PEP: 512
Title: Migrating from hg.python.org to GitHub
Version: $Revision$
Last-Modified: $Date$
Author: Brett Cannon <brett@python.org>
Discussions-To: core-workflow@python.org
Status: Final
Type: Process
Content-Type: text/x-rst
Created: 17-Jan-2015
Post-History: 17-Jan-2016, 19-Jan-2016, 23-Jan-2016


.. note::

   CPython's development process moved to https://github.com/python/cpython
   on 2017-02-10.

Abstract
========

This PEP outlines the steps required to migrate Python's development
process from Mercurial [#hg]_ as hosted at
hg.python.org [#h.p.o]_ to Git [#git]_ on GitHub [#GitHub]_. Meeting
the minimum goals of this PEP should allow for the development
process of Python to be as productive as it currently is, and meeting
its extended goals should improve the development process from its
status quo.


Rationale
=========

In 2014, it became obvious that Python's custom development
process was becoming a hindrance. As an example, for an external
contributor to submit a fix for a bug that eventually was committed,
the basic steps were:

1. Open an issue for the bug at bugs.python.org [#b.p.o]_.
2. Checkout out the CPython source code from hg.python.org [#h.p.o]_.
3. Make the fix.
4. Upload a patch.
5. Have a core developer review the patch using our fork of the
   Rietveld code review tool [#rietveld]_.
6. Download the patch to make sure it still applies cleanly.
7. Run the test suite manually.
8. Update the ``NEWS``, ``ACKS``, and "What's New" document as necessary
9. Pull changes to avoid a merge race.
10. Commit the change manually.
11. If the change was for a bugfix release, merge into the
    in-development branch.
12. Run the test suite manually again.
13. Commit the merge.
14. Push the changes.

This is a very heavy, manual process for core developers. Even in the
simple case, you could only possibly skip the code review step, as you
would still need to build the documentation. This led to patches
languishing on the issue tracker due to core developers not being
able to work through the backlog fast enough to keep up with
submissions. In turn, that led to a side-effect issue of discouraging
outside contribution due to frustration from lack of attention, which
is a dangerous problem for an open source project with no corporate
backing as it runs counter to having a viable future for the project.
While allowing patches to be uploaded to bugs.python.org [#b.p.o]_ is
potentially simple for an external contributor, it is as slow and
burdensome as it gets for a core developer to work with.

Hence the decision was made in late 2014 that a move to a new
development process was needed. A request for PEPs
proposing new workflows was made, in the end leading to two:
:pep:`481` and :pep:`507` proposing GitHub [#github]_ and
GitLab [#gitlab]_, respectively.

The year 2015 was spent off-and-on working on those proposals and
trying to tease out details of what made them different from each
other on the core-workflow mailing list [#core-workflow]_.
PyCon US 2015 also showed that the community was a bit frustrated
with our process due to both cognitive overhead for new contributors
and how long it was taking for core developers to
look at a patch (see the end of Guido van Rossum's
keynote at PyCon US 2015 [#guido-keynote]_ as an example of the
frustration).

On January 1, 2016, the decision was made by Brett Cannon to move the
development process to GitHub. The key reasons for choosing GitHub
were [#reasons]_:

* Maintaining custom infrastructure has been a burden on volunteers
  (e.g., an unmaintained, custom fork of Rietveld [#rietveld]_
  is currently being used).
* The custom workflow is very time-consuming for core developers
  (not enough automated tooling built to help support it).
* The custom workflow is a hindrance to external contributors
  (acts as a barrier of entry due to time required to ramp up on
  development process unique to CPython itself).
* There is no feature differentiating GitLab from GitHub beyond
  GitLab being open source.
* Familiarity with GitHub is far higher among core developers and
  external contributors than with GitLab.
* Our BDFL prefers GitHub (who would be the first person to tell
  you that his opinion shouldn't matter, but the person making the
  decision felt it was important that the BDFL feel comfortable with
  the workflow of his own programming language to encourage his
  continued participation).

There's even already an unofficial logo to represent the
migration to GitHub [#pythocat]_.

The overarching goal of this migration is to improve the development
process to the extent that a core developer can go from external
contribution submission through all the steps leading to committing
said contribution from within a browser on a tablet with WiFi
using *some* development process (this does not inherently mean
GitHub's default workflow). The final solution will also allow
an external contributor to contribute even if they chose not to use
GitHub (although there is not guarantee in feature parity).


