3. Lifecycle of a Pull Request

3.1. Creating

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 GitHub, and than create a pull request. GitHub’s help pages are good and there are tons of pages out there for help with Git. As such, this document does not go into any great detail as the assumption is there is a resource out there which will explain things in a way that makes sense for you personally when it comes to general Git and GitHub details.

3.1.1. Tool Setup

If you have not already done so, you will want to fork the CPython repository. You can read GitHub’s documentation on how to fork a repository if you are not already familiar with how to do this. This will make sure that you have a clone of your fork of CPython on your computer and the appropriate remote repositories.

You will then want to create a branch to contain your work. GitHub has instructions on how to create a branch within your fork through their website.

3.1.2. Preparation

When creating a patch for submission, there are several things that you should do to help ensure that your patch 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 commits your patch. But if you have systematic deviations from the style guides your patch 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 patch will make the final call on whether something is acceptable, thinking about backwards-compatibility early will help prevent having your patch rejected on these grounds. Put yourself in the shoes of someone whose code will be broken by the change(s) introduced by the patch. 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 patch works as expected. Patches 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.

3.1.3. Generation

To perform a quick sanity check on your patch, you can run:

make patchcheck

This will check and/or fix various common things people forget to do for patches, such as adding any new files needed for the patch to work (note that not all checks apply to non-core developers). On Windows, use this command (after any successful build of Python):

python.bat Tools/scripts/patchcheck.py

3.2. Licensing

For non-trivial changes, we must have your formal approval for distributing your work under the PSF license. Therefore, you need to fill out a contributor form 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.

You will also need to specify you GitHub username on the issue tracker on the Your Details link found on the homepage.

3.3. 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 patch 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 patch reviewers from having to delay reviewing your patch because of lack of information.

3.4. 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 patch 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 patch.

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 patches 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.

3.4.1. 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 committed to 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 patch 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. Apply the pull request (GitHub has instructions with each pull request on how to do this).
4. If the patch affects 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 patch 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 committed to the Python repository). You should also try to see if there are any corner cases in this or related issue 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 patch as ‘commit-ready’, you should always make sure the entire test suite passes.

3.5. Committing/Rejecting

Once your patch has reached an acceptable state (and thus considered “accepted”), it will either be committed or rejected. If it is rejected, please do not take it personally! Your work is still appreciated regardless of whether your patch is committed. 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 committed 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 commit believes it is warranted.

3.6. Crediting

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