of what the purpose of the commit is. If this is not enough detail for a commit, a new paragraph(s) can be added to explain in proper depth what has happened (detail should be good enough that a core developer reading the commit message understands the justification for the change).

Check the git bootcamp for further instructions on how the commit should look like when merging a pull request.

3.7. Licensing

To accept your change we must have your formal approval for distributing your work under the PSF license. Therefore, you need to sign a contributor agreement which allows the Python Software Foundation to license your code for use with Python (you retain the copyright).

Note

You only have to sign this document once, it will then apply to all your further contributions to Python.

Here are the steps needed in order to sign the CLA:

  1. If you don’t have an account on bugs.python.org (aka b.p.o), please register to create one.
  2. Make sure your GitHub username is listed in the “Your Details” section at b.p.o.
  3. Fill out and sign the PSF contributor form. The “bugs.python.org username” requested by the form is the “Login name” field under “Your Details”.

After signing the CLA, please wait at least one US business day and then check “Your Details” on b.p.o to see if your account has been marked as having signed the CLA (the delay is due to a person having to manually check your signed CLA). Once you have verified that your b.p.o account reflects your signing of the CLA, you can either ask for the CLA check to be run again or wait for it to be run automatically the next time you push changes to your PR.

3.8. Submitting

Once you are satisfied with your work you will want to commit your changes to your branch. In general you can run git commit -a and that will commit everything. You can always run git status to see what changes are outstanding.

When all of your changes are committed (i.e. git status doesn’t list anything), you will want to push your branch to your fork:

git push origin <branch name>

This will get your changes up to GitHub.

Now you want to create a pull request from your fork. If this is a pull request in response to a pre-existing issue on the issue tracker, please make sure to reference the issue number using bpo-NNNN in the pull request title or message.

If this is a pull request for an unreported issue (assuming you already performed a search on the issue tracker for a pre-existing issue), create a new issue and reference it in the pull request. Please fill in as much relevant detail as possible to prevent reviewers from having to delay reviewing your pull request because of lack of information.

If this issue is so simple that there’s no need for an issue to track any discussion of what the pull request is trying to solve (e.g. fixing a spelling mistake), then the pull request needs to have the “skip issue” label added to it.

Your pull request may involve several commits as a result of addressing code review comments. Please keep the commit history in the pull request intact by not squashing, amending, or anything that would require a force push to GitHub. A detailed commit history allows reviewers to view the diff of one commit to another so they can easily verify whether their comments have been addressed. The commits will be squashed when the pull request is merged.

3.9. Converting an Existing Patch from b.p.o to GitHub

When a patch exists in the issue tracker that should be converted into a GitHub pull request, please first ask the original patch author to prepare their own pull request. If the author does not respond after a week, it is acceptable for another contributor to prepare the pull request based on the existing patch. In this case, both parties should sign the CLA. When creating a pull request based on another person’s patch, provide attribution to the original patch author by adding “Original patch by <author name>.” to the pull request description and commit message.

See also Applying a Patch from Mercurial to Git.

3.10. Reviewing

To begin with, please be patient! There are many more people submitting pull requests than there are people capable of reviewing your pull request. Getting your pull request reviewed requires a reviewer to have the spare time and motivation to look at your pull request (we cannot force anyone to review pull requests and no one is employed to look at pull requests). If your pull request has not received any notice from reviewers (i.e., no comment made) after one month, first “ping” the issue on the issue tracker to remind the nosy list that the pull request needs a review. If you don’t get a response within a week after pinging the issue, then you can try emailing python-dev@python.org to ask for someone to review your pull request.

When someone does manage to find the time to look at your pull request they will most likely make comments about how it can be improved (don’t worry, even core developers of Python have their pull requests sent back to them for changes). It is then expected that you update your pull request to address these comments, and the review process will thus iterate until a satisfactory solution has emerged.

3.10.1. How to Review a Pull Request

One of the bottlenecks in the Python development process is the lack of code reviews. If you browse the bug tracker, you will see that numerous issues have a fix, but cannot be merged into the main source code repository, because no one has reviewed the proposed solution. Reviewing a pull request can be just as informative as providing a pull request and it will allow you to give constructive comments on another developer’s work. This guide provides a checklist for submitting a code review. It is a common misconception that in order to be useful, a code review has to be perfect. This is not the case at all! It is helpful to just test the pull request and/or play around with the code and leave comments in the pull request or issue tracker.

  1. If you have not already done so, get a copy of the CPython repository by following the setup guide, build it and run the tests.
  2. Check the bug tracker to see what steps are necessary to reproduce the issue and confirm that you can reproduce the issue in your version of the Python REPL (the interactive shell prompt), which you can launch by executing ./python inside the repository.
  3. Checkout and apply the pull request (Please refer to the instruction Downloading Other’s Patches)
  4. If the changes affect any C file, run the build again.
  5. Launch the Python REPL (the interactive shell prompt) and check if you can reproduce the issue. Now that the pull request has been applied, the issue should be fixed (in theory, but mistakes do happen! A good review aims to catch these before the code is merged into the Python repository). You should also try to see if there are any corner cases in this or related issues that the author of the fix may have missed.
  6. If you have time, run the entire test suite. If you are pressed for time, run the tests for the module(s) where changes were applied. However, please be aware that if you are recommending a pull request as ‘merge-ready’, you should always make sure the entire test suite passes.

3.11. Dismissing Review from Another Core Developer

A core developer can dismiss another core developer’s review if they confirmed that the requested changes have been made. When a core developer has assigned the PR to themselves, then it is a sign that they are actively looking after the PR, and their review should not be dismissed.

3.12. Committing/Rejecting

Once your pull request has reached an acceptable state (and thus considered “accepted”), it will either be merged or rejected. If it is rejected, please do not take it personally! Your work is still appreciated regardless of whether your pull request is merged. Balancing what does and does not go into Python is tricky and we simply cannot accept everyone’s contributions.

But if your pull request is merged it will then go into Python’s VCS to be released with the next major release of Python. It may also be backported to older versions of Python as a bugfix if the core developer doing the merge believes it is warranted.

3.13. Crediting

Non-trivial contributions are credited in the Misc/ACKS file (and, most often, in a contribution’s news entry as well). You may be asked to make these edits on the behalf of the core developer who accepts your pull request.