Repositories to Migrate
=======================

While hg.python.org [#h.p.o]_ hosts many repositories, there are only
five key repositories that need to move:

1. devinabox [#devinabox-repo]_ (done)
2. benchmarks [#benchmarks-repo]_ (skipped)
3. peps [#peps-repo]_ (done)
4. devguide [#devguide-repo]_ (done)
5. cpython [#cpython-repo]_

The devinabox repository is code-only.
The peps and devguide repositories involve the generation of webpages.
And the cpython repository has special requirements for integration
with bugs.python.org [#b.p.o]_.


Migration Plan
==============

The migration plan is separated into sections based on what is
required to migrate the repositories listed in the
`Repositories to Migrate`_ section. Completion of requirements
outlined in each section should unblock the migration of the related
repositories. The sections are expected to be completed in order, but
not necessarily the requirements within a section.


Requirements for Code-Only Repositories
---------------------------------------

Completion of the requirements in this section will allow the
devinabox repository to move to GitHub.


Create a 'Python core' team
'''''''''''''''''''''''''''

To manage permissions, a 'Python core' team will be created as part of
the python organization [#github-python-org]_. Any repository that is
moved will have the 'Python core' team added to it with write
permissions [#github-org-perms]_. Anyone who previously had rights to
manage SSH keys on hg.python.org will become a team maintainer for the
'Python core' team.


Define commands to move a Mercurial repository to Git
'''''''''''''''''''''''''''''''''''''''''''''''''''''

Since moving to GitHub also entails moving to Git [#git]_, we must
decide what tools and commands we will run to translate a Mercurial
repository to Git. The tools developed specifically for this migration
are hosted at https://github.com/orsenthil/cpython-hg-to-git .


CLA enforcement
'''''''''''''''

A key part of any open source project is making sure that its source
code can be properly licensed. This requires making sure all people
making contributions have signed a contributor license agreement
(CLA) [#cla]_. Up until now, enforcement of CLA signing of
contributed code has been enforced by core developers checking
whether someone had an ``*`` by their username on
bugs.python.org [#b.p.o]_. With this migration, the plan is to start
off with automated checking and enforcement of contributors signing
the CLA.


Adding GitHub username support to bugs.python.org
+++++++++++++++++++++++++++++++++++++++++++++++++

To keep tracking of CLA signing under the direct control of the PSF,
tracking who has signed the PSF CLA will be continued by marking that
fact as part of someone's bugs.python.org user profile. What this
means is that an association will be needed between a person's
bugs.python.org [#b.p.o]_ account and their GitHub account, which
will be done through a new field in a user's profile. This does
implicitly require that contributors will need both a
GitHub [#github]_ and bugs.python.org account in order to sign the
CLA and contribute through GitHub.

An API is provided to query bugs.python.org to see if a GitHub
username corresponds to someone who has signed the CLA. Making a GET
request to e.g.
http://bugs.python.org/user?@template=clacheck&github_names=brettcannon,notanuser
returns a JSON dictionary with the keys of the usernames requested
and a ``true`` value if they have signed the CLA, ``false`` if they
have not, and ``null`` if no corresponding GitHub username was found.


A bot to enforce CLA signing
++++++++++++++++++++++++++++

With an association between someone's GitHub account and their
bugs.python.org [#b.p.o]_ account, which has the data as to whether
someone has signed the CLA, a bot can monitor pull requests on
GitHub and denote whether the contributor has signed the CLA.

If the user has signed the CLA, the bot will add a positive label to
the issue to denote the pull request has no CLA issues (e.g., a green
label stating, "CLA signed"). If the contributor has not signed a CLA,
a negative label will be added to the pull request will be blocked
using GitHub's status API (e.g., a red label stating, "CLA not signed").
If a contributor lacks a bugs.python.org account, that will lead to
the negative label being used as well. Using a label for both
positive and negative cases provides a fallback signal if the
bot happens to fail, preventing potential false-positives or
false-negatives. It also allows for an easy way to trigger the bot
again by simply removing a CLA-related label (this is in contrast to
using a GitHub status check [#gh-status-check]_ which is only
triggered on code changes).

As no pre-existing bot exists to meet our needs, it will be hosted on
Heroku [#heroku]_ and written to target Python 3.5 to act as a
showcase for asynchronous programming. The code for the bot is hosted
in the Knights Who Say Ni project [#ni]_.


Make old repository read-only
'''''''''''''''''''''''''''''

Updating ``.hg/hgrc`` in the now-old Mercurial repository in the ``[hooks]``
section with::

  pretxnchangegroup.reject = echo " * This repo has been migrated to github.com/python/peps and does not accept new commits in Mercurial!" 2>&1; exit 1

will make the repository read-only.


Requirements for Web-Related Repositories
-----------------------------------------

Due to their use for generating webpages, the
devguide [#devguide-repo]_ and peps [#peps-repo]_ repositories need
their respective processes updated to pull from their new Git
repositories.


