python-peps/pep-0481.txt

PEP: 481
Title: Migrate CPython to Git, Github, and Phabricator
Version: $Revision$
Last-Modified: $Date$
Author: Donald Stufft <donald@stufft.io>
Status: Draft
Type: Process
Content-Type: text/x-rst
Created: 29-Nov-2014
Post-History: 29-Nov-2014


Abstract
========

This PEP proposes migrating the repository hosting of CPython and the
supporting repositories to Git and Github. It also proposes adding Phabricator
as an alternative to Github Pull Requests to handle reviewing changes. This
particular PEP is offered as an alternative to PEP 474 and PEP 462 which aims
to achieve the same overall benefits but restricts itself to tools that support
Mercurial and are completely Open Source.


Rationale
=========

CPython is an open source project which relies on a number of volunteers
donating their time. As an open source project it relies on attracting new
volunteers as well as retaining existing ones in order to continue to have
a healthy amount of manpower available. In addition to increasing the amount of
manpower that is available to the project, it also needs to allow for effective
use of what manpower *is* available.

The current toolchain of the CPython project is a custom and unique combination
of tools which mandates a workflow that is similar to one found in a lot of
older projects, but which is becoming less and less popular as time goes on.

The one-off nature of the CPython toolchain and workflow means that any new
contributor is going to need spend time learning the tools and workflow before
they can start contributing to CPython. Once a new contributor goes through
the process of learning the CPython workflow they also are unlikely to be able
to take that knowledge and apply it to future projects they wish to contribute
to. This acts as a barrier to contribution which will scare off potential new
contributors.

In addition the tooling that CPython uses is under-maintained, antiquated,
and it lacks important features that enable committers to more effectively use
their time when reviewing and approving changes. The fact that it is
under-maintained means that bugs are likely to last for longer, if they ever
get fixed, as well as it's more likely to go down for extended periods of time.
The fact that it is antiquated means that it doesn't effectively harness the
capabilities of the modern web platform. Finally the fact that it lacks several
important features such as a lack of pre-testing commits and the lack of an
automatic merge tool means that committers have to do needless busy work to
commit even the simplest of changes.


Version Control System
----------------------

The first decision that needs to be made is the VCS of the primary server side
repository. Currently the CPython repository, as well as a number of supporting
repositories, uses Mercurial. When evaluating the VCS we must consider the
capabilities of the VCS itself as well as the network effect and mindshare of
the community around that VCS.

There are really only two real options for this, Mercurial and Git. Between the
two of them the technical capabilities are largely equivilant. For this reason
this PEP will largely ignore the technical arguments about the VCS system and
will instead focus on the social aspects.

It is not possible to get exact numbers for the number of projects or people
which are using a particular VCS, however we can infer this by looking at
several sources of information for what VCS projects are using.

The Open Hub (previously Ohloh) statistics [#openhub-stats]_ show that 37% of
the repositories indexed by The Open Hub are using Git (second only to SVN
which has 48%) while Mercurial has just 2% (beating only bazaar which has 1%).
This has Git being just over 18 times as popular as Mercurial on The Open Hub.

Another source of information on the popular of the difference VCSs is PyPI
itself. This source is more targeted at the Python community itself since it
represents projects developed for Python. Unfortunately PyPI does not have a
standard location for representing this information, so this requires manual
processing. If we limit our search to the top 100 projects on PyPI (ordered
by download counts) we can see that 62% of them use Git while 22% of them use
Mercurial while 13% use something else. This has Git being just under 3 times
as popular as Mercurial for the top 100 projects on PyPI.

Obviously from these numbers Git is by far the more popular DVCS for open
source projects and choosing the more popular VCS has a number of positive
benefits.

For new contributors it increases the likelihood that they will have already
learned the basics of Git as part of working with another project or if they
are just now learning Git, that they'll be able to take that knowledge and
apply it to other projects. Additionally a larger community means more people
writing how to guides, answering questions, and writing articles about Git
which makes it easier for a new user to find answers and information about
the tool they are trying to learn.

Another benefit is that by nature of having a larger community, there will be
more tooling written *around* it. This increases options for everything from
GUI clients, helper scripts, repository hosting, etc.


Repository Hosting
------------------

This PEP proposes allowing GitHub Pull Requests to be submitted, however GitHub
does not have a way to submit Pull Requests against a repository that is not
hosted on GitHub. This PEP also proposes that in addition to GitHub Pull
Requests Phabricator's Differential app can also be used to submit proposed
changes and Phabricator *does* allow submitting changes against a repository
that is not hosted on Phabricator.

