How to make a code contribution#

This document outlines the process for contributing code to the Astropy project.

Already experienced with git? Contributed before? Jump right to Astropy Guidelines for git.


Before following the steps in this document you need:

  • an account on GitHub

  • a local copy of the astropy source. Instructions for doing that, including the basics you need for setting up git and GitHub, are at Try the development version.

New to git?#

Some git resources#

If you have never used git or have limited experience with it, take a few minutes to look at Git Basics, part of a much longer git book.

In practice, you need only a handful of git commands to make contributions to Astropy. There is a more extensive list of Git resources if you want more background.

Double check your setup#

Before going further, make sure you have set up astropy as described in Try the development version.

In a terminal window, change directory to the one containing your clone of Astropy. Then, run git remote; the output should look something like this:


If that works, also run git fetch --all. If it runs without errors then your installation is working and you have a complete list of all branches in your clone, origin and astropy.

About names in git#

git is designed to be a distributed version control system. Each clone of a repository is, itself, a repository. That can lead to some confusion, especially for the branch called main. If you list all of the branches your clone of git knows about with git branch -a you will see there are three different branches called main:

* main                              # this is main in your local repo
remotes/astropy/main                # the official development branch of Astropy
remotes/origin/main                 # main on your fork of Astropy on GitHub

The naming scheme used by git will also be used here. A plain branch name, like main means a branch in your local copy of Astropy. A branch on a remote, like astropy , is labeled by that remote, astropy/main.

This duplication of names can get very confusing when working with pull requests, especially when the official main branch, astropy/main, changes due to other contributions before your contributions are merged in. As a result, you should never do any work in your main branch, main. Always work on a branch instead.

Essential git commands#

A full git tutorial is beyond the scope of this document but this list describes the few git commands you are likely to encounter in contributing to Astropy:

  • git fetch gets the latest development version of Astropy, which you will use as the basis for making your changes.

  • git branch makes a logically separate copy of Astropy to keep track of your changes.

  • git add stages files you have changed or created for addition to git.

  • git commit adds your staged changes to the repository.

  • git push copies the changes you committed to GitHub

  • git status to see a list of files that have been modified or created.


A good graphical interface to git makes some of these steps much easier. You might find this list of GUI Clients to be helpful.

If something goes wrong#

git provides a number of ways to recover from errors. If you end up making a git mistake, do not hesitate to ask for help. An additional resource that walks you through recovering from git mistakes is the git choose-your-own-adventure.

Astropy Guidelines for git#


It is strongly suggested that you automate the code-style checks using the provided pre-commit hook, see Pre-commit below for details.

  • Don’t use your main branch for anything. Consider Deleting your main branch.

  • Make a new branch, called a feature branch, for each separable set of changes: “one task, one branch” (ipython git workflow).

  • Start that new feature branch from the most current development version of astropy (instructions are below).

  • Name your branch for the purpose of the changes, for example bugfix-for-issue-14 or refactor-database-code.

  • Make frequent commits, and always include a commit message. Each commit should represent one logical set of changes.

  • Ask on the astropy-dev mailing list if you get stuck.

  • Never merge changes from astropy/main into your feature branch. If changes in the development version require changes to our code you can Rebase if necessary.

  • If you need to edit .mailmap and know how to do it then you can open a pull request for that. Please run git shortlog -es locally first with your changes to make sure your edit is correct, and you only appear in the list once.

In addition, there is a git naming convention used in this document:

  • Name the remote that is the primary Astropy repository astropy; in prior versions of this documentation it was referred to as upstream.


All of the coding style checks described in Coding Style/Conventions can be performed automatically when you make a git commit using our provided pre-commit hook for git, for more information see git hooks. We encourage you to setup and use these hooks to ensure that your code always meets our coding style standards. The easiest way to do this is by installing pre-commit:

python -m pip install pre-commit

Or if you prefer conda:

conda install pre-commit

This next command needs be done by installing pre-commit in the root of your astropy repository by running:

pre-commit install

For more detailed instructions on installing pre-commit, see the install guide. Once this installation is complete, all the coding style checks will be run each time you commit and the necessary changes will automatically be applied to your code if possible.


The changes made by pre-commit will not be automatically staged, so you will need to review and re-stage any files that pre-commit has changed.

In general, git will not allow you to commit until the pre-commit hook has run successfully. If you need to make a commit which fails the pre-commit checks, you can skip these checks by running:

git commit --no-verify

If you do not want to use pre-commit as part of your git workflow, you can still run the checks manually (see, Coding Style/Conventions) using:

tox -e codestyle

Or this will run whether you did pre-commit install or not:

pre-commit run

Again, this will automatically apply the necessary changes to your code if possible.


Once you have made a pull-request the bot is available to assist you with fixing any issues with your code style, see Fixing coding style issues for details on how to use this bot.


These, conceptually, are the steps you will follow in contributing to Astropy:

  1. Fetch the latest Astropy

  2. Make a new feature branch; you will make your changes on this branch.

  3. Install your branch

  4. Follow The editing workflow to write/edit/document/test code - make frequent, small commits.

  5. Add a changelog entry

  6. Push your changes to GitHub

  7. From GitHub, Ask for your changes to be reviewed to let the Astropy maintainers know you have contributions to review.

  8. Revise and push as necessary in response to comments on the pull request. Pushing those changes to GitHub automatically updates the pull request.

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.

A worked example that follows these steps for fixing an Astropy issue is at Contributing code to Astropy, a worked example.

Some additional topics related to git are in Some other things you might want to do.

Deleting your main branch#

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

Fetch the latest Astropy#

From time to time you should fetch the development version (i.e., Astropy astropy/main) changes from GitHub:

git fetch astropy --tags

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

Make a new feature branch#

Make the new 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 a 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. Branch names like add-ability-to-fly or bugfix-for-issue-42 clearly describe the purpose of the branch.

Always make your branch from astropy/main so that you are basing your changes on the latest version of Astropy:

# Update the mirror of trunk
git fetch astropy --tags

# Make new feature branch starting at astropy/main
git branch my-new-feature astropy/main
git checkout my-new-feature

Connect the branch to GitHub#

At this point you have made and checked out a new branch, but git does not know it should be connected to your fork on GitHub. You need that connection for your proposed changes to be managed by the Astropy maintainers on GitHub.

The most convenient way for connecting your local branch to GitHub is to git push this new branch up to your GitHub repo with 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 your GitHub fork of Astropy.

You will still need to Push your changes to GitHub periodically. The setup in this section will make that easier because any following pushes of this branch can be performed without having to write out the remote and branch names, but it never hurts to be explicit in typing out the commands.

Install your branch#

Ideally you should set up a Python virtual environment just for this fix; instructions for doing to are at Python virtual environments. Doing so ensures you will not corrupt your main astropy install and makes it very easy to recover from mistakes, and thus, is recommended before you proceed.

Assuming you have set up and activated this virtual environment, you need to install the version of astropy you are working on into it. Do that with:

python -m pip install --editable ".[test]"

This will install astropy itself, along with a few packages which will be useful for testing the changes you will make down the road.

For more details on building astropy from source, see Building Astropy and its Subpackages.

The editing workflow#

Conceptually, you will:

  1. Make changes to one or more files and/or add a new file.

  2. Check that your changes do not break existing code.

  3. Add documentation to your code and, as appropriate, to the Astropy documentation.

  4. Ideally, also make sure your changes do not break the documentation.

  5. Add tests of the code you contribute.

  6. Commit your changes in git.

  7. Repeat as necessary.

In more detail#

  1. Make some changes to one or more files. You should follow the Astropy Coding Guidelines. Each logical set of changes should be treated as one commit. For example, if you are fixing a known bug in Astropy and notice a different bug while implementing your fix, implement the fix to that new bug as a different set of changes.

  2. Test that your changes do not lead to regressions, i.e. that your changes do not break existing code, by running the Astropy tests. You can run all of the Astropy tests from ipython with:

    import astropy

    If your change involves only a small part of Astropy, e.g. Time, you can run just those tests:

    import astropy

    Tests can also be run from the command line while in the package root directory, e.g.:


    To run the tests in only a single package, e.g. Time, you can do:

    pytest -P time

    For more details on running tests, please see Testing Guidelines.

  3. Make sure your code includes appropriate docstrings, in the Numpydoc format. If appropriate, as when you are adding a new feature, you should update the appropriate documentation in the docs directory; a detailed description is in Writing Documentation.

  4. If you have sphinx installed, you can also check that the documentation builds and looks correct by running, from the astropy directory:

    cd docs
    make html

    The last line should just state build succeeded, and should not mention any warnings. (For more details, see Writing Documentation.)

  5. Add tests of your new code, if appropriate. Some changes (e.g. to documentation) do not need tests. Detailed instructions are at Writing tests, but if you have no experience writing tests or with the pytest testing framework submit your changes without adding tests, but mention in the pull request that you have not written tests. An example of writing a test is in Stop and think: Any more tests or other changes?.

  6. Stage your changes using git add and commit them using git commit. An example of doing that, based on the fix for an actual Astropy issue, is at Contributing code to Astropy, a worked example.


    Make your git commit messages short and descriptive. If a commit fixes an issue, include, on the second or later line of the commit message, the issue number in the commit message, like this: Closes #123. Doing so will automatically close the issue when the pull request is accepted.

  7. Some modifications require more than one commit; if in doubt, break your changes into a few, smaller, commits rather than one large commit that does many things at once. Repeat the steps above as necessary!

Add a changelog entry#

Add a changelog fragment briefly describing the change you made by creating a new file in docs/changes/<sub-package>/. The file should be named like <PULL REQUEST>.<TYPE>.rst, where <PULL REQUEST> is a pull request number, and <TYPE> is one of:

  • feature: New feature.

  • api: API change.

  • bugfix: Bug fix.

  • other: Other changes and additions.

An example entry, for the changes in PR 1845, the file would be docs/changes/wcs/1845.bugfix.rst and would contain:

``astropy.wcs.Wcs.printwcs`` will no longer warn that ``cdelt`` is
being ignored when none was present in the FITS file.

If you are opening a new pull request, you may not know its number yet, but you can add it after you make the pull request. If you’re not sure where to put the changelog entry, wait at least until a maintainer has reviewed your PR and assigned it to a milestone.

When writing changelog entries, do not attempt to make API reference links by using single-backticks. This is because the changelog (in its current format) runs for the history of the project, and API references you make today may not be valid in a future version of Astropy. However, use of double-backticks for monospace rendering of module/class/function/argument names and the like is encouraged.

Push your changes to GitHub#

To push your changes on a branch named my-new-feature to your GitHub fork of astropy with the same branch name:

git push origin my-new-feature

Ask for your changes to be reviewed#

A pull request on GitHub is a request to merge the changes you have made into another repository.

When you are ready to ask for someone to review your code and consider merging it into Astropy:

  1. Go to the URL of your fork of Astropy, e.g.,

  2. Use the ‘Switch Branches’ dropdown menu to select the branch with your changes:

  3. Click on the ‘Pull request’ button:


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

    If you don’t think your request is ready to be merged, just say so in your pull request message. This is still a good way to start a preliminary code review.

    You may also opt to open a work-in-progress pull request. If you do so, instead of clicking “Create pull request”, click on the small down arrow next to it and select “Create draft pull request”. This will let the maintainers know that your work is not ready for a full review nor to be merged yet. In addition, if your commits are not ready for CI testing, you should also use [ci skip] or [skip ci] directive in your commit message. For usage of pre-commit hooks and directives, see Pre-commit and Fixing coding style issues.

Revise and push as necessary#

You may be asked to make changes in the discussion of the pull request. Make those changes in your local copy, commit them to your local repo and push them to GitHub. GitHub will automatically update your pull request.

