Using GitHub to make a Pull Request

Any contributions to MNE, whether bug fixes, improvements to the documentation, or new functionality, can be done via pull requests on GitHub. The workflow for this is described here. [Many thanks to Astropy for providing clear instructions that we have adapted for our use here!]

The only absolutely necessary configuration step is identifying yourself and your contact info:

$ git config --global user.name "Your Name"
$ git config --global user.email you@yourdomain.example.com

If you are going to Set up and configure a GitHub account eventually, this email address should be the same as the one used to sign up for a GitHub account. For more information about configuring your git installation, see Customizing git.

The following sections cover the installation of the git software, the basic configuration, and links to resources to learn more about using git. However, you can also directly go to the GitHub help pages which offer a great introduction to git and GitHub.

In the present document, we refer to the mne-python/master branch, as the trunk.

Creating a fork

You need to do this only once for each package you want to contribute to. The instructions here are very similar to the instructions at https://help.github.com/fork-a-repo/ – please see that page for more details. We’re repeating some of it here just to give the specifics for the mne-python project, and to suggest some default names.

Set up and configure a GitHub account

If you don’t have a GitHub account, go to the GitHub page, and make one.

You then need to configure your account to allow write access – see the Generating SSH keys help on GitHub Help.

Create your own fork of a repository

Now you should fork the core mne-python.git repository:

  1. Log into your GitHub account.

  2. Go to the mne-python GitHub home.

  3. Click on the fork button:

    _images/forking_button.png

    Now, after a short pause and some ‘Hardcore forking action’, you should find yourself at the home page for your own forked copy of mne-python.

Setting up the fork and the working directory

Briefly, this is done using:

$ git clone git@github.com:your-user-name/mne-python.git
$ cd mne-python
$ git remote add upstream git://github.com/mne-tools/mne-python.git

These steps can be broken out to be more explicit as:

  1. Clone your fork to the local computer:

    $ git clone git@github.com:your-user-name/mne-python.git
    
  2. Change directory to your new repo:

    $ cd mne-python
    

    Then type:

    $ git branch -a
    

    to show you all branches. You’ll get something like:

    * master
    remotes/origin/master
    

    This tells you that you are currently on the master branch, and that you also have a remote connection to origin/master. What remote repository is remote/origin? Try git remote -v to see the URLs for the remote. They will point to your GitHub fork.

    Now you want to connect to the mne-python repository, so you can merge in changes from the trunk:

    $ cd mne-python
    $ git remote add upstream git://github.com/mne-tools/mne-python.git
    

    upstream here is just the arbitrary name we’re using to refer to the main mne-python repository.

    Note that we’ve used git:// for the URL rather than git@. The git:// URL is read only. This means we that we can’t accidentally (or deliberately) write to the upstream repo, and we are only going to use it to merge into our own code.

    Just for your own satisfaction, show yourself that you now have a new ‘remote’, with git remote -v show, giving you something like:

    upstream   git://github.com/mne-tools/mne-python.git (fetch)
    upstream   git://github.com/mne-tools/mne-python.git (push)
    origin     git@github.com:your-user-name/mne-python.git (fetch)
    origin     git@github.com:your-user-name/mne-python.git (push)
    

    Your fork is now set up correctly.

  3. Install mne with editing permissions using the installed folder:

    To be able to conveniently edit your files after installing mne, install using the following setting:

    $ python setup.py develop --user
    

    To make changes in the code, edit the relevant files and restart the ipython kernel for changes to take effect.

  4. Ensure unit tests pass

    Make sure before starting to code that all unit tests pass with pytest:

    $ make test
    

    That’s it! You are now ready to hack away.

Workflow summary

This section gives a summary of the workflow once you have successfully forked the repository, and details are given for each of these steps in the following sections.

  • Don’t use your master branch for anything. Consider deleting it.
  • When you are starting a new set of changes, fetch any changes from the trunk, and start a new feature branch from that.
  • Make a new branch for each separable set of changes – “one task, one branch” (ipython git workflow).
  • Name your branch for the purpose of the changes - e.g. bugfix-for-issue-14 or refactor-database-code.
  • If you can possibly avoid it, avoid merging trunk or any other branches into your feature branch while you are working.
  • If you do find yourself merging from the trunk, consider Rebasing on trunk
  • Ensure all tests still pass. Make travis happy.
  • Ask for code review!

This way of working helps to keep work well organized, with readable history. This in turn makes it easier for project maintainers (that might be you) to see what you’ve done, and why you did it.

See linux git workflow and ipython git workflow for some explanation.

Deleting your master branch

It may sound strange, but deleting your own master branch can help reduce confusion about which branch you are on. See deleting master on github for details.

Updating the mirror of trunk

From time to time you should fetch the upstream (trunk) changes from GitHub:

$ git fetch upstream

This will pull down any commits you don’t have, and set the remote branches to point to the right commit. For example, ‘trunk’ is the branch referred to by (remote/branchname) upstream/master - and if there have been commits since you last checked, upstream/master will change after you do the fetch.

Making a new feature branch

When you are ready to make some changes to the code, you should start a new branch. Branches that are for a collection of related edits are often called ‘feature branches’.