For this reason this PEP proposes using GitHub as the canonical location of
the repository with a read-only mirror located in Phabricator. If at some point
in the future GitHub is no longer desired, then repository hosting can easily
be moved to solely in Phabricator and the ability to accept GitHub Pull
Requests dropped.

In addition to hosting the repositories on Github, a read only copy of all
repositories will also be mirrored onto the PSF Infrastructure.


Code Review
-----------

Currently CPython uses a custom fork of Rietveld which has been modified to
not run on Google App Engine which is really only able to be maintained
currently by one person. In addition it is missing out on features that are
present in many modern code review tools.

This PEP proposes allowing both Github Pull Requests and Phabricator changes
to propose changes and review code. It suggests both so that contributors can
select which tool best enables them to submit changes, and reviewers can focus
on reviewing changes in the tooling they like best.


GitHub Pull Requests
~~~~~~~~~~~~~~~~~~~~

GitHub is a very popular code hosting site and is increasingly becoming the
primary place people look to contribute to a project. Enabling users to
contribute through GitHub is enabling contributors to contribute using tooling
that they are likely already familiar with and if they are not they are likely
to be able to apply to another project.

GitHub Pull Requests have a fairly major advantage over the older "submit a
patch to a bug tracker" model. It allows developers to work completely within
their VCS using standard VCS tooling so it does not require creating a patch
file and figuring out what the right location is to upload it to. This lowers
the barrier for sending a change to be reviewed.

On the reviewing side, GitHub Pull Requests are far easier to review, they have
nice syntax highlighted diffs which can operate in either unified or side by
side views. They allow expanding the context on a diff up to and including the
entire file. Finally they allow commenting inline and on the pull request as
a whole and they present that in a nice unified way which will also hide
comments which no longer apply. Github also provides a "rendered diff" view
which enables easily viewing a diff of rendered markup (such as rst) instead
of needing to review the diff of the raw markup.

The Pull Request work flow also makes it trivial to enable the ability to
pre-test a change before actually merging it. Any particular pull request can
have any number of different types of "commit statuses" applied to it, marking
the commit (and thus the pull request) as either in a pending, successful,
errored, or failure state. This makes it easy to see inline if the pull request
is passing all of the tests, if the contributor has signed a CLA, etc.

Actually merging a Github Pull Request is quite simple, a core reviewer simply
needs to press the "Merge" button once the status of all the checks on the
Pull Request are green for successful.

GitHub also has a good workflow for submitting pull requests to a project
completely through their web interface. This would enable the Python
documentation to have "Edit on GitHub" buttons on every page and people who
discover things like typos, inaccuracies, or just want to make improvements to
the docs they are currently writing can simply hit that button and get an in
browser editor that will let them make changes and submit a pull request all
from the comfort of their browser.


Phabricator
~~~~~~~~~~~

In addition to GitHub Pull Requests this PEP also proposes setting up a
Phabricator instance and pointing it at the GitHub hosted repositories. This
will allow utilizing the Phabricator review applications of Differential and
Audit.

Differential functions similarly to GitHub pull requests except that they
require installing the ``arc`` command line tool to upload patches to
Phabricator.

Whether to enable Phabricator for any particular repository can be chosen on
a case by case basis, this PEP only proposes that it must be enabled for the
CPython repository, however for smaller repositories such as the PEP repository
it may not be worth the effort.


Criticism
=========

X is not written in Python
--------------------------

One feature that the current tooling (Mercurial, Rietveld) has is that the
primary language for all of the pieces are written in Python. It is this PEPs
belief that we should focus on the *best* tools for the job and not the *best*
tools that happen to be written in Python. Volunteer time is a precious
resource to any open source project and we can best respect and utilize that
time by focusing on the benefits and downsides of the tools themselves rather
than what language their authors happened to write them in.

One concern is the ability to modify tools to work for us, however one of
the Goals here is to *not* modify software to work for us and instead adapt
ourselves to a more standard workflow. This standardization pays off in the
ability to re-use tools out of the box freeing up developer time to actually
work on Python itself as well as enabling knowledge sharing between projects.

However if we do need to modify the tooling, Git itself is largely written in
C the same as CPython itself is. It can also have commands written for it using
any language, including Python. Phabricator is written in PHP which is a fairly
common language in the web world and fairly easy to pick up. GitHub itself is
largely written in Ruby but given that it's not Open Source there is no ability
to modify it so it's implementation language is completely meaningless.