Requirements for the cpython Repository
---------------------------------------

Obviously the most active and important repository currently hosted
at hg.python.org [#h.p.o]_ is the cpython
repository [#cpython-repo]_. Because of its importance and
high-frequency use, it requires more tooling before being moved to GitHub
compared to the other repositories mentioned in this PEP.


Document steps to commit a pull request
'''''''''''''''''''''''''''''''''''''''

During the process of choosing a new development workflow, it was
decided that a linear history is desired. People preferred having a
single commit representing a single change instead of having a set of
unrelated commits lead to a merge commit that represented a single
change. This means that the convenient "Merge" button in GitHub pull
requests will be set to only do *squash* commits and not merge
commits.

A second set of recommended commands will also be written for
committing a contribution from a patch file uploaded to
bugs.python.org [#b.p.o]_. This will obviously help keep the linear
history, but it will need to be made to have attribution to the patch
author.

The exact sequence of commands that will be given as guidelines to
core developers is an open issue:
`Git CLI commands for committing a pull request to cpython`_.


Linking pull requests to issues
'''''''''''''''''''''''''''''''

Historically, external contributions were attached to an issue on
bugs.python.org [#b.p.o]_ thanks to the fact that all external
contributions were uploaded as a file. For changes committed by a
core developer who committed a change directly, the specifying of an
issue number in the commit message of the format ``Issue #`` at the
start of the message led to a comment being posted to the issue
linking to the commit.


Linking a pull request to an issue
++++++++++++++++++++++++++++++++++

An association between a pull request and an issue is needed to track
when a fix has been proposed. The association needs to be many-to-one
as there can take multiple pull requests to solve a single issue
(technically it should be a many-to-many association for when a
single fix solves multiple issues, but this is fairly rare and issues
can be merged into one using the ``Superseder`` field on the issue
tracker).

The association between a pull request and an issue will be done based
on detecting an issue number. If the issue is specified in either the
title or in the body of a message on a pull request then a connection
will be made on bugs.python.org [#b.p.o]_. Some visible notification
-- e.g. label or message -- will be made to the pull request to
notify that the association was successfully made.


Notify the issue if a commit is made
++++++++++++++++++++++++++++++++++++

Once a commit is made, the corresponding issue should be updated to
reflect this fact. This should work regardless of whether the commit
came from a pull request or a direct commit.


Update the linking service for mapping commit IDs to URLs
'''''''''''''''''''''''''''''''''''''''''''''''''''''''''

Currently you can use https://hg.python.org/lookup/ with a revision
ID from either the Subversion or Mercurial copies of the
cpython repo [#cpython-repo]_ to get redirected to the URL for that
revision in the Mercurial repository. The URL rewriter will need to
be updated to redirect to the Git repository and to support the new
revision IDs created for the Git repository.

The most likely design is to statically know all the Mercurial
changeset numbers once the migration has occurred. The lookup code
will then be updated to accept hashes from 7 to 40 hexadecimal digits.
Any hexadecimal of length 12 or 40 will be compared against the
Mercurial changeset numbers. If the number doesn't match or is of some
other length between 7 and 40 then it will be assumed to be a Git hash.

The `bugs.python.org commit number rewriter <https://hg.python.org/tracker/python-dev/file/tip/extensions/local_replace.py#l76>`__
will also need to be updated to accept hashes as short as 7 digits as
Git will match on hashes that short or longer.

Deprecate sys._mercurial
''''''''''''''''''''''''

Once Python is no longer kept in Mercurial, the ``sys._mercurial``
attribute will need to be changed to return ``('CPython', '', '')``.
An equivalent ``sys._git`` attribute will be added which fulfills the
same use-cases.


Update the devguide
'''''''''''''''''''

The devguide will need to be updated with details of the new
workflow. Mostly likely work will take place in a separate branch
until the migration actually occurs.


Update PEP 101
''''''''''''''

The release process will need to be updated as necessary.


Optional, Planned Features
--------------------------

Once the cpython repository [#cpython-repo]_ is migrated, all
repositories will have been moved to GitHub [#github]_ and the
development process should be on equal footing as before the move. But
a key reason for this migration is to improve the development process,
making it better than it has ever been. This section outlines some
plans on how to improve things.

It should be mentioned that overall feature planning for
bugs.python.org [#b.p.o]_ -- which includes plans independent of this
migration -- are tracked on their own wiki page [#tracker-plans]_.


