Lifecycle of a pull request#

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).

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:

Create an issue that describes your change [*]
Create a new branch in Git from the main branch
Work on changes (e.g. fix a bug or add a new feature)
Run tests and make patchcheck
Commit and push changes to your GitHub fork
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! :)

Note

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.

Step-by-step guide#

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

Update data from your upstream repository:
```
git fetch upstream
```
Create a new branch in your local clone from the main branch in the upstream repository:
```
git checkout -b <branch-name> upstream/main
```
Note

Almost all changes to Python, including bug fixes, should first be made against the main branch. See here for more details.
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 and 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>
```
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:
```
git status
```
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 main branch 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 main or the change requires a different approach in an older Python version compared to main.
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.

Note

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`#

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

On Unix (including macOS):

make patchcheck

On Windows (after any successful build):

.\python.bat Tools\patchcheck\patchcheck.py

The automated patch checklist runs through:

Are there any whitespace problems in Python files? (using Tools/patchcheck/reindent.py)
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?
Has an entry under Misc/NEWS.d/next been added? (using blurb-it, or the blurb tool)
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.

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.

Note

How to Write a Git Commit Message is a nice article that describes how to write a good commit message.

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:

Create a change and submit it as a pull request.
When cpython-cla-bot comments 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.

Lifecycle of a pull request#

Introduction#

Quick guide#

Step-by-step guide#

Resolving merge conflicts#

Making good PRs#

patchcheck#

Making good commits#

Licensing#

`patchcheck`#