Do Not Create a Merge Commit#

If your branch associated with the pull request falls behind the main branch of astropy/astropy, GitHub might offer you the option to catch up or resolve conflicts via its web interface, but do not use this. Using the web interface might create a “merge commit” in your commit history, which is undesirable, as a “merge commit” can introduce maintenance overhead for the release manager as well as undesirable branch structure complexity. Do not use the git pull command either.

Instead, in your local checkout, do a fetch and then a rebase, and resolve conflicts as necessary. See Rebase if necessary and How to rebase for further information.

Rebase if necessary#

Rebasing means taking your changes and applying them to the latest version of the main branch of the official astropy repository as though that was the version you had originally branched from. Each individual commit remains visible, but with new commit hashes.

When to rebase#

Pull requests must be rebased (but not necessarily squashed to a single commit) if at least one of the following conditions is met:

  • There are conflicts with main.

  • There are commits in main (after the PR branch point) needed for continuous integration tests to run correctly.

  • There are merge commits from astropy/main in the PR commit history (merge commits from PRs to the user’s fork are fine).

How to rebase#

It is easier to make mistakes rebasing than other areas of git, so before you start make a branch to serve as a backup copy of your work:

git branch tmp my-new-feature # make temporary branch--will be deleted later

After altering the history, e.g., with git rebase, a normal git push is prevented, and a --force option will be required.


Do not update your branch with git pull. Pulling changes from astropy/main includes merging the branches, which combines them in a way that preserves the commit history of both. The purpose of rebasing is rewriting the commit history of your branch, not preserving it.

Behind the scenes, git is deleting the changes and branch you made, making the changes others made to the development branch of Astropy, then re-making your branch from the development branch and applying your changes to your branch.

First, fetch the latest development astropy and go to your branch of interest:

git fetch astropy main
git checkout my-new-feature

Now, do the rebase:

git rebase astropy/main

You are more likely to run into conflicts here — places where the changes you made conflict with changes that someone else made — than anywhere else. Ask for help if you need it. Instructions are available on how to resolve merge conflicts after a Git rebase.

Squash if necessary#

Squashing refers to combining multiple commits into a single commit. This can be done using the git rebase command or via Github ‘Squash and merge’.

An Astropy maintainer will be happy to guide you through this process.

When to squash#

If a pull request contains commits with large (approximately > 10KB) intermediate changes which are ultimately removed or modified in the PR, the intermediate diffs must be squashed. An example is adding a large data file in one commit and then removing it in a subsequent commit. The goal is to avoid an unnecessary increase in the size of the astropy repository.

Squashing commits is encouraged when there are numerous commits which do not add value to the commit history. Most small to medium pull requests can be done with a few commits. Some large or intricate pull requests may require more commits to capture the logical progression of the changes.

In general, commits that reflect a specific atomic change (e.g., “Fixed bug revealed by tests for feature X”) may have value for the history.

Commits that are good candidates for squashing include but not limited to:

  • Content that gets removed later, most commonly changes in the implementation or temporary debugging code or, especially, data files (see above).

  • Non-specific commits; e.g., “Added more code and fixed stuff.”

  • Fixes of typos, linting fixes, or other inconsequential changes.

  • Commit messages that violate the code of conduct.

How to squash#

In many cases, squashing can be done by a maintainer using the Github ‘Squash and merge’ button on the GitHub pull request page. If this is not possible, we typically squash using git rebase –interactive. In particular, you can rebase and squash within the existing branch using:

git fetch astropy
git rebase -i astropy/main

The last command will open an editor with all your commits, allowing you to squash several commits together, rename them, etc. Helpfully, the file you are editing has the instructions on what to do.

How to push#

After using git rebase you will still need to push your changes to GitHub so that they are visible to others and the pull request can be updated. Use of a simple git push will be prevented because of the changed history, and will need to be manually overridden using:

git push origin my-new-feature --force

If you run into any problems, do not hesitate to ask. A more detailed conceptual discussing of rebasing is at Rebasing on trunk.

Once the modifications and new git history are successfully pushed to GitHub you can delete any backup branches that may have been created:

git branch -D tmp