Handling Misc/NEWS
''''''''''''''''''

Traditionally the ``Misc/NEWS`` file [#news-file]_ has been
problematic for changes which spanned Python releases. Oftentimes
there will be merge conflicts when committing a change between e.g.,
3.5 and 3.6 only in the ``Misc/NEWS`` file. It's so common, in fact,
that the example instructions in the devguide explicitly mention how
to resolve conflicts in the ``Misc/NEWS`` file
[#devguide-merge-across-branches]_. As part of our tool
modernization, working with the ``Misc/NEWS`` file will be
simplified.

The planned approach is to use an individual file per news entry,
containing the text for the entry. In this scenario, each feature
release would have its own directory for news entries and a separate
file would be created in that directory that was either named after
the issue it closed or a timestamp value (which prevents collisions).
Merges across branches would have no issue as the news entry file
would still be uniquely named and in the directory of the latest
version that contained the fix. A script would collect all news entry
files no matter what directory they reside in and create an
appropriate news file (the release directory can be ignored as the
mere fact that the file exists is enough to represent that the entry
belongs to the release). Classification can either be done by keyword
in the new entry file itself or by using subdirectories representing
each news entry classification in each release directory (or
classification of news entries could be dropped since critical
information is captured by the "What's New" documents which are
organized). The benefit of this approach is that it keeps the changes
with the code that was actually changed. It also ties the message to
being part of the commit which introduced the change. For a commit
made through the CLI, a script could be provided to help generate the
file. In a bot-driven scenario, the merge bot could have a way to
specify a specific news entry and create the file as part of its
flattened commit (while most likely also supporting using the first
line of the commit message if no specific news entry was specified).
If a web-based workflow is used then a status check could be used to
verify that a new entry file is in the pull request to act as a
reminder that the file is missing. Code for this approach has been
written previously for the Mercurial workflow at
http://bugs.python.org/issue18967. There is also tools from the
community like https://pypi.python.org/pypi/towncrier,
https://github.com/twisted/newsbuilder, and
http://docs.openstack.org/developer/reno/.

Discussions at the Sep 2016 Python core-dev sprints led to this
decision compared to the rejected approaches outlined in the
``Rejected Ideas`` section of this PEP. The separate files approach
seems to have the right balance of flexibility and potential tooling
out of the various options while solving the motivating problem.

Work for this is being tracked at
https://github.com/python/core-workflow/issues/6.


Handling Misc/ACKS
''''''''''''''''''

Traditionally the ``Misc/ACKS`` file [#acks-file]_ has been managed
by hand. But thanks to Git supporting an ``author`` value as well as
a ``committer`` value per commit, authorship of a commit can be part
of the history of the code itself.

As such, manual management of ``Misc/ACKS`` will become optional. A
script will be written that will collect all author and committer
names and merge them into ``Misc/ACKS`` with all of the names listed
prior to the move to Git. Running this script will become part of the
release process.

The script should also generate a list of all people who contributed
since the last execution. This will allow having a list of those who
contributed to a specific release so they can be explicitly thanked.

Work for this is being tracked at
https://github.com/python/core-workflow/issues/7.


Create ``https://git.python.org``
'''''''''''''''''''''''''''''''''

Just as hg.python.org [#h.p.o]_ currently points to the Mercurial
repository for Python, git.python.org should do the equivalent for
the Git repository.


Backup of pull request data
'''''''''''''''''''''''''''

Since GitHub [#github]_ is going to be used for code hosting and code
review, those two things need to be backed up. In the case of code
hosting, the backup is implicit as all non-shallow Git [#git]_ clones
contain the full history of the repository, hence there will be many
backups of the repository.

The code review history does not have the same implicit backup
mechanism as the repository itself. That means a daily backup of code
review history should be done so that it is not lost in case of any
issues with GitHub. It also helps guarantee that a migration from
GitHub to some other code review system is feasible were GitHub to
disappear overnight.


Bot to generate cherry-pick pull requests
''''''''''''''''''''''''''''''''''''''''''

Since the decision has been made to work with cherry-picks instead of
forward merging of branches, it would be convenient to have a bot that
would generate pull requests based on cherry-picking for any pull
requests that affect multiple branches. The most likely design is a
bot that monitors merged pull requests with key labels applied that
delineate what branches the pull request should be cherry-picked into.
The bot would then generate cherry-pick pull requests for each label
and remove the labels as the pull requests are created (this allows
for easy detection when automatic cherry-picking failed).

Work for this is being tracked at
https://github.com/python/core-workflow/issues/8.


Pull request commit queue
''''''''''''''''''''''''''

This would linearly apply accepted pull requests and verify that the
commits did not interfere with each other by running the test suite
and backing out commits if the test run failed. To help facilitate
the speed of testing, all patches committed since the last test run
can be applied at once under a single test run as the optimistic
assumption is that the patches will work in tandem. Some mechanism to
re-run the tests in case of test flakiness will be needed, whether it
is from removing a "test failed" label, web interface for core
developers to trigger another testing event, etc.

Inspiration or basis of the bot could be taken from pre-existing bots
such as Homu [#homu]_ or Zuul [#zuul]_.

The name given to this bot in order to give it commands is an open
issue: `Naming the bots`_.


A CI service
''''''''''''

There are various CI services that provide free support for open
source projects hosted on GitHub [#github]_. After experimenting
with a couple CI services, the decision was made to go with
Travis [#travis]_.

The current CI service for Python is Pypatcher [#pypatcher]_. A
request can be made in IRC to try a patch from
bugs.python.org [#b.p.o]_. The results can be viewed at
https://ci.centos.org/job/cPython-build-patch/ .

Work for this is being tracked at
https://github.com/python/core-workflow/issues/1.


Test coverage report
''''''''''''''''''''

Getting an up-to-date test coverage report for Python's standard
library would be extremely beneficial as generating such a report can
take quite a while to produce.

There are a couple pre-existing services that provide free test
coverage for open source projects. In the end, Codecov [#codecov]_ was
chosen as the best option.

Work for this is being tracked at
https://github.com/python/core-workflow/issues/2.


Notifying issues of pull request comments
'''''''''''''''''''''''''''''''''''''''''

The current development process does not include notifying an issue
on bugs.python.org [#b.p.o]_ when a review comment is left on
Rietveld [#rietveld]_. It would be nice to fix this so that people
can subscribe only to comments at bugs.python.org and not
GitHub [#github]_ and yet still know when something occurs on GitHub
in terms of review comments on relevant pull requests. Current
thinking is to post a comment to bugs.python.org to the relevant
issue when at least one review comment has been made over a certain
period of time (e.g., 15 or 30 minutes, although with GitHub now
supporting
`reviews <https://help.github.com/articles/reviewing-changes-in-pull-requests/>`_
the time aspect may be unnecessary). This keeps the email volume
down for those that receive both GitHub and bugs.python.org email
notifications while still making sure that those only following
bugs.python.org know when there might be a review comment to address.


Allow bugs.python.org to use GitHub as a login provider
'''''''''''''''''''''''''''''''''''''''''''''''''''''''

As of right now, bugs.python.org [#b.p.o]_ allows people to log in
using Google, Launchpad, or OpenID credentials. It would be good to
expand this to GitHub credentials.


Web hooks for re-generating web content
'''''''''''''''''''''''''''''''''''''''

The content at https://docs.python.org/,
https://docs.python.org/devguide, and
https://www.python.org/dev/peps/ are all derived from files kept in
one of the repositories to be moved as part of this migration. As
such, it would be nice to set up appropriate webhooks to trigger
rebuilding the appropriate web content when the files they are based
on change instead of having to wait for, e.g., a cronjob to trigger.

This can partially be solved if the documentation is a Sphinx project
as then the site can have an unofficial mirror on
`Read the Docs <https://readthedocs.org/>`_, e.g.
http://cpython-devguide.readthedocs.io/.

Work for this is being tracked at
https://github.com/python/core-workflow/issues/9.


Link web content back to files that it is generated from
''''''''''''''''''''''''''''''''''''''''''''''''''''''''

It would be helpful for people who find issues with any of the
documentation that is generated from a file to have a link on each
page which points back to the file on GitHub [#github]_ that stores
the content of the page. That would allow for quick pull requests to
fix simple things such as spelling mistakes.

Work for this is being tracked at
http://bugs.python.org/issue28929.


Splitting out parts of the documentation into their own repositories
''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''

While certain parts of the documentation at https://docs.python.org
change with the code, other parts are fairly static and are not
tightly bound to the CPython code itself. The following sections of
the documentation fit this category of slow-changing,
loosely-coupled:

* `Tutorial <https://docs.python.org/3/tutorial/index.html>`__
* `Python Setup and Usage <https://docs.python.org/3/using/index.html>`__
* `HOWTOs <https://docs.python.org/3/howto/index.html>`__
* `Installing Python Modules <https://docs.python.org/3/installing/index.html>`__
* `Distributing Python Modules <https://docs.python.org/3/distributing/index.html>`__
* `Extending and Embedding <https://docs.python.org/3/extending/index.html>`__
* `FAQs <https://docs.python.org/3/faq/index.html>`__

These parts of the documentation could be broken out into their own
repositories to simplify their maintenance and to expand who has
commit rights to them to ease in their maintenance.

It has also been suggested to split out the
`What's New <https://docs.python.org/3/whatsnew/index.html>`__
documents. That would require deciding whether a workflow could be
developed where it would be difficult to forget to update
What's New (potentially through a label added to PRs, like
"What's New needed").


