PEP: 481 Title: Migrate Some Supporting Repositories to Git and Github Version: $Revision$ Last-Modified: $Date$ Author: Donald Stufft Status: Draft Type: Process Content-Type: text/x-rst Created: 29-Nov-2014 Post-History: 29-Nov-2014 Abstract ======== This PEP proposes migrating to Git and Github for certain supporting repositories (such as the repository for Python Enhancement Proposals) in a way that is more accessible to new contributors, and easier to manage for core developers. This is offered as an alternative to PEP 474 which aims to achieve the same overall benefits but while continuing to use the Mercurial DVCS and without relying on a commerical entity. In particular this PEP proposes changes to the following repositories: * https://hg.python.org/devguide/ * https://hg.python.org/devinabox/ * https://hg.python.org/peps/ This PEP does not propose any changes to the core development workflow for CPython itself. Rationale ========= As PEP 474 mentions, there are currently a number of repositories hosted on hg.python.org which are not directly used for the development of CPython but instead are supporting or ancillary repositories. These supporting repositories do not typically have complex workflows or often branches at all other than the primary integration branch. This simplicity makes them very good targets for the "Pull Request" workflow that is commonly found on sites like Github. However whereas PEP 474 proposes to continue to use Mercurial and restricts itself to only solutions which are OSS and self-hosted, this PEP expands the scope of that to include migrating to Git and using Github. The existing method of contributing to these repositories generally includes generating a patch and either uploading them to bugs.python.org or emailing them to peps@python.org. This process is unfriendly towards non-comitter contributors as well as cumbersome for comitters seeking to accept the patches sent by users. In contrast, the Pull Request workflow style enables non technical contributors, especially those who do not know their way around the DVCS of choice, to contribute using the web based editor. On the committer side, the Pull Requests enable them to tell, before merging, whether or not a particular Pull Request will break anything. It also enables them to do a simple "push button" merge which does not require them to check out the changes locally. Another such feature that is useful in particular for docs, is the ability to view a "prose" diff. This Github-specific feature enables a committer to view a diff of the rendered output which will hide things like reformatting a paragraph and show you what the actual "meat" of the change actually is. Why Git? -------- Looking at the variety of DVCS which are available today, it becomes fairly clear that git has the largest mindshare. The Open Hub (previously Ohloh) statistics [#openhub-stats]_ show that currently 37% of the repositories indexed by Open Hub are using git which is second only to SVN (which has 48%), while Mercurial has just 2% of the indexed repositories (beating only bazaar which has 1%). In additon to the Open Hub statistics, a look at the top 100 projects on PyPI (ordered by total download counts) shows that within the Python space itself, the majority of projects use git. === ========= ========== ====== === ==== Git Mercurial Subversion Bazaar CVS None === ========= ========== ====== === ==== 62 22 7 4 1 1 === ========= ========== ====== === ==== Chosing a DVCS which has the larger mindshare will make it more likely that any particular person who has experience with DVCS at all will be able to meaningfully contribute without having to learn a new tool. In addition to simply making it more likely that any individual will already know how to use git, the number of projects and people using it means that the resources for learning the tool are likely to be more fully fleshed out. When you run into problems, the likelihood that someone else had that problem and posted a question and recieved an answer is also far higher. Thirdly, by using a more popular tool you also increase your options for tooling *around* the DVCS itself. Looking at the various options for hosting repositories, it's extremely rare to find a hosting solution (whether OSS or commerical) that supports Mercurial but does not support Git. On the flip side, there are a number of tools which support Git but do not support Mercurial. Therefore the popularity of git increases the flexibility of our options going into the future for what toolchain these projects use. Also, by moving to the more popular DVCS, we increase the likelihood that the knowledge that the person has learned in contributing to these support repositories will transfer to projects outside of the immediate CPython project such as to the larger Python community which is primarily using Git hosted on Github. In previous years there was concern about how well supported git was on Windows in comparison to Mercurial. However, git has grown to support Windows as a first class citizen. In addition to that, for Windows users who are not well acquainted with the Windows command line, there are GUI options