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sandbox/morph-devguide / patch.rst

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Lifecycle of a Patch


Tool Usage

Mercurial allows for various workflows according to each person's or project's preference. We present here a very simple solution based on mq (Mercurial Queues). You are welcome to use any approach you like (including a svn-like approach of simply using hg diff to create a patch based on uncommitted changes in your working copy). Usage of mq is merely a suggestion; it's a balance between being able to do everything needed while allowing for more powerful usage if desired in the future.

First make sure that the extension has been turned on in your .hgrc or Mercurial.ini file:

mq =

You can verify this is working properly by running hg help mq.

Before you start modifying things in your working copy, type:

hg qnew mywork

where mywork is a descriptive name for what you are going to work on. This will create a patch in your patch queue. Whenever you have reached a point that you want to save what you have done, run:

hg qrefresh

This will update the patch to contain all of the changes you have made up to this point. If you have added or removed any file, use hg add or hg remove, respectively, before running hg qrefresh.

Later on, we will explain :ref:`how to generate a patch <patch-generation>`.

If you want to delete your changes irrevocably (either because they were committed, or they ended up uninteresting), use:

hg qpop mywork
hg qdelete mywork


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 discrepencies 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, having you think about backwards-compatibility early will help prevent having your patch rejected on these grounds. Basically just put yourself in the shoes of someone whose code will be broken by a change to pre-existing semantics. It is guaranteed 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 somewhere to update their code (this obviously does not apply to new semantics).

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 :ref:`runs <runtests>` 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 :ref:`documentation <documenting>` additions/changes should be included.

Sixth, if you are not already in the Misc/ACKS file then add your name. If you have taken the time to diagnose a problem, invent a solution, code it up, and submit a patch you deserve to be recognized as having contributed to Python. This also means 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).


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:

./python.exe Tools/scripts/patchcheck.py

Assuming you are using the :ref:`mq approach <mq-workflow>` suggested earlier, first check that all your local changes have been recorded (using hg qrefresh), then type the following:

hg qdiff > 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.

Also, please make sure your patch is whitespace normalized. patchcheck will check this for you.


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.


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 a substantial amount of time then you may email python-dev@python.org asking for someone to take a look at 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.


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 :abbr:`VCS (version control system)` 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.