Backup of Git repositories
''''''''''''''''''''''''''

While not necessary, it would be good to have official backups of the
various Git repositories for disaster protection. It will be up to
the PSF infrastructure committee to decide if this is worthwhile or
unnecessary.


Identify potential new core developers
''''''''''''''''''''''''''''''''''''''

The Python development team has long-standing guidelines for
selecting new core developers. The key part of the guidelines is that
a person needs to have contributed multiple patches which have been
accepted and are high enough quality and size to demonstrate an
understanding of Python's development process. A bot could be written
which tracks patch acceptance rates and generates a report to help
identify contributors who warrant consideration for becoming core
developers. This work doesn't even necessarily require GitHub
integration as long as the committer field in all git commits is
filled in properly.

Work is being tracked at
https://github.com/python/core-workflow/issues/10.


Status
======

Requirements for migrating the devinabox [#devinabox-repo]_
repository:

* Completed

  - `Adding GitHub username support to bugs.python.org`_
    (Maciej Szulik and Ezio Melotti)
  - `A bot to enforce CLA signing`_:
    https://github.com/python/the-knights-who-say-ni (Brett Cannon)
  - `Create a 'Python core' team`_
  - `Define commands to move a Mercurial repository to Git`_:
    https://github.com/orsenthil/cpython-hg-to-git (Senthil Kumaran)


Repositories whose build steps need updating:

* Completed

  - peps [#peps-repo]_
  - devguide [#devguide-repo]_


cpython repo [#cpython-repo]_
-----------------------------

Required:

* Not started

  - `Update PEP 101`_ (commitment from Ned Deily to do this;
    **non-blocker**)

* In progress

  - `Deprecate sys._mercurial`_
    (http://bugs.python.org/issue27593;
    review committal from Ned Deily;
    **non-blocker**)
  - `Update the linking service for mapping commit IDs to URLs`_
    (code ready, needs deployment once the hg repository is made read-only;
    https://gist.github.com/brettcannon/f8d97c92b0df264cd4db008ffd32daf9;
    **post-migration**)

* Completed

  - `Notify the issue if a commit is made`_
    (http://psf.upfronthosting.co.za/roundup/meta/issue611)
  - Track PR status in appropriate issue
    (http://psf.upfronthosting.co.za/roundup/meta/issue590)
  - `Update the devguide`_, including `Document steps to commit a pull request`_
    (https://github.com/python/devguide/milestone/1)
  - Update commit hash detection on b.p.o to support 10- and 11-character hashes
    (http://psf.upfronthosting.co.za/roundup/meta/issue610)
  - `Linking a pull request to an issue`_
    (http://psf.upfronthosting.co.za/roundup/meta/issue589)
  - Email python-checkins for each commit (PR or direct)
    (https://help.github.com/articles/managing-notifications-for-pushes-to-a-repository/)
  - Message #python-dev for each commit (PR or direct)
    (https://github.com/python/cpython/settings/hooks/new?service=irc)
  - Get docs built from git
    (https://github.com/python/docsbuild-scripts/blob/main/build_docs.py already
    updated; https://github.com/python/psf-salt/pull/91 to switch)
  - Migrate buildbots to be triggered and pull from GitHub


Optional features:

* Not started

  - Check for whitespace abnormalities as part of CI
  - `Create https://git.python.org`_
  - `Backup of pull request data`_
  - `Handling Misc/ACKS`_
  - `Pull request commit queue`_
  - `Allow bugs.python.org to use GitHub as a login provider`_
  - `Web hooks for re-generating web content`_
  - `Splitting out parts of the documentation into their own repositories`_
  - `Backup of Git repositories`_

* In progress

  - `Notifying issues of pull request comments`_
    (http://psf.upfronthosting.co.za/roundup/meta/issue592)
  - Convert b.p.o patches to GitHub PRs
    (http://psf.upfronthosting.co.za/roundup/meta/issue600)

* Completed

  - `A CI Service`_
  - `Test coverage report`_
  - `Link web content back to files that it is generated from`_
  - `Handling Misc/NEWS`_
  - `Bot to generate cherry-pick pull requests`_
  - Write ``.github/CONTRIBUTING.md``
    (to prevent PRs that are inappropriate from even showing up and pointing to the devguide)



Open Issues
===========

For this PEP, open issues are ones where a decision needs to be made
to how to approach or solve a problem. Open issues do not entail
coordination issues such as who is going to write a certain bit of
code.


The fate of hg.python.org
-------------------------

With the code repositories moving over to Git [#git]_, there is no
technical need to keep hg.python.org [#h.p.o]_ running. Having said
that, some in the community would like to have it stay functioning as
a Mercurial [#hg]_ mirror of the Git repositories. Others have said
that they still want a mirror, but one using Git.

