Contributing code to Astropy, a worked example¶
The issue title was “len() does not work for coordinates” with description
“It would be nice to be able to use
len on coordinate arrays to know how
many coordinates are present.”
This particular example was chosen because it was tagged as easy in GitHub; seemed like the best place to start out!
Before you begin¶
Make sure you have a local copy of astropy set up as described in
Try the development version. In a nutshell, the output of
git remote -v, run in the
directory where your local of
astropy resides, should be something like this:
astropy firstname.lastname@example.org:astropy/astropy.git (fetch) astropy email@example.com:astropy/astropy.git (push) your-user-name firstname.lastname@example.org:your-user-name/astropy.git (fetch) your-user-name email@example.com:your-user-name/astropy.git (push)
The precise form of the URLs for
your-user-name depends on the
authentication method you set up with GitHub.
The important point is that
astropy should point to the official
your-user-name should point to your copy of
astropy on GitHub.
Grab the latest updates to astropy¶
A few steps in this tutorial take only a single command. They are broken out separately to outline the process in words as well as code.
Inform your local copy of
astropy about the latest changes in the development
git fetch astropy --tags
Set up an isolated workspace¶
Make a new git branch for fixing this issue and switch to the branch:
git checkout astropy/master -b fix-1761
Make a Python environment just for this fix and switch to that environment. The example below shows the necessary steps in the Miniconda/Anaconda Python distribution:
conda create -n apy-1761 python=3.9 # replace 3.9 with desired version conda activate apy-1761
If you are using a different distribution, see Python virtual environments for instructions for creating and activating a new environment.
Install our branch in this environment with:
pip install -e .[test]
Do you really have to set up a separate Python environment for each fix? No, but you definitely want to have a Python environment for your work on code contributions. Making new environments is fast, does not take much space, and provide a way to keep your work organized.
If installation fails, try to upgrade
pip install pip -U
command. It is also a good practice to keep your
conda up-to-date by
conda update conda -n base when prompted to do so; maybe
Test first, please¶
It would be hard to overstate the importance of testing in Astropy. Tests are what gives you confidence that new code does what it should and that it does not break old code.
You should at least run the relevant tests before you make any changes to make sure that your Python environment is set up properly.
The first challenge is figuring out where to look for relevant tests. Issue
1761 is a problem in the
coordinates package, so the tests for
it are in
astropy/coordinates/tests. The rest of
astropy has a similar
layout, as described at Testing Guidelines.
Run the current tests in that directory with:
If the bug you are working on involves remote data access, you need to run
the tests with an extra flag, i.e.,
pytest ... --remote-data.
In the event where all the tests passed with the bug present, new tests are needed to expose this bug.
A subpackage organizes its tests into multiple test modules; e.g.:
$ ls astropy/coordinates/tests test_angles.py test_angular_separation.py test_api_ape5.py test_arrays.py ...
The goal at this point may be a little counter-intuitive: write a test that we
know will fail with the current code. This test allows
astropy to check,
in an automated way, whether our fix actually works and to prevent
regression (i.e., make sure future changes to code do not break our fix).
Looking over the existing code in
test_arrays.py, each test is a function
with a name that starts with
test_. An appropriate place to add the test is
after the last test function in the file.
Give the test a reasonably clear name; e.g.,
easiest way to figure out what you need to import and how to set up the test
is to look at other tests. The full test is in the traceback below and in
pull request 1917.
Write the test, then see if it works as expected; remember, in this case we
expect it to fail without the patch from pull request 1917.
pytest astropy/coordinates/tests/test_arrays.py would give the expected failure;
an excerpt from the output is:
================= FAILURES ============================= ______________ test_array_len __________________________ def test_array_len(): from .. import ICRS input_length = 5 ra = np.linspace(0, 360, input_length) dec = np.linspace(0, 90, input_length) c = ICRS(ra, dec, unit=(u.degree, u.degree)) > assert len(c) == input_length E TypeError: object of type 'ICRS' has no len() astropy/coordinates/tests/test_arrays.py:291: TypeError
Keep git commits small and focused on one logical piece at a time. The test we just wrote is one logical change, so we will commit it. You could, if you prefer, wait and commit this test along with your fix.
For this tutorial, we will commit the test separately. If you are not sure what to do, ask on astropy-dev mailing list.
Check what was changed¶
We can see what has changed with
$ git status On branch fix-1761 Your branch is up-to-date with 'astropy/master'. Changes not staged for commit: (use "git add <file>..." to update what will be committed) (use "git checkout -- <file>..." to discard changes in working directory) modified: astropy/coordinates/tests/test_arrays.py no changes added to commit (use "git add" and/or "git commit -a")
There are two bits of information here:
one file changed; i.e.,
this file has not been added to git’s staging area yet, so it is listed under
Changes not staged for commit.
git diff to see what changes have been made:
$ git diff diff --git a/astropy/coordinates/tests/test_arrays.py b/astropy/coordinates/test index 2785b59..7eecfbb 100644 --- a/astropy/coordinates/tests/test_arrays.py +++ b/astropy/coordinates/tests/test_arrays.py @@ -278,3 +278,14 @@ def test_array_indexing(): assert c2.equinox == c1.equinox assert c3.equinox == c1.equinox assert c4.equinox == c1.equinox + + +def test_array_len(): + from .. import ICRS + + input_length = 5 + ra = np.linspace(0, 360, input_length) + dec = np.linspace(0, 90, input_length) + + c = ICRS(ra, dec, unit=(u.degree, u.degree)) + + assert len(c) == input_length
A graphical interface to git makes keeping track of these sorts of changes even easier; see Get a git GUI (optional) if you are interested.
Stage the change¶
git requires you to add changes in two steps:
stage the change with
git add ...; this adds the file to the list of items that will be added to the repo when you are ready to commit.
commit the change with
git commit ...; this actually adds the changes to your repo.
These can be combined into one step (not recommended); the advantage of doing it in two steps
is that it is easier to undo staging than committing. As we will see later,
git status even tells you how to do it.
Staging can be very handy if you are making changes in a couple of different places that you want to commit at the same time. Make your first changes, stage it, then make your second change and stage that. Once everything is staged, commit the changes as one commit.
In this case, first stage the change:
git add astropy/coordinates/tests/test_arrays.py
You get no notice at the command line that anything has changed, but
git status will let you know:
$ git status On branch fix-1761 Your branch is up-to-date with 'astropy/master'. Changes to be committed: (use "git reset HEAD <file>..." to unstage) modified: astropy/coordinates/tests/test_arrays.py
Note that git helpfully includes the command necessary to unstage the change if you want to.
Commit your change¶
Next, we will commit the test without the fix:
$ git commit -m "Add test for array coordinate length (issue #1761)" [fix-1761 dd4ef8c] Add test for array coordinate length (issue #1761) 1 file changed, 12 insertions(+)
Commit messages should be concise. Including the GitHub issue number allows GitHub to automatically create links to the relevant issue.
git status to get a recap of where we are so far:
$ git status On branch fix-1761 Your branch is ahead of 'astropy/master' by 1 commit. (use "git push" to publish your local commits) nothing to commit, working directory clean
In other words, we have made a change to our local copy of
astropy but we
have not pushed (transferred) that change to our GitHub account.
Fix the issue¶
Write the code¶
Now that we have a test written, we will fix the issue. A full discussion of
the fix is beyond the scope of this tutorial, but the fix is to add a
__len__ method to
coordsystems.py (the code has since been refactored, if you try to look
for it). All of the spherical coordinate systems inherit from
this base class and it is this base class that implements the
__getitem__ method that allows indexing of coordinate arrays.
See pull request 1917 to view the changes to the code.
Test your change¶
There are a few levels at which you want to test:
Does this code change make the test we wrote succeed now? Check by running
pytest astropy/coordinates/tests/test_arrays.py. In this case, yes!
Do the rest of the coordinate tests still pass? Check by running
pytest astropy/coordinates/. In this case, yes, we have not broken anything!
Do all of the astropy tests still succeed? Check by running
pytestfrom the top-level directory. This may take a while depending on the speed of your system. Success again!
Tests that are skipped or xfailed are fine. A fail or an error is not fine. If you get stuck, ask on astropy-dev mailing list for help!
Stage and commit your change¶
Add the file to your git repo in two steps: stage, then commit.
To make this a little different than the commit we did above, make sure you
are still in the top level directory and check the
$ git status On branch fix-1761 Your branch is ahead of 'astropy/master' by 1 commit. (use "git push" to publish your local commits) Changes not staged for commit: (use "git add <file>..." to update what will be committed) (use "git checkout -- <file>..." to discard changes in working directory) modified: astropy/coordinates/coordsystems.py no changes added to commit (use "git add" and/or "git commit -a")
Note that git knows what has changed no matter what directory you are in (as long as you are in one of the directories in the repo, that is).
Stage the change with:
git add astropy/coordinates/coordsystems.py
For this commit, it is helpful to use a multi-line commit message that will automatically close the issue on GitHub when this change is accepted. The snippet below accomplishes that in bash (and similar shells):
$ git commit -m" > Add len() to coordinates > > Closes #1761" [fix-1761 f196771] Add len() to coordinates 1 file changed, 4 insertions(+)
Another option for multi-line commit message is to use a Git GUI or to
git commit without a message to get prompted by an editor.