Making an new branch for each set of related changes will make it easier for someone reviewing your branch to see what you are doing.

Choose an informative name for the branch to remind yourself and the rest of us what the changes in the branch are for. For example add-ability-to-fly, or buxfix-for-issue-42.

# Update the mirror of trunk
$ git fetch upstream

# Make new feature branch starting at current trunk
$ git branch my-new-feature upstream/master
$ git checkout my-new-feature

Generally, you will want to keep your feature branches on your public GitHub fork. To do this, you git push this new branch up to your github repo. Generally (if you followed the instructions in these pages, and by default), git will have a link to your GitHub repo, called origin. You push up to your own repo on GitHub with:

$ git push origin my-new-feature

In git > 1.7 you can ensure that the link is correctly set by using the --set-upstream option:

$ git push --set-upstream origin my-new-feature

From now on git will know that my-new-feature is related to the my-new-feature branch in the GitHub repo.

The editing workflow

Overview

$ git add my_new_file
$ git commit -am 'FIX: some message'
$ git push

In more detail

  1. Make some changes

  2. See which files have changed with git status (see git status). You’ll see a listing like this one:

    # On branch ny-new-feature
    # Changed but not updated:
    #   (use "git add <file>..." to update what will be committed)
    #   (use "git checkout -- <file>..." to discard changes in working directory)
    #
    #    modified:   README
    #
    # Untracked files:
    #   (use "git add <file>..." to include in what will be committed)
    #
    #    INSTALL
    no changes added to commit (use "git add" and/or "git commit -a")
    
  3. Check what the actual changes are with git diff (git diff).

  4. Add any new files to version control git add new_file_name (see git add).

  5. Add any modified files that you want to commit using git add modified_file_name (see git add).

  6. Once you are ready to commit, check with git status which files are about to be committed:

    # Changes to be committed:
    #   (use "git reset HEAD <file>..." to unstage)
    #
    #    modified:   README
    

    Then use git commit -m 'A commit message'. The m flag just signals that you’re going to type a message on the command line. The git commit manual page might also be useful.

    It is also good practice to prefix commits with the type of change, such as FIX:, STY:, or ENH: for fixes, style changes, or enhancements.

  7. To push the changes up to your forked repo on GitHub, do a git push (see git push).

Asking for your changes to be reviewed or merged

When you are ready to ask for someone to review your code and consider a merge:

  1. Go to the URL of your forked repo, say https://github.com/your-user-name/mne-python.

  2. Use the ‘Switch Branches’ dropdown menu near the top left of the page to select the branch with your changes:

    _images/branch_dropdown.png
  3. Click on the ‘Pull request’ button:

    _images/pull_button.png

    Enter a title for the set of changes, and some explanation of what you’ve done. Say if there is anything you’d like particular attention for - like a complicated change or some code you are not happy with.

    If you don’t think your request is ready to be merged, prefix WIP: to the title of the pull request, and note it also in your pull request message. This is still a good way of getting some preliminary code review. Submitting a pull request early on in feature development can save a great deal of time for you, as the code maintainers may have “suggestions” about how the code should be written (features, style, etc.) that are easier to implement from the start.

  4. Finally, make travis happy. Ensure that builds in all four jobs pass. To make code python3 compatible, refer to externals/six.py. Use virtual environments to test code on different python versions. Please remember that travis only runs a subset of the tests and is thus not a substitute for running the entire test suite locally.

  5. For the code to be mergeable, please rebase w.r.t master branch.

  6. Once, you are ready, prefix MRG: to the title of the pull request to indicate that you are ready for the pull request to be merged.

If you are uncertain about what would or would not be appropriate to contribute to MNE, don’t hesitate to either send a pull request, or open an issue on the mne-python GitHub site to discuss potential changes.

Rebasing on trunk

Let’s say you thought of some work you’d like to do. You Updating the mirror of trunk and Making a new feature branch called cool-feature. At this stage trunk is at some commit, let’s call it E. Now you make some new commits on your cool-feature branch, let’s call them A, B, C. Maybe your changes take a while, or you come back to them after a while. In the meantime, trunk has progressed from commit E to commit (say) G:

      A---B---C cool-feature
     /
D---E---F---G trunk

At this stage you consider merging trunk into your feature branch, and you remember that this here page sternly advises you not to do that, because the history will get messy. Most of the time you can just ask for a review, and not worry that trunk has got a little ahead. But sometimes, the changes in trunk might affect your changes, and you need to harmonize them. In this situation you may prefer to do a rebase.

Rebase takes your changes (A, B, C) and replays them as if they had been made to the current state of trunk. In other words, in this case, it takes the changes represented by A, B, C and replays them on top of G. After the rebase, your history will look like this:

              A'--B'--C' cool-feature
             /
D---E---F---G trunk

See rebase without tears for more detail.

To do a rebase on trunk:

# Update the mirror of trunk
$ git fetch upstream

# Go to the feature branch
$ git checkout cool-feature

# Make a backup in case you mess up
$ git branch tmp cool-feature

# Rebase cool-feature onto trunk
$ git rebase --onto upstream/master upstream/master cool-feature

In this situation, where you are already on branch cool-feature, the last command can be written more succinctly as:

$ git rebase upstream/master

When all looks good you can delete your backup branch:

$ git branch -D tmp

If it doesn’t look good you may need to have a look at Recovering from mess-ups.

If you have made changes to files that have also changed in trunk, this may generate merge conflicts that you need to resolve - see the git rebase man page for some instructions at the end of the “Description” section. There is some related help on merging in the git user manual - see resolving a merge.

If your feature branch is already on GitHub and you rebase, you will have to force push the branch; a normal push would give an error. If the branch you rebased is called cool-feature and your GitHub fork is available as the remote called origin, you use this command to force-push:

$ git push -f origin cool-feature

Note that this will overwrite the branch on GitHub, i.e. this is one of the few ways you can actually lose commits with git. Also note that it is never allowed to force push to the main mne-python repo (typically called upstream), because this would re-write commit history and thus cause problems for all others.

Recovering from mess-ups

Sometimes, you mess up merges or rebases. Luckily, in git it is relatively straightforward to recover from such mistakes.

If you mess up during a rebase:

$ git rebase --abort

If you notice you messed up after the rebase:

# Reset branch back to the saved point
$ git reset --hard tmp

If you forgot to make a backup branch:

# Look at the reflog of the branch
$ git reflog show cool-feature

8630830 cool-feature@{0}: commit: BUG: io: close file handles immediately
278dd2a cool-feature@{1}: rebase finished: refs/heads/my-feature-branch onto 11ee694744f2552d
26aa21a cool-feature@{2}: commit: BUG: lib: make seek_gzip_factory not leak gzip obj
...

# Reset the branch to where it was before the botched rebase
$ git reset --hard cool-feature@{2}

Otherwise, googling the issue may be helpful (especially links to Stack Overflow).

Rewriting commit history

Note

Do this only for your own feature branches.

There’s an embarrassing typo in a commit you made? Or perhaps the you made several false starts you would like the posterity not to see.

This can be done via interactive rebasing.

Suppose that the commit history looks like this:

$ git log --oneline
eadc391 Fix some remaining bugs
a815645 Modify it so that it works
2dec1ac Fix a few bugs + disable
13d7934 First implementation
6ad92e5 * masked is now an instance of a new object, MaskedConstant
29001ed Add pre-nep for a copule of structured_array_extensions.
...

and 6ad92e5 is the last commit in the cool-feature branch. Suppose we want to make the following changes:

  • Rewrite the commit message for 13d7934 to something more sensible.
  • Combine the commits 2dec1ac, a815645, eadc391 into a single one.

We do as follows:

# make a backup of the current state
$ git branch tmp HEAD
# interactive rebase
$ git rebase -i 6ad92e5

This will open an editor with the following text in it:

pick 13d7934 First implementation
pick 2dec1ac Fix a few bugs + disable
pick a815645 Modify it so that it works
pick eadc391 Fix some remaining bugs

# Rebase 6ad92e5..eadc391 onto 6ad92e5
#
# Commands:
#  p, pick = use commit
#  r, reword = use commit, but edit the commit message
#  e, edit = use commit, but stop for amending
#  s, squash = use commit, but meld into previous commit
#  f, fixup = like "squash", but discard this commit's log message
#
# If you remove a line here THAT COMMIT WILL BE LOST.
# However, if you remove everything, the rebase will be aborted.
#

To achieve what we want, we will make the following changes to it:

r 13d7934 First implementation
pick 2dec1ac Fix a few bugs + disable
f a815645 Modify it so that it works
f eadc391 Fix some remaining bugs

This means that (i) we want to edit the commit message for 13d7934, and (ii) collapse the last three commits into one. Now we save and quit the editor.

Git will then immediately bring up an editor for editing the commit message. After revising it, we get the output:

[detached HEAD 721fc64] FOO: First implementation
 2 files changed, 199 insertions(+), 66 deletions(-)
[detached HEAD 0f22701] Fix a few bugs + disable
 1 files changed, 79 insertions(+), 61 deletions(-)
Successfully rebased and updated refs/heads/my-feature-branch.

and the history looks now like this:

0f22701 Fix a few bugs + disable
721fc64 ENH: Sophisticated feature
6ad92e5 * masked is now an instance of a new object, MaskedConstant

If it went wrong, recovery is again possible as explained above.

Fetching a pull request

To fetch a pull request on the main repository to your local working directory as a new branch, just do:

$ git fetch upstream pull/<pull request number>/head:<local-branch>

As an example, to pull the realtime pull request which has a url https://github.com/mne-tools/mne-python/pull/615/, do:

$ git fetch upstream pull/615/head:realtime

If you want to fetch a pull request to your own fork, replace upstream with origin. That’s it!

Skipping a build

The builds when the pull request is in WIP state can be safely skipped. The important thing is to ensure that the builds pass when the PR is ready to be merged. To skip a Travis build, add [ci skip] to the commit message:

FIX: some changes [ci skip]

This will help prevent clogging up Travis and Appveyor and also save the environment.