Lifecycle of a pull request#
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 (
Here is a quick overview of how you can contribute to CPython:
Create a new branch in Git from the
Work on changes (e.g. fix a bug or add a new feature)
Run tests and
Create Pull Request on GitHub to merge a branch from your fork
Make sure the continuous integration checks on your Pull Request are green (i.e. successful)
Review and address comments on your Pull Request
When your changes are merged, you can delete the PR branch
Celebrate contributing to CPython! :)
In order to keep the commit history intact, please avoid squashing or amending history and then force-pushing to the PR. Reviewers often want to look at individual commits.
Update data from your
git fetch upstream
Create a new branch in your local clone from the
mainbranch in the upstream repository:
git checkout -b <branch-name> upstream/main
Almost all changes to Python, including bug fixes, should first be made against the
mainbranch. See here for more details.
Make changes to the code, and use
git diffto 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
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>
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 switch <branch-name> # make changes and run tests git add <filenames> git commit -m '<message>' git push origin <branch-name>
If a core developer reviewing your PR pushed one or more commits to your PR branch, then after checking out your branch and before editing, run:
git pull origin <branch-name> # pull = fetch + merge
If you have made local changes that have not been pushed to your fork and there are merge conflicts, Git will warn you about this and enter conflict resolution mode. See Resolving merge conflicts below.
If time passes and there are merge conflicts with the main branch, GitHub will show a warning to this end and you may be asked to address this. Merge the changes from the main branch while resolving the conflicts locally:
git switch <branch-name> git pull upstream main # pull = fetch + merge # resolve conflicts: see "Resolving Merge Conflicts" below 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
Resolving merge conflicts#
When merging changes from different branches (or variants of a branch on different repos), the two branches may contain incompatible changes to one or more files. These are called “merge conflicts” and need to be manually resolved as follows:
Check which files have merge conflicts:
Edit the affected files and bring them to their intended final state. Make sure to remove the special “conflict markers” inserted by Git.
Commit the affected files:
git add <filenames> git merge --continue
When running the final command, Git may open an editor for writing a commit message. It is usually okay to leave that as-is and close the editor.
See the merge command’s documentation for a detailed technical explanation.
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.
Make your change against the right version of Python. In general all changes are made against the
mainbranch first. This includes bug fixes. After the change is merged there, it will be ported back to older maintenance releases as well. That way we ensure all affected versions are handled. Therefore, basing a new change directly on a maintenance branch is only used in specific circumstances, for instance when that change does not apply to
mainor the change requires a different approach in an older Python version compared to
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.
Pull requests with only code formatting changes are usually rejected. On the other hand, fixes for typos and grammar errors in documents and docstrings are welcome.
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.
Make sure you have proper tests to verify your pull request works as expected. Pull requests will not be accepted without the proper tests!
Make sure all tests pass. The entire test suite needs to run 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.
Proper documentation additions/changes should be included.
patchcheck is a simple automated patch checklist that guides a developer
through the common patch generation checks. To run
On Unix (including macOS):
On Windows (after any successful build):
The automated patch checklist runs through:
Are there any whitespace problems in Python files? (using
Are there any whitespace problems in C files?
Are there any whitespace problems in the documentation?
Has the documentation been updated?
Has the test suite been updated?
configurebeen regenerated, if necessary?
pyconfig.h.inbeen 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.
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:
Make the spam module more spammy 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. The imperative form (used in the example
above) is strongly preferred to a descriptive form such as ‘the spam module is
now more spammy’. Use
git log --oneline to see existing title lines.
Furthermore, the first line should not end in a period.
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 message should look like when merging a pull request.
How to Write a Git Commit Message is a nice article that describes how to write a good commit message.
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).
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:
Create a change and submit it as a pull request.
cpython-cla-botcomments on your pull request that commit authors are required to sign a Contributor License Agreement, click on the button in the comment to sign it. It’s enough to log in through GitHub. The process is automatic.
After signing, the comment by
cpython-cla-botwill update to indicate that “all commit authors signed the Contributor License Agreement.
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
gh-NNNNN: prefix in the pull request title and
#NNNNN in the description.
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 by someone with commit access.
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.
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 “Co-authored-by: Author Name <email_address> .” to the pull request description and commit message. See the GitHub article on how to properly add the co-author info.
See also Applying a Patch to Git.
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 subscribers that the pull request needs a review. If you don’t get a response within a week after pinging the issue, you can post on the Core Development Discourse category 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.
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.
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.
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.
Checkout and apply the pull request (Please refer to the instruction Downloading other’s patches)
If the changes affect any C file, run the build again.
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.
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.
Leaving a pull request review on GitHub#
When you review a pull request, you should provide additional details and context of your review process.
Instead of simply “approving” the pull request, leave comments. For example:
If you tested the PR, report the result and the system and version tested on, such as ‘Windows 10’, ‘Ubuntu 16.4’, or ‘Mac High Sierra’.
If you request changes, try to suggest how.
Comment on what is “good” about the pull request, not just the “bad”. Doing so will make it easier for the PR author to find the good in your comments.
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.
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.
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.