The message after committing should look like this when you inspect with
Add len() to coordinates Closes #1761
If the commit message does not look right, run
git commit --amend.
If you still run into problems, please ask about fixing it at
astropy-dev mailing list.
At this point, none of the Astropy maintainers know anything about your changes.
We will take care of that in a moment with a “pull request”, but first, see Stop and think: Any more tests or other changes?.
Stop and think: Any more tests or other changes?¶
It never hurts to pause at this point and review whether your proposed changes are complete. In this case, there are some more tests that could be included, such as:
What happens when
len()is called on a coordinate that is not an array?
len()work when the coordinate is an array with one entry?
The second case is easier, so it will be handled first following the development cycle we used above:
Make the change in
Test the change
The test passed; but rather than committing this one change, we will also implement the check for the scalar case.
One could imagine two different desirable outcomes here:
len(scalar_coordinate)behaves just like
len(scalar_angle), raising a
TypeErrorfor a scalar coordinate.
len(scalar_coordinate)returns 1 since there is one coordinate.
If you encounter a case like this and are not sure what to do, ask. The best place to ask is on GitHub on the page for the issue you are fixing.
Alternatively, make a choice and be clear in your pull request on GitHub what you chose and why; instructions for that are below.
Testing for an expected error¶
In this case, we opted for raising a
the user needs to know that the coordinate they created is not going to
behave like an array of one coordinate if they try to index it later on.
The pytest framework makes testing for an exception relatively
easy; you put the code you expect to fail in a
c = ICRS(0, 0, unit=(u.degree, u.degree)) with pytest.raises(TypeError): len(c)
A test like this can be added
In your own work, you may also choose to put that into a new test function,
if you wish.
Aside: Python lesson–let others do your work¶
The actual fix to this issue was very, very short. In
lines were added:
def __len__(self): return len(self.lonangle)
lonangle contains the
Angle``s that represent longitude (sometimes this
is an RA, sometimes a longitude). By simply calling ``len() on one of the
angles in the array you get, for free, whatever behavior has been defined in
Angle class for handling the case of a scalar.
Adding an explicit check for the case of a scalar here would have the very
big downside of having two things that need to be kept in sync: handling of
Angle and in coordinates.
Commit any additional changes¶
Continue to follow the development cycle above for other files that you need to modify, including changelog (see Edit the changelog) and documentation, as needed:
Check that all
astropytests still pass; see Test your change
git statusto see what needs to be staged and committed
git add ...to stage the changes
git commit ...to commit the changes
git logto inspect the change history
The git commands, without their output, are:
git status git add astropy/coordinates/tests/test_arrays.py git commit -m "Add tests of len() for scalar coordinate and length 1 coordinate" git log
Edit the changelog¶
Keeping the list of changes up to date is nearly impossible unless each contributor makes the appropriate updates as they propose changes.
Changes are in the file
CHANGES.rst in the top-level directory (the
setup.py is). Put the change under the list that matches
the milestone (a.k.a. release) that is set by a maintainer for the issue in GitHub. If you are
proposing a new feature in a pull request, you may need to wait on this change
until the pull request is discussed.
This issue was tagged for
astropy 0.3.1, as shown in the image below, so the changelog
entry went there.
The entry in
CHANGES.rst should summarize what you did and include the
pull request number. For writing changelog entries, you do not need to know
much about the markup language being used (though you can read as much as
you want about it at the Sphinx primer); look at other entries and
For this issue, the entry was the line that started with
astropy.coordinates ^^^^^^^^^^^^^^^^^^^ - Implemented ``len()`` for coordinate objects. [#1761]
Starting the line with a
- makes a bulleted list item, making
it a sub-list of
astropy.coordinates and putting
double-backtick makes that text render in a monospaced font.
Commit your changes to the CHANGES.rst¶
You can use
git status as above or jump right to staging and committing:
git add CHANGES.rst git commit -m "Add changelog entry for 1761"
Push your changes to your GitHub fork of astropy¶
Use this command to push your local changes out to your copy of
on GitHub before asking for the changes to be reviewed:
git push your-user-name fix-1761
Propose your changes as a pull request¶
This stage requires going to your GitHub account and navigate to your copy
astropy; the url will be something like
Once there, select the branch that contains your fix from the branches dropdown:
After selecting the correct branch, click on the “Pull Request” button, as shown below:
Name your pull request something sensible. Include the issue number with a
# in the description of the pull request so that a link is
created to the original issue, as stated in
astropy’s pull request template.
Please see pull request 1917 for the pull request showcased in this tutorial.
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.