32. Git Bootcamp and Cheat Sheet

In this section, we’ll go over some commonly used Git commands that are relevant to CPython’s workflow.

32.1. Forking CPython GitHub Repository

You’ll only need to do this once.

  1. Go to https://github.com/python/cpython.
  2. Press Fork on the top right.
  3. When asked where to fork the repository, choose to fork it to your username.
  4. Your fork will be created at https://github.com/<username>/cpython.

32.2. Cloning The Forked CPython Repository

You’ll only need to do this once. From your command line:

$ git clone git@github.com:<username>/cpython.git
$ cd cpython
$ git remote add upstream git@github.com:python/cpython.git

32.3. Listing the Remote Repositories

To list the remote repositories that are configured, along with their urls:

$ git remote -v

32.4. Creating and Switching Branches

Note

Never commit directly to the master branch.

Create a new branch and switch to it:

# creates a new branch off master and switch to it
$ git checkout -b <branch-name> master

This is equivalent to:

# create a new branch off 'master', without checking it out
$ git branch <branch-name> master
# check out the branch
$ git checkout <branch-name>

To find the branch you are currently on:

$ git branch

The current branch will have an asterisk next to the branch name. Note, this will only list all of your local branches.

To list all the branches, including the remote branches:

$ git branch -a

To switch to a different branch:

$ git checkout <another-branch-name>

32.5. Deleting Local Branches

To delete a branch that you no longer need:

$ git branch -D <branch-name>

You may specify more than one branch for deletion.

32.6. Staging and Committing Files

  1. To show the current changes:

    $ git status
    
  2. To stage the files to be included in your commit:

    $ git add path/to/file1 path/to/file2 path/to/file3
    
  3. To commit the files that have been staged (done in step 2):

    $ git commit -m "bpo-XXXX: This is the commit message."
    

32.7. Reverting Changes

To revert changes to a file that has not been committed yet:

$ git checkout path/to/file

If the change has been committed, and now you want to reset it to whatever the origin is at:

$ git reset --hard HEAD

32.8. Stashing Changes

To stash away changes that are not ready to be committed yet:

$ git stash

To re-apply the last stashed change:

$ git stash pop

32.9. Pushing Changes

Once your changes are ready for a review or a pull request, you’ll need to push them to the remote repository.

$ git checkout <branch-name>
$ git push origin <branch-name>

32.10. Creating a Pull Request

  1. Go to https://github.com/python/cpython.
  2. Click compare across forks link.
  3. Select the base fork: python/cpython and base branch: master.
  4. Select the head fork: <username>/cpython and base branch: the branch containing your changes.
  5. Press Create Pull Request button.

32.11. Syncing With Upstream

Scenario:

  • You forked the CPython repository some time ago.
  • Time passes.
  • There have been new commits made in upstream CPython repository.
  • Your forked CPython repository is no longer up to date.
  • You now want to update your forked CPython repository to be the same as upstream.

Solution:

$ git checkout master
$ git pull --rebase upstream master
$ git push origin master

The --rebase option is only needed if you have local changes to the branch.

Another scenario:

  • You created some-branch some time ago.
  • Time passes.
  • You made some commits to some-branch.
  • Meanwhile, there are recent changes from upstream CPython repository.
  • You want to incorporate the recent changes from upstream into some-branch.

Solution:

$ git checkout some-branch
$ git fetch upstream
$ git rebase upstream/master

32.12. Backporting Merged Changes

A pull request may need to be backported into one of the maintenance branches after it has been accepted and merged into master. It is usually indicated by the label needs backport to X.Y on the pull request itself.

Use the utility script cherry_picker.py from the core-workflow repository to backport the commit.

32.13. Applying a Patch from Mercurial to Git

Scenario:

  • A Mercurial patch exists but there is no pull request for it.

Solution:

  1. Download the patch locally.

  2. Apply the patch:

    $ git apply /path/to/issueNNNN-git.patch
    

    If there are errors, update to a revision from when the patch was created and then try the git apply again:

    $ git checkout `git rev-list -n 1 --before="yyyy-mm-dd hh:mm:ss" master`
    $ git apply /path/to/issueNNNN-git.patch
    

    If the patch still won’t apply, then a patch tool will not be able to apply the patch and it will need to be re-implemented manually.

  3. If the apply was successful, create a new branch and switch to it.

  4. Stage and commit the changes.

  5. If the patch was applied to an old revision, it needs to be updated and merge conflicts need to be resolved:

    $ git rebase master
    $ git mergetool
    
  6. Push the changes and open a pull request.

32.14. Downloading Other’s Patches

Scenario:

  • A contributor made a pull request to CPython.
  • Before merging it, you want to be able to test their changes locally.

On Unix and MacOS, set up the following git alias:

$ git config --global alias.pr '!sh -c "git fetch upstream pull/${1}/head:pr_${1} && git checkout pr_${1}" -'

On Windows, reverse the single () and double () quotes:

git config --global alias.pr "!sh -c 'git fetch upstream pull/${1}/head:pr_${1} && git checkout pr_${1}' -"

The alias only needs to be done once. After the alias is set up, you can get a local copy of a pull request as follows:

$ git pr <pr_number>

32.15. Accepting and Merging A Pull Request

Pull requests can be accepted and merged by a Python Core Developer.

  1. At the bottom of the pull request page, click the Squash and merge button.

  2. Adjust and clean up the commit message. Replace the reference to GitHub PR #XXX into GH-XXX.

    Example of good commit message:

    bpo-12345: Improve the spam module (GH-777)
    
    * Add method A to the spam module
    * Update the documentation of the spam module
    

    Example of bad commit message:

    bpo-12345: Improve the spam module (#777)
    
    * Improve the spam module
    * merge from master
    * adjust code based on review comment
    * rebased
    
  3. Press the Confirm squash and merge button.

32.16. Editing a Pull Request Prior to Merging

When a pull request submitter has enabled the Allow edits from maintainers option, Python Core Developers may decide to make any remaining edits needed prior to merging themselves, rather than asking the submitter to do them. This can be particularly appropriate when the remaining changes are bookkeeping items like updating Misc/ACKS and Misc/NEWS.

To edit an open pull request that targets master:

  1. In the pull request page, under the description, there is some information about the contributor’s fork and branch name that will be useful later:

    <contributor> wants to merge 1 commit into python:master from <contributor>:<branch_name>
    
  2. Fetch the pull request, using the git pr alias:

    $ git pr <pr_number>
    

    This will checkout the contributor’s branch at pr_XXX.

  3. Make and commit your changes on the branch. For example, merge in changes made to master since the PR was submitted (any merge commits will be removed by the later Squash and Merge when accepting the change):

    $ git fetch upstream
    $ git merge upstream/master
    $ git add <filename>
    $ git commit -m "<commit message>"
    
  4. Push the changes back to the contributor’s PR branch:

    $ git push git@github.com:<contributor>/cpython <pr_XXX>:<branch_name>
    
  5. Optionally, delete the local PR branch:

    $ git checkout master
    $ git branch -D <pr_XXX>