As maintaining hg.python.org is not necessary, it will be up to the
PSF infrastructure committee to decide if they want to spend the
time and resources to keep it running. They may also choose whether
they want to host a Git mirror on PSF infrastructure.

Depending on the decision reached, other ancillary repositories will
either be forced to migration or they can choose to simply stay on
hg.python.org.


Git CLI commands for committing a pull request to cpython
---------------------------------------------------------

Because Git [#git]_ may be a new version control system for core
developers, the commands people are expected to run will need to be
written down. These commands also need to keep a linear history while
giving proper attribution to the pull request author.

Another set of commands will also be necessary for when working with
a patch file uploaded to bugs.python.org [#b.p.o]_. Here the linear
history will be kept implicitly, but it will need to make sure to
keep/add attribution.


Naming the bots
---------------

As naming things can lead to bikeshedding of epic proportions, Brett
Cannon will choose the final name of the various bots (the name of
the project for the bots themselves can be anything, this is purely
for the name used in giving commands to the bot or the account name).
The names must come from Monty Python, which is only fitting since
Python is named after the comedy troupe.


Rejected Ideas
==============

Separate Python 2 and Python 3 repositories
-------------------------------------------

It was discussed whether separate repositories for Python 2 and
Python 3 were desired. The thinking was that this would shrink the
overall repository size which benefits people with slow Internet
connections or small bandwidth caps.

In the end it was decided that it was easier logistically to simply
keep all of CPython's history in a single repository.


Commit multi-release changes in bugfix branch first
---------------------------------------------------

As the current development process has changes committed in the
oldest branch first and then merged up to the default branch, the
question came up as to whether this workflow should be perpetuated.
In the end it was decided that committing in the newest branch and
then cherry-picking changes into older branches would work best as
most people will instinctively work off the newest branch and it is a
more common workflow when using Git [#git]_.

Cherry-picking is also more bot-friendly for an in-browser workflow.
In the merge-up scenario, if you were to request a bot to do a merge
and it failed, then you would have to make sure to immediately solve
the merge conflicts if you still allowed the main commit, else you
would need to postpone the entire commit until all merges could be
handled. With a cherry-picking workflow, the main commit could
proceed while postponing the merge-failing cherry-picks. This allows
for possibly distributing the work of managing conflicting merges.

Lastly, cherry-picking should help avoid merge races. Currently, when
one is doing work that spans branches, it takes time to commit in the
older branch, possibly push to another clone representing the
``default`` branch, merge the change, and then push upstream.
Cherry-picking should decouple this so that you don't have to rush
your multi-branch changes as the cherry-pick can be done separately.


Deriving ``Misc/NEWS`` from the commit logs
-------------------------------------------

As part of the discussion surrounding `Handling Misc/NEWS`_, the
suggestion has come up of deriving the file from the commit logs
itself. In this scenario, the first line of a commit message would be
taken to represent the news entry for the change. Some heuristic to
tie in whether a change warranted a news entry would be used, e.g.,
whether an issue number is listed.

This idea has been rejected due to some core developers preferring to
write a news entry separate from the commit message. The argument is
the first line of a commit message compared to that of a news entry
have different requirements in terms of brevity, what should be said,
etc.


Deriving ``Misc/NEWS`` from bugs.python.org
-------------------------------------------

A rejected solution to the ``NEWS`` file problem was to specify the
entry on  bugs.python.org [#b.p.o]_. This would mean an issue that is
marked as "resolved" could not be closed until a news entry is added
in the "news" field in the issue tracker. The benefit of tying the
news entry to the issue is it makes sure that all changes worthy of a
news entry have an accompanying issue. It also makes classifying a
news entry automatic thanks to the Component field of the issue. The
Versions field of the issue also ties the news entry to which Python
releases were affected. A script would be written to query
bugs.python.org for relevant new entries for a release and to produce
the output needed to be checked into the code repository. This
approach is agnostic to whether a commit was done by CLI or bot. A
drawback is that there's a disconnect between the actual commit that
made the change and the news entry by having them live in separate
places (in this case, GitHub and bugs.python.org). This would mean
making a commit would then require remembering to go back to
bugs.python.org to add the news entry.