GitHub is not Free/Open Source
------------------------------

GitHub is a big part of this proposal and someone who tends more to ideology
rather than practicality may be opposed to this PEP on that grounds alone. It
is this PEPs belief that while using entirely Free/Open Source software is an
attractive idea and a noble goal, that valuing the time of the contributors by
giving them good tooling that is well maintained and that they either already
know or if they learn it they can apply to other projects is a more important
concern than treating whether something is Free/Open Source is a hard
requirement.

However, history has shown us that sometimes benevolent proprietary companies
can stop being benevolent. This is hedged against in a few ways:

* We are not utilizing the GitHub Issue Tracker, both because it is not
  powerful enough for CPython but also because for the primary CPython
  repository the ability to take our issues and put them somewhere else if we
  ever need to leave GitHub relies on GitHub continuing to allow API access.

* We are utilizing the GitHub Pull Request workflow, however all of those
  changes live inside of Git. So a mirror of the GitHub repositories can easily
  contain all of those Pull Requests. We would potentially lose any comments if
  GitHub suddenly turned "evil", but the changes themselves would still exist.

* We are utilizing the GitHub repository hosting feature, however since this is
  just git moving away from GitHub is as simple as pushing the repository to
  a different location. Data portability for the repository itself is extremely
  high.

* We are also utilizing Phabricator to provide an alternative for people who
  do not wish to use GitHub. This also acts as a fallback option which will
  already be in place if we ever need to stop using GitHub.

Relying on GitHub comes with a number of benefits beyond just the benefits of
the platform itself. Since it is a commercially backed venture it has a full
time staff responsible for maintaining its services. This includes making sure
they stay up, making sure they stay patched for various security
vulnerabilities, and further improving the software and infrastructure as time
goes on.


Mercurial is better than Git
----------------------------

Whether Mercurial or Git is better on a technical level is a highly subjective
opinion. This PEP does not state whether the mechanics of Git or Mercurial is
better and instead focuses on the network effect that is available for either
option. Since this PEP proposes switching to Git this leaves the people who
prefer Mercurial out, however those users can easily continue to work with
Mercurial by using the hg-git [#hg-git]_ extension for Mercurial which will
let it work with a repository which is Git on the serverside.


CPython Workflow is too Complicated
-----------------------------------

One sentiment that came out of previous discussions was that the multi branch
model of CPython was too complicated for Github Pull Requests. It is the belief
of this PEP that statement is not accurate.

Currently any particular change requires manually creating a patch for 2.7 and
3.x which won't change at all in this regards.

If someone submits a fix for the current stable branch (currently 3.4) the
GitHub Pull Request workflow can be used to create, in the browser, a Pull
Request to merge the current stable branch into the master branch (assuming
there is no merge conflicts). If there is a merge conflict that would need to
be handled locally. This provides an improvement over the current situation
where the merge must always happen locally.

Finally if someone submits a fix for the current development branch currently
then this has to be manually applied to the stable branch if it desired to
include it there as well. This must also happen locally as well in the new
workflow, however for minor changes it could easily be accomplished in the
GitHub web editor.

Looking at this, I do not believe that *any* system can hide the complexities
involved in maintaining several long running branches. The only thing that the
tooling can do is make it as easy as possible to submit changes.


Example: Scientific Python
==========================

One of the key ideas behind the move to both git and Github is that a feature
of a DVCS, the repository hosting, and the workflow used is the social network
and size of the community using said tools. We can see this is true by looking
at an example from a sub-community of the Python community: The Scientific
Python community. They have already migrated most of the key pieces of the
SciPy stack onto Github using the Pull Request based workflow. This process
started with IPython, and as more projects moved over it became a natural
default for new projects in the community.

They claim to have seen a great benefit from this move, in that it enables
casual contributors to easily move between different projects within their
sub-community without having to learn a special, bespoke workflow and a
different toolchain for each project. They've found that when people can use
their limited time on actually contributing instead of learning the different
tools and workflows, not only do they contribute more to one project, but
that they also expand out and contribute to other projects. This move has also
been attributed to the increased tendency for members of that community to go
so far as publishing their research and educational materials on Github as
well.

This example showcases the real power behind moving to a highly popular
toolchain and workflow, as each variance introduces yet another hurdle for new
and casual contributors to get past and it makes the time spent learning that
workflow less reusable with other projects.


References
==========

.. [#openhub-stats] `Open Hub Statistics <https://www.openhub.net/repositories/compare>`
.. [#hg-git] `Hg-Git mercurial plugin <https://hg-git.github.io/>`


Copyright
=========

This document has been placed in the public domain.



..
   Local Variables:
   mode: indented-text
   indent-tabs-mode: nil
   sentence-end-double-space: t
   fill-column: 70
   coding: utf-8
   End: