3. Lifecycle of a Patch

3.1. Creating

3.1.1. Tool Usage

Mercurial allows for various workflows according to each person’s or project’s preference. It is out of this guide’s scope to present them all, so we will stick to a basic workflow where you work on a patch in your working copy without ever making any local commits.

If you use this workflow, and your work adds or removes files to the source tree, you will have to temporarily hg add or hg remove them, respectively, before generating a patch.

To generate a patch, just invoke hg diff which will print out a patch of the working copy’s changes against the current revision:

hg diff > mywork.patch

If you want to undo your changes, you can revert them from the working copy:

hg revert -a

You can later re-apply the changes if you want to continue working on the patch:

hg import --no-commit mywork.patch

See also

Refer to the FAQ for more information on using Mercurial.

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

Assuming you are using the basic approach suggested earlier, just type the following:

hg diff > mywork.patch

If you are using another approach, you probably need to find out the right invocation of hg diff for your purposes; see hg help diff and hg help revisions. Just please make sure that you generate a single, condensed patch rather than a series of several changesets.

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).


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

3.3. Submitting

If this is a patch in response to a pre-existing issue on the issue tracker, attach the patch to the issue; use the Choose File button on the tracker web page for the issue to upload your patch file. Please provide any details about your patch that would be relevant to the discussion of the issue or your patch.

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 attach your patch. 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 patches than there are people capable of reviewing your patch. Getting your patch reviewed requires a reviewer to have the spare time and motivation to look at your patch (we cannot force anyone to review patches). If your patch 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 few days after pinging the issue, then you can try emailing python-dev@python.org asking for someone to review your patch.

When someone does manage to find the time to look at your patch 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 post a new patch addressing these comments, and the review process will thus iterate until a satisfactory solution has emerged.

3.4.1. How to Review a Patch

One of the bottlenecks in the Python development process is the lack of patch reviews. If you browse the bug tracker, you will see that numerous issues have a patch, but cannot be commited to the main source code repository, because no one has reviewed the proposed patch. Reviewing a patch can be just as informative as providing a patch and it will allow you to give constructive comments on another developer’s work. This guide provides a checklist for submitting a patch review. It is a common misconception that in order to be useful, a patch 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 bug 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 patch you saved from the bug tracker. If you are not sure how to apply a patch, please check the Lifecycle of a Patch documentation.
  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 patch 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 patch 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). This is something the core developer will do when committing your patch, you don’t have to propose the addition by yourself.