as well. Why Github? ----------- There are a number of software projects or web services which offer functionality similar to that of Github. These range from commerical web services such as Bitbucket to self-hosted OSS solutions such as Kallithea or Gitlab. This PEP proposes that we move these repositories to Github. There are two primary reasons for selecting Github: Popularity and Quality/Polish. Github is currently the most popular hosted repository hosting according to Alexa, where it currently has a global rank of 121. Much like for Git itself, by choosing the most popular tool we gain benefits in increasing the likelihood that a new contributor will have already experienced the toolchain, the quality and availablity of the help, more and better tooling being built around it, and the knowledge transfer to other projects. A look again at the top 100 projects by download counts on PyPI shows the following hosting locations: ====== ========= =========== ========= =========== ========== GitHub BitBucket Google Code Launchpad SourceForge Other/Self ====== ========= =========== ========= =========== ========== 62 18 6 4 3 7 ====== ========= =========== ========= =========== ========== In addition to all of those reasons, Github also has the benefit that while many of the options have similar features when you look at them in a feature matrix the Github version of each of those features tend to work better and be far more polished. This is hard to quantify objectively however it is a fairly common sentiment if you go around and ask people who are using these services often. Finally, a reason to choose a web service at all over something that is self-hosted is to be able to more efficiently use volunteer time and donated resources. Every additional service hosted on the PSF infrastructure by the PSF infrastructure team further spreads out the amount of time that the volunteers on that team have to spend and uses some chunk of resources that could potentially be used for something where there is no free or affordable hosted solution available. One concern that people do have with using a hosted service is that there is a lack of control and that at some point in the future the service may no longer be suitable. It is the opinion of this PEP that Github does not currently and has not in the past engaged in any attempts to lock people into their platform and that if at some point in the future Github is no longer suitable for one reason or another, then at that point we can look at migrating away from Github onto a different solution. In other words, we'll cross that bridge if and when we come to it. 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. Migration ========= Through the use of hg-git [#hg-git]_ we can easily convert a Mercurial repository to a Git repository by simply pushing the Mercurial repository to the Git repository. People who wish to continue to use Mercurial locally can then use hg-git going into the future using the new Github URL. However they will need to re-clone their repositories as using Git as the server seems to trigger a one time change of the changeset ids. As none of the selected repositories have any tags, branches, or bookmarks other than the ``default`` branch the migration will simply map the ``default`` branch in Mercurial to the ``master`` branch in git. In addition, since none of the selected projects have any great need of a complex bug tracker, they will also migrate their issue handling to using the GitHub issues. In addition to the migration of the repository hosting itself there are a number of locations for each particular repository which will require updating. The bulk of these will simply be changing commands from the hg equivalent to the git equivalent. In particular this will include: * Updating www.python.org to generate PEPs using a git clone and link to Github. * Updating docs.python.org to pull from Github instead of hg.python.org for the devguide. * Enabling the ability to send an email to python-checkins@python.org for each push. * Enabling the ability to send an IRC message to #python-dev on Freenode for each push. * Migrate any issues for these projects to their respective bug tracker on Github. * Use hg-git to provide a read-only mirror on hg.python.org which will enable read-only uses of the hg.python.org instances of the specified repositories to remain the same. This will restore these repositories to similar functionality as they currently have. In addition to this the migration will also include enabling testing for each pull request using Travis CI [#travisci]_ where possible to ensure that a new PR does not break the ability to render the documentation or PEPs. User Access =========== Moving to Github would involve adding an additional user account that will need to be managed, however it also offers finer grained control, allowing the ability to grant someone access to only one particular repository instead of the coarser grained ACLs available on hg.python.org. References ========== .. [#openhub-stats] `Open Hub Statistics ` .. [#hg-git] `hg-git ` .. [#travisci] `Travis CI ` 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: