Git bootcamp and cheat sheet¶

Note

This section provides instructions on common tasks in CPython’s workflow. It’s designed to assist new contributors who have some familiarity with Git and GitHub.

If you are new to Git and GitHub, please become comfortable with these instructions before submitting a pull request. As there are several ways to accomplish these tasks using Git and GitHub, this section reflects one method suitable for new contributors. Experienced contributors may desire a different approach.

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

Note

Setting up Git aliases for common tasks can be useful to you. You can get more information about that in Git documentation

Forking CPython GitHub repository¶

You will only need to do this once.

Go to https://github.com/python/cpython.
Press Fork located near the top right of the page.
Uncheck “Copy the main branch only”.
Press the Create fork button.
Your forked CPython repository will be created at https://github.com/<username>/cpython.

Cloning a forked CPython repository¶

You will only need to do this once per machine. From your command line:

$ git clone git@github.com:<username>/cpython.git

It is also recommended to configure an upstream remote repository:

$ cd cpython
$ git remote add upstream https://github.com/python/cpython

You can also use SSH-based or HTTPS-based URLs.

Configure the remotes¶

Configure git to pull main from the upstream remote:

$ git config --local branch.main.remote upstream

Since one should never attempt to push to upstream, configure git to push always to origin:

$ git remote set-url --push upstream git@github.com:<username>/cpython.git

Listing the remote repositories¶

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

$ git remote -v

You should have two remote repositories: origin pointing to your forked CPython repository, and upstream pointing to the official CPython repository:

origin  git@github.com:<username>/cpython.git (fetch)
origin  git@github.com:<username>/cpython.git (push)
upstream        https://github.com/python/cpython (fetch)
upstream        git@github.com:<username>/cpython.git (push)

To verify the upstream for main:

$ git config branch.main.remote

It should emit upstream, indicating to track/pull changes for main from the upstream remote.

Once this is verified, update your local clone with the upstream branches:

$ git fetch upstream

Setting up your name and email address¶

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

The --global flag sets these parameters globally while the --local flag sets them only for the current project.

Enabling autocrlf on Windows¶

The autocrlf option will fix automatically any Windows-specific line endings. This should be enabled on Windows, since the public repository has a hook which will reject all commits having the wrong line endings:

$ git config --global core.autocrlf input

Creating and switching branches¶

Important

Never commit directly to the main branch.

Create a new branch from main and switch to it:

$ git switch -c <branch-name> main

This is equivalent to:

$ # create a new branch from main
$ git branch <branch-name> main
$ # switch to the new branch
$ git switch <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 switch <another-branch-name>

Other releases are just branches in the repository. For example, to work on the 3.12 release from the upstream remote:

$ git switch -c 3.12 upstream/3.12

Deleting branches¶

To delete a local branch that you no longer need:

$ git switch main
$ git branch -D <branch-name>

To delete a remote branch:

$ git push origin -d <branch-name>

You may specify more than one branch for deletion.

Renaming branch¶

The CPython repository’s default branch was renamed from master to main after the Python 3.10b1 release.

If you have a fork on GitHub (as described in Forking CPython GitHub repository) that was created before the rename, you should visit the GitHub page for your fork to rename the branch there. You only have to do this once. GitHub should provide you with a dialog for this. If it doesn’t (or the dialog was already dismissed), you can rename the branch in your fork manually by following these GitHub instructions.

After renaming the branch in your fork, you need to update any local clones as well. This only has to be done once per clone:

$ git branch -m master main
$ git fetch origin
$ git branch -u origin/main main
$ git remote set-head origin -a

(GitHub also provides these instructions after you rename the branch.)

If you do not have a fork on GitHub, but rather a direct clone of the main repo created before the branch rename, you still have to update your local clones. This still only has to be done once per clone. In that case, you can rename your local branch as follows:

$ git branch -m master main
$ git fetch upstream
$ git branch -u upstream/main main

Staging and committing files¶

To show the current changes:

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

$ git add -p # to review and add changes to existing files $ git add <filename1> <filename2> # to add new files
To commit the files that have been staged (done in step 2):

git commit -m "This is the commit message."