References
==========

.. [#h.p.o] https://hg.python.org

.. [#GitHub] GitHub (https://github.com)

.. [#hg] Mercurial (https://www.mercurial-scm.org/)

.. [#git] Git (https://git-scm.com/)

.. [#b.p.o]  https://bugs.python.org

.. [#rietveld] Rietveld (https://github.com/rietveld-codereview/rietveld)

.. [#gitlab] GitLab (https://about.gitlab.com/)

.. [#core-workflow] core-workflow mailing list (https://mail.python.org/mailman/listinfo/core-workflow)

.. [#guido-keynote] Guido van Rossum's keynote at PyCon US (https://www.youtube.com/watch?v=G-uKNd5TSBw)

.. [#reasons] Email to core-workflow outlining reasons why GitHub was selected
   (https://mail.python.org/pipermail/core-workflow/2016-January/000345.html)

.. [#benchmarks-repo] Mercurial repository for the Unified Benchmark Suite
   (https://hg.python.org/benchmarks/)

.. [#devinabox-repo] Mercurial repository for devinabox (https://hg.python.org/devinabox/)

.. [#peps-repo] Mercurial repository of the Python Enhancement Proposals (https://hg.python.org/peps/)

.. [#devguide-repo] Mercurial repository for the Python Developer's Guide (https://hg.python.org/devguide/)

.. [#cpython-repo] Mercurial repository for CPython (https://hg.python.org/cpython/)

.. [#github-python-org] Python organization on GitHub (https://github.com/python)

.. [#github-org-perms] GitHub repository permission levels
   (https://help.github.com/enterprise/2.4/user/articles/repository-permission-levels-for-an-organization/)

.. [#cla] Python Software Foundation Contributor Agreement (https://www.python.org/psf/contrib/contrib-form/)

.. [#news-file] ``Misc/NEWS`` (https://hg.python.org/cpython/file/default/Misc/NEWS)

.. [#acks-file] ``Misc/ACKS`` (https://hg.python.org/cpython/file/default/Misc/ACKS)

.. [#devguide-merge-across-branches] Devguide instructions on how to merge across branches
   (https://docs.python.org/devguide/committing.html#merging-between-different-branches-within-the-same-major-version)

.. [#pythocat] Pythocat (https://octodex.github.com/pythocat)

.. [#tracker-plans] Wiki page for bugs.python.org feature development
   (https://wiki.python.org/moin/TrackerDevelopmentPlanning)

.. [#black-knight-sketch] The "Black Knight" sketch from "Monty Python and the Holy Grail"
   (https://www.youtube.com/watch?v=dhRUe-gz690)

.. [#bridge-of-death-sketch] The "Bridge of Death" sketch from "Monty Python and the Holy Grail"
   (https://www.youtube.com/watch?v=cV0tCphFMr8)

.. [#holy-grail] "Monty Python and the Holy Grail" sketches
   (https://www.youtube.com/playlist?list=PL-Qryc-SVnnu1MvN3r94Y9atpaRuIoGmp)

.. [#killer-rabbit-sketch] "Killer rabbit" sketch from "Monty Python and the Holy Grail"
   (https://www.youtube.com/watch?v=Nvs5pqf-DMA&list=PL-Qryc-SVnnu1MvN3r94Y9atpaRuIoGmp&index=11)

.. [#french-taunter-sketch] "French Taunter" from "Monty Python and the Holy Grail"
   (https://www.youtube.com/watch?v=A8yjNbcKkNY&list=PL-Qryc-SVnnu1MvN3r94Y9atpaRuIoGmp&index=13)

.. [#constitutional-peasants-sketch] "Constitutional Peasants" from "Monty Python and the Holy Grail"
   (https://www.youtube.com/watch?v=JvKIWjnEPNY&list=PL-Qryc-SVnnu1MvN3r94Y9atpaRuIoGmp&index=14)

.. [#ni-sketch] "Knights Who Say Ni" from "Monty Python and the Holy Grail"
   (https://www.youtube.com/watch?v=zIV4poUZAQo&list=PL-Qryc-SVnnu1MvN3r94Y9atpaRuIoGmp&index=15)

.. [#homu] Homu (http://homu.io/)

.. [#zuul] Zuul (http://docs.openstack.org/infra/zuul/)

.. [#travis] Travis (https://travis-ci.org/)

.. [#codeship] Codeship (https://codeship.com/)

.. [#coverage] coverage.py (https://pypi.python.org/pypi/coverage)

.. [#coveralls] Coveralls (https://coveralls.io/)

.. [#codecov] Codecov (https://codecov.io/)

.. [#pypatcher] Pypatcher (https://github.com/kushaldas/pypatcher)

.. [#heroku] Heroku (https://www.heroku.com/)

.. [#gh-status-check] GitHub status checks
   (https://developer.github.com/v3/repos/statuses/)

.. [#ni] The Knights Who Say Ni project
   (https://github.com/python/the-knights-who-say-ni)


Copyright
=========

This document has been placed in the public domain.



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