Workflow for Developers

In this document, we refer to the Astropy 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 — please see that page for more details. We’re repeating some of it here just to give the specifics for the Astropy 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

The following example shows how to fork the core astropy repository, but the same applies to other packages:

  1. Log into your GitHub account.

  2. Go to the Astropy GitHub home page.

  3. Click on the fork button:


    After a short pause and an animation of Octocat scanning a book on a flatbed scanner, you should find yourself at the home page for your own forked copy of Astropy.

Setting up the fork to work on


This is done using:

git clone
cd astropy
git remote add upstream git://

In detail

  1. Clone your fork to the local computer:

    git clone
  2. Change directory to your new repo:

    cd astropy

    Then type:

    git branch -a

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

    * 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 connections. They will point to your GitHub fork.

    Now connect to the Astropy repository, so you can merge in changes from the trunk:

    cd astropy
    git remote add upstream git://

    upstream is just the arbitrary name we’re using to refer to the main Astropy repository.

    Note that we’ve used git:// for the URL rather than git@. The git:// URL is read only. This means 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 connection with git remote -v show, which should give you something like:

    upstream   git:// (fetch)
    upstream   git:// (push)
    origin (fetch)
    origin (push)

    Your fork is now set up correctly, and you are ready to hack away.

Installing Astropy in develop mode

Astropy is designed so that it can generally be used directly out of the source tree by using import astropy when running Python in the source of an Astropy repository clone.

  1. Install Astropy in develop mode:

    $ python develop

    This semi-permanently installs Astropy on your path in such a way that astropy is always imported from your repository clone regardless of your working directory. This way any edits you make to the code in your repository will always be immediately available next time you start a Python interpreter and import astropy.

  2. Develop mode can be easily disabled again by running python develop -u. Without having the package installed in develop mode, if you change directories from the root of the repository clone the astropy package will no longer be importable, and other features may not work properly.

    Another useful technique to avoid having to switch develop mode on and off without disrupting production use of Astropy is to use virtualenv as explained below.

    It should be noted that python develop will not work with Python 3, as Astropy currently uses 2to3 to convert the source code to a format compatible with Python 3. Because of this it is not possible to import astropy directly from a source checkout with Python 3. In the future Astropy may move to a cross-compatible coding style that works natively in both Python 2 and 3.


When switching branches it is not generally required to re-run python develop, though it may be necessary to run python build_ext --inplace to build C extensions in “inplace” mode (which copies the built shared libraries into the source tree) in case you think there might be differences in the C extensions between the two branches.

To check that the installation worked correctly, run the Astropy tests.

Workflow summary

This section gives a summary of the workflow to follow once you have successfully forked the repository. The details for each of these steps are given 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, then 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, for example bugfix-for-issue-14 or refactor-database-code.
  • If you can possibly avoid it, don’t merge the 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
  • Ask on the astropy-dev mailing list if you get stuck.
  • Once your code is nearing completion, run the test suite to ensure you have not accidentally caused regressions, and add new tests to ensure your contribution behaves correctly (see Testing Guidelines).
  • Issue a pull request on github!
  • As the code is converging to a final state, ensure your documentation follows the guidelines (see Writing Documentation).
  • Once your code is ready to be accepted, please add an entry to the changelog (see Updating and Maintaining the Changelog). If you’re sure where to put this, please at least suggest a brief (one or two sentence) description of your change so that another Astropy developer can add it to the changelog.

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 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. 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


Make changes, test, and:

git add my_new_file
git commit -m 'NF - some message'
git push

In more detail

  1. Make some changes

  2. Once you are a bit further along, test your changes do not lead to regressions, and add new tests (see Testing Guidelines). For example, if you are working on time:

    import astropy

    If you have sphinx installed, you can also check that the documentation builds and looks correct:

    python build_sphinx

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

  3. 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")
  4. Check what the actual changes are with git diff (see git diff).

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

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

  7. 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.

  8. Push the changes up to your forked repo on GitHub with 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, e.g.,

  2. Use the ‘Switch Branches’ dropdown menu near the top left of the page 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.

Using virtualenv

virtualenv is a tool for creating and activating isolated Python environments that allow installing and experimenting with Python packages without disrupting your production Python environment. When using commands such as python develop, for example, it is strongly recommended to do so within a virtualenv. This is generally preferable to installing a development version of Astropy into your system site-packages and having to keep track of whether or not your environment is in a “known good” configuration for production/science use.

Using a virtualenv is also a good way to try out new versions of software that you’re not actively doing development work on without disrupting your normal production environment.

We won’t provide a full tutorial on using virtualenv here — the virtualenv documentation linked to above is a better place to start. But here is a quick overview on how to set up a virtualenv for Astropy development with your default Python version:

  1. Install virtualenv:

    $ pip install virtualenv


    $ easy_install virtualenv

    or (on Debian/Ubuntu):

    $ sudo apt-get install python-virtualenv


  2. (Recommended) Create a root directory for all your virtualenvs under a path you have write access to. For example:

    $ mkdir ~/.virtualenvs
  3. Create the Astropy virtualenv:

    $ virtualenv --distribute --system-site-packages ~/.virtualenvs/astropy-dev

    The --system-site-packages option inherits all packages already installed in your system site-packages directory; this frees you from having to reinstall packages like Numpy and Scipy in the virtualenv. However, if you would like your virtualenv to use a development version of Numpy, for example, you can still install Numpy into the virtualenv and it will take precedence over the version installed in site-packages.

