SNCosmo follows the same general development workflow as astropy and many other open-source software projects. The astropy development workflow page documents the process in some detail. While you should indeed read that page, it can seem a bit overwhelming at first. So, we present here a rough outline of the process, and try to explain the reasoning behind it.

The process is centered around git and GitHub, so you need to know how to use basic git commands and also have a GitHub account. There is a “blessed” copy of the repository at Individual contributors make changes to their own copy (or “fork” or “clone” in git parlance) of the repository, e.g.,, then ask that their changes be merged into the “blessed” copy via a Pull Request (PR) on GitHub. A maintainer (currently Kyle) will review the changes in the PR, possibly ask for alterations, and then eventually merge the change.

This seems overly complex at first glance, but there are two main benefits to this process: (1) Anyone is free to try out any crazy change they want and share it with the world on their own GitHub account, without affecting the “blessed” repository, and (2) Any proposed changes are reviewed and discussed by at least one person (the maintainer) before being merged in.

Detailed steps

Do once:

  1. Hit the “fork” button in the upper right hand corner of the page. This creates a clone of the repository on your personal github account.

  2. Get it onto your computer (replace username with your GitHub username):

    git clone
  3. Add the “blessed” version as a remote:

    git remote add upstream

    This will allow you to update your version to reflect new changes to the blessed repository that others have made).

  4. Check that everything is OK:

    $ git remote -v
    origin (fetch)
    origin (push)
    upstream (fetch)
    upstream (push)

    You can call the remotes anything you want. “origin” and “upstream” have no intrinsic meaning for git; they’re just nicknames. The astropy documentation calls them “your-github-username” and “astropy” respectively.

  5. Install the SNCosmo package in development mode. From the git directory:

    pip install -e .

    If you are only editing Python code, the latest code will be used when you import sncosmo in a Python interpreter for the first time. If you are editing any of the Cython code in SNCosmo (files with .c or .pyx extensions), then you will need to run this command again to compile that code for your changes to be picked up.

Every time you want to make a contribution:

  1. Ensure that the clone of the repository on your local machine is up-to-date with the latest upstream changes by doing git fetch upstream. This updates your local “remote tracking branch”, called upstream/master.

  2. Create a “topic branch” for the change you want to make. If you plan to make enhancements to the simulation code, name the branch something like “simulation-enhancements”:

    git branch simulation-enhancements upstream/master

    (upstream/master is where the branch branches off from.)

  3. Move to the branch you just created:

    git checkout simulation-enhancements
  4. Make changes, ensure that they work, etc. Make commits as you go.

  5. Once you’re happy with the state of your branch, push it to your GitHub account for the world to see:

    git push origin simulation-enhancements
  6. Create a PR: Go to your copy on github ( select the branch you just pushed in the upper left-ish of the page, and hit the green button next to it. (It has a mouse-over “compare, review, create a pull request”)

What happens when the upstream branch is updated?

Suppose that you are following the above workflow: you created a topic branch simulation-enhancements and made a few commits on that branch. You now want to create a pull request, but there’s a problem: while you were working, more commmits were added to the upstream/master branch on GitHub. The history of your branch has now diverged from the main development branch! What to do?

  1. Fetch the changes made to the upstream branch on so that you can deal with the changes locally:

    git fetch upstream

    This will update your local branch upstream/master (and any other upstream branches) to the match the state of the upstream branch on GitHub. It doesn’t do any merging or resolving, it just makes the new changes to upstream/master visible locally.

  2. There are two options for this next step: merge or rebase with the latter being preferred for this purpose. Assuming you are on your branch simulation-enhancements, you could do git merge upstream/master. This would create a merge commit that merges the diverged histories back together. This works, but it can end up creating a confusing commit history, particularly if you repeat this process several times while working on your new branch. Instead, you can do:

    git rebase upstream/master

    This actually rewrites your commits to make it look like they started from where upstream/master now is, rather than where it was when you started work on your simulation-enhancements branch. Your branch will have the exact same contents as if you had used git merge, but the history will be different than it would have been if you had merged. In particular, there is no merge commit created, because the history has been rewritten so that your branch starts where upstream/master ends, and there is no divergent history to resolve. This means you can rebase again and again without creating a convoluted history full of merges back and forth between the branches.

Trying out new ideas

git branches are the best way to try out new ideas for code modifications or additions. You don’t even have to tell anyone about your bad ideas, since branches are local! They only become world visible when you push them to GitHub. If, after making several commits, you decide that your new branch simulation-enhancements sucks, you can just create a new branch starting from upstream/master again. If it is a really terrible idea you never want to see again, you can delete it by doing git branch -D simulation-enhancements.

Obviously this isn’t a complete guide to git, but hopefully it jump-starts the git learning process.


SNCosmo uses pytest to check that all of the code is running as expected. When you add new functionality to SNCosmo, you should write a test for that functionality. All of the tests can be found in the sncosmo/tests directory.

When a new PR is created, the testsuite will be run automatically on a range of different machines and conditions using tox. You can run these same tests locally using tox. First, install tox:

pip install tox

From within the SNCosmo directory, run the test suite:

tox -e py3

The previous command will run the core test suite with the currently installed version of Python. You can run the full test suite with all of the optional dependencies by adding the -alldeps tag:

tox -e py3-alldeps

Running the tests with the -cov tag will generate a coverage report:

tox -e py3-cov

SNCosmo includes hundreds of builtin bandpasses and sources that are downloaded from external sites when they are loaded. tox can be used to check that all of these builtins are accessible with the following command:

tox -e builtins

tox can also be used to check the code style:

tox -e codestyle

or to build the documentation:

tox -e build_docs

tox uses virtual environments for testing which can be somewhat slow. You can alternatively run the test in your own Python environment. First, install all of the testing dependencies from the test section of setup.cfg. This can be done automatically when installing SNCosmo with the following command:

pip install -e .[test]

The tests can then be run with the following command:

pytest --pyargs sncosmo

Developer’s documentation: release procedure

The release procedure is automated through GitHub Actions. To create a new release:

  • Update docs/history.rst with a summary of the new version’s changes.

  • Bump version in sncosmo/

  • Ensure that the tests have all completed successfully and that the docs are looking good.

  • Create a new release through the releases tab on GitHub, and tag it with the latest version.

  • Copy the change list into the release description.

  • Publish the release.

Packaging and Docs

  • GitHub Actions will trigger after each release and build compiled wheels and source distributions. These will then be pushed to PyPI.

  • A conda build should start (with some delay) via a bot pull request at Merge the PR once it passes all tests.

  • The docs for the release will show up on as the new stable version.

Bumping Minimum Supported Python Version

Versions are hardcoded in

  • tox.ini - update the envlist = py{...} line.

  • setup.cfg - update python_requires, install_requires, and oldestdeps as needed.

  • .github/workflows/run_tests.yml - update the python and toxenv lines

  • docs/install.rst - Ensure that the first line “SNCosmo works on Python 3.x+” is correct