Reverting changes¶

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

$ git checkout <filename>

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

$ git reset --hard HEAD

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

Comparing changes¶

View all non-commited changes:

$ git diff

Compare to the main branch:

$ git diff main

Exclude generated files from diff using an attr pathspec (note the single quotes):

$ git diff main ':(attr:!generated)'

Exclude generated files from diff by default:

$ git config diff.generated.binary true

The generated attribute is defined in .gitattributes, found in the repository root.

Pushing changes¶

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

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

Creating a pull request¶

Go to https://github.com/python/cpython.
Press the New pull request button.
Click the compare across forks link.
Select the base repository: python/cpython and base branch: main.
Select the head repository: <username>/cpython and head branch: the branch containing your changes.
Press the Create pull request button.

You should include the issue number in the title of the PR, in the format gh-NNNNN: <PR Title>.

Linking to issues and pull requests¶

You can link to issues and pull requests using gh-NNNNN (this form is preferred over #NNNNN). If the reference appears in a list, the link will be expanded to show the status and title of the issue/PR.

When you create a PR that includes gh-NNNNN in the title, bedevere will automatically add a link to the issue in the first message.

In addition, pull requests support special keywords that can be used to link to an issue and automatically close it when the PR is merged. However, issues often require multiple PRs before they can be closed (for example, backports to other branches), so this features is only useful if you know for sure that a single PR is enough to address and close the issue.

Updating your CPython fork¶

Scenario:

You forked the CPython repository some time ago.
Time passes.
There have been new commits made in the 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 the upstream CPython repository.

Please do not try to solve this by creating a pull request from python:main to <username>:main as the authors of the pull requests will get notified unnecessarily.

Solution:

$ git switch main
$ git pull upstream main
$ git push origin main

Note

For the above commands to work, please follow the instructions found in the Get the source code section.

Another scenario:

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

Solution:

$ git switch some-branch
$ git fetch upstream
$ git merge upstream/main
$ git push origin some-branch

You may see error messages like “CONFLICT” and “Automatic merge failed;” when you run git merge upstream/main.

When it happens, you need to resolve conflict. See these articles about resolving conflicts:

Applying a patch to Git¶

Scenario:

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

Solution:

Download the patch locally.
Apply the patch:

$ git apply /path/to/patch.diff

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" main) $ git apply /path/to/patch.diff

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.
If the apply was successful, create a new branch and switch to it.
Stage and commit the changes.
If the patch was applied to an old revision, it needs to be updated and merge conflicts need to be resolved:

$ git rebase main $ git mergetool

For very old changes, git merge --no-ff may be easier than a rebase, with regards to resolving conflicts.
Push the changes and open a pull request.

Checking out others’ pull requests¶

Scenario:

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

If you’ve got GitHub CLI or hub installed, you can do:

$ gh co <pr_number>  # GitHub CLI
$ hub pr checkout <pr_number>  # hub

Both of these tools will configure a remote URL for the branch, so you can git push if the pull request author checked “Allow edits from maintainers” when creating the pull request.

Otherwise, you can run the following commands:

$ git fetch upstream pull/NNNNN/head:pr_NNNNN
$ git switch pr_NNNNN

Or set up a Git alias:

Unix/macOS

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

Windows cmd

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

Windows Powershell

git config --global alias.pr '!f() { git fetch upstream pull/$1/head:pr_$1 && git checkout pr_$1; }; f'

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>

Accepting and merging a pull request¶

Pull requests can be accepted and merged by a Python Core Developer. You can read more about what to look for before accepting a change here.

All pull requests have required checks that need to pass before a change can be merged. See “Keeping CI green” for some simple things you can do to help the checks turn green.

At any point, a core developer can schedule an automatic merge of the change by clicking the gray Enable auto-merge (squash) button. You will find it at the bottom of the pull request page. The auto-merge will only happen if all the required checks pass, but the PR does not need to have been approved for a successful auto-merge to take place.

If all required checks are already finished on a PR you’re reviewing, in place of the gray Enable auto-merge button you will find a green Squash and merge button.

In either case, adjust and clean up the commit message.

✅ Here’s an example of a good commit message:

gh-12345: Improve the spam module (GH-777)

* Add method A to the spam module
* Update the documentation of the spam module

❌ Here’s an example of a bad commit message:

gh-12345: Improve the spam module (#777)

* Improve the spam module
* merge from main
* adjust code based on review comment
* rebased

The bad example contains bullet points that are a direct effect of the PR life cycle, while being irrelevant to the final change.