  4. Activate the virtualenv:

    $ source ~/.virtualenvs/astropy-dev/bin/activate

    or if you’re using a csh-variant:

    $ source ~/.virtualenvs/astropy-dev/bin/activate.csh

    virtualenv works on Windows too — see the documentation for details.

  5. If the virtualenv successfully activated its name should appear in your shell prompt:

    (astropy-dev) $

    The virtualenv can be disabled at any time by entering:

    (astropy-dev) $ deactivate
  6. Now as long as the virtualenv is activated, packages you install with pip, easy_install, or by manually running python install will automatically install into your virtualenv instead of the system site-packages. Consider installing Astropy in develop mode into the virtualenv as described above.

Using virtualenv with IPython


As of IPython 0.13 this functionality is built into IPython and these steps are not necessary for IPython to recognize that it’s running with a virtualenv enabled.

Each virtualenv has its own bin/, and as IPython is written in pure Python one can always install IPython directly into a virtualenv. However, if you would rather not have to install IPython every time you create a virtualenv, it also suffices to make IPython virtualenv-aware.

  1. Check to see if you already have an IPython profile in ~/.ipython/profile_default/; if not, create one:

    $ ipython profile create
  2. Edit ~/.ipython/profile_default/ and add the following to the end:

    import os
    execfile(os.path.join(os.environ['HOME'], '.ipython', ''))
  3. Finally, create the ~/.ipython/ module:

    import site
    from os import environ
    from os.path import join
    from sys import version_info
    if 'VIRTUAL_ENV' in environ:
        virtual_env = join(environ.get('VIRTUAL_ENV'),
                           'python%d.%d' % version_info[:2],
        print 'VIRTUAL_ENV ->', virtual_env
        del virtual_env
    del site, environ, join, version_info

Now IPython will import all packages from your virtualenv where applicable.


This is not magic. If you switch to a virtualenv that uses a different Python version from your main IPython installation this won’t help you — instead use the appropriate IPython installation for the Python version in question.


virtualenvwrapper is a set of enhancements to virtualenv mostly implemented through simple shell scripts and aliases. It automatically organizes all your virtualenvs under a single directory (as suggested above). To create a new virtualenv you can just use the mkvirtualenv command and it will automatically create a new virtualenv of that name in the default location.

To activate a virtualenv with virtualenvwrapper you don’t need to think about the environment’s location of the filesystem or which activate script to run. Simply run workon. You can also list all virtualenvs with lsvirtualenv. That just scratches the surface of the goodies included with virtualenvwrapper.

The one caveat is that it does not support csh-like shells. For csh-like shells there exists virtualenvwrapper-csh, which implements most of the virtualenvwrapper functionality and is otherwise compatible with the original. There also exists virtualenvwrapper-win, which ports virtualenvwrapper to Windows batch scripts.


virtualenv is so commonly used in the Python development community that its functionality was finally added to the standard library in Python 3.3 under the name venv. venv has not gained wide use yet and is not explicitly supported by tools like virtualenvwrapper, but it is expected to see wider adoption in the future.

Some other things you might want to do

Delete a branch on GitHub

# change to the master branch (if you still have one, otherwise change to
# another branch)
git checkout master

# delete branch locally
git branch -D my-unwanted-branch

# delete branch on GitHub
git push origin :my-unwanted-branch

(Note the colon : before test-branch. See also:

Several people sharing a single repository

If you want to work on some stuff with other people, where you are all committing into the same repository, or even the same branch, then just share it via GitHub.

First fork Astropy into your account, as from Creating a fork.

Then, go to your forked repository GitHub page, e.g.,

Click on the ‘Admin’ button, and add anyone else to the repo as a collaborator:


Now all those people can do:

git clone

Remember that links starting with git@ use the ssh protocol and are read-write; links starting with git:// are read-only.

Your collaborators can then commit directly into that repo with the usual:

git commit -am 'ENH - much better code'
git push origin master # pushes directly into your repo

Explore your repository

To see a graphical representation of the repository branches and commits:

gitk --all

To see a linear list of commits for this branch:

git log

You can also look at the network graph visualizer for your GitHub repo.

Finally the Fancy log output lg alias will give you a reasonable text-based graph of the repository.

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 astropy 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}

Rewriting commit history


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 couple 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.

Converting a GitHub issue to a pull request

Sometimes you have a branch in your own GitHub repository designed to fix one particular issue. If that issue is listed on GitHub, a natural way to address it is to convert the issue to a pull request by attaching code containing the fix for the issue. This can currently only be done using the GitHub API (there’s no button or anything on the web site that does it, at least as of 2/6/2012). There are two options to do this:

  • You can use the script at, which will do this for you automatically — just download the script and run it as a python command-line script, using the python --help option to determine the precise usage.

  • You can use the hub command-line utility provided here by GitHub. Once installed, you can attach a branch to a pull request by doing:

    hub pull-request -i <ID> -b astropy:master -h <USER>:<BRANCH>

    where <ID> is the ID of the issue, <USER> is the username, and <BRANCH> is the name of the branch you want to attach to the issue. For example:

    hub pull-request -i 42 -b astropy:master -h galahad:feature

    will attach the feature branch from galahad‘s Astropy repository to issue 42.

    The hub command can do a lot more to interact with GitHub, so be sure to read their documentation. For example, you can fetch all branches of a repository for a given user by doing:

    hub fetch <USER>