Note

How to Write a Git Commit Message is a nice article describing how to write a good commit message.

Finally, press the Confirm squash and merge button.

Cancelling an automatic merge¶

If you notice a problem with a pull request that was accepted and where auto-merge was enabled, you can still cancel the workflow before GitHub automatically merges the change.

Press the gray Disable auto-merge button on the bottom of the pull request page to disable automatic merging entirely. This is the recommended approach.

To pause automatic merging, apply the “DO-NOT-MERGE” label to the PR or submit a review requesting changes. The latter will put an “awaiting changes” label on the PR, which pauses the auto-merge similarly to “DO-NOT-MERGE”. After the author submits a fix and re-requests review, you can resume the auto-merge process either by submitting an approving review or by dismissing your previous review that requested changes.

Note that pushing new changes after the auto-merge flow was enabled does NOT stop it.

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 main. It is usually indicated by the label needs backport to X.Y on the pull request itself.

Use the utility script cherry_picker.py to backport the commit.

The commit hash for backporting is the squashed commit that was merged to the main branch. On the merged pull request, scroll to the bottom of the page. Find the event that says something like:

<core_developer> merged commit <commit_sha1> into python:main <sometime> ago.

By following the link to <commit_sha1>, you will get the full commit hash.

Alternatively, the commit hash can also be obtained by the following Git commands:

$ git fetch upstream
$ git rev-parse ":/gh-12345"

The above commands will print out the hash of the commit containing "gh-12345" as part of the commit message.

When formatting the commit message for a backport commit: leave the original one as is and delete the number of the backport pull request.

✅ Example of good backport commit message:

 gh-12345: Improve the spam module (GH-777)

 * Add method A to the spam module
 * Update the documentation of the spam module

 (cherry picked from commit 62adc55)

❌ Example of bad backport commit message:

 gh-12345: Improve the spam module (GH-777) (#888)

 * Add method A to the spam module
 * Update the documentation of the spam module

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 a news entry.

To edit an open pull request that targets main:

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

<contributor> wants to merge 1 commit into python:main from <contributor>:<branch_name>

Fetch the pull request, using the git pr alias:

$ git pr <pr_number>

This will checkout the contributor’s branch at <pr_number>.
Make and commit your changes on the branch. For example, merge in changes made to main 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/main $ git add <filename> $ git commit -m "<message>"
Push the changes back to the contributor’s PR branch:

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

GitHub CLI¶

GitHub CLI is a command-line interface that allows you to create, update, and check GitHub issues and pull requests.

You can install GitHub CLI by following these instructions. After installing, you need to authenticate:

$ gh auth login

Examples of useful commands:

Create a PR:

$ gh pr create
Check out another PR:

$ gh co <pr-id>
Set ssh as the Git protocol:

$ gh config set git_protocol ssh
Set the browser:

$ gh config set browser <browser-path>

Git worktree¶

With Git worktrees, you can have multiple isolated working trees associated with a single repository (the .git directory). This allows you to work simultaneously on different version branches, eliminating the need for multiple independent clones that need to be maintained and updated separately. In addition, it reduces cloning overhead and saves disk space.

Setting up Git worktree¶

With an existing CPython clone (see Cloning a forked CPython repository), rename the cpython directory to main and move it into a new cpython directory, so we have a structure like:

cpython
└── main (.git is here)

Next, create worktrees for the other branches:

$ cd cpython/main
$ git worktree add -b 3.11 ../3.11 upstream/3.11
$ git worktree add -b 3.12 ../3.12 upstream/3.12

This gives a structure like this, with the code for each branch checked out in its own directory:

cpython
├── 3.11
├── 3.12
└── main

Using Git worktree¶

List your worktrees, for example:

$ git worktree list
/Users/my-name/cpython/main  b3d24c40df [main]
/Users/my-name/cpython/3.11  da1736b06a [3.11]
/Users/my-name/cpython/3.12  cf29a2f25e [3.12]

Change into a directory to work from that branch. For example:

$ cd ../3.12
$ git switch -c my-3.12-bugfix-branch  # create new branch
$ # make changes, test them, commit
$ git push origin my-3.12-bugfix-branch
$ # create PR
$ git switch 3.12  # switch back to the 3.12 branch
...