Writing your first patch for Django
Interested in giving back to the community a little? Maybe you’ve found a bug in Django that you’d like to see fixed, or maybe there’s a small feature you want added.
Contributing back to Django itself is the best way to see your own concerns addressed. This may seem daunting at first, but it’s a well-traveled path with documentation, tooling, and a community to support you. We’ll walk you through the entire process, so you can learn by example.
Who’s this tutorial for?
If you are looking for a reference on how to submit patches, see the Submitting patches documentation.
For this tutorial, we expect that you have at least a basic understanding of how Django works. This means you should be comfortable going through the existing tutorials on writing your first Django app. In addition, you should have a good understanding of Python itself. But if you don’t, Dive Into Python is a fantastic (and free) online book for beginning Python programmers.
Those of you who are unfamiliar with version control systems and Trac will find that this tutorial and its links include just enough information to get started. However, you’ll probably want to read some more about these different tools if you plan on contributing to Django regularly.
For the most part though, this tutorial tries to explain as much as possible, so that it can be of use to the widest audience.
Where to get help:
What does this tutorial cover?
We’ll be walking you through contributing a patch to Django for the first time. By the end of this tutorial, you should have a basic understanding of both the tools and the processes involved. Specifically, we’ll be covering the following:
- Installing Git.
- Downloading a copy of Django’s development version.
- Running Django’s test suite.
- Writing a test for your patch.
- Writing the code for your patch.
- Testing your patch.
- Submitting a pull request.
- Where to look for more information.
Once you’re done with the tutorial, you can look through the rest of Django’s documentation on contributing. It contains lots of great information and is a must read for anyone who’d like to become a regular contributor to Django. If you’ve got questions, it’s probably got the answers.
Python 3 required!
For Windows users
Code of Conduct
For this tutorial, you’ll need Git installed to download the current development version of Django and to generate patch files for the changes you make.
To check whether or not you have Git installed, enter
git into the command line. If you get messages saying that this command could not be found, you’ll have to download and install it, see Git’s download page.
If you’re not that familiar with Git, you can always find out more about its commands (once it’s installed) by typing
git help into the command line.
Getting a copy of Django’s development version
The first step to contributing to Django is to get a copy of the source code. First, fork Django on GitHub. Then, from the command line, use the
cd command to navigate to the directory where you’ll want your local copy of Django to live.
Download the Django source code repository using the following command:
$ git clone https://github.com/YourGitHubName/django.git
Low bandwidth connection?
You can add the
--depth 1 argument to
git clone to skip downloading all of Django’s commit history, which reduces data transfer from ~250 MB to ~70 MB.
Now that you have a local copy of Django, you can install it just like you would install any package using
pip. The most convenient way to do so is by using a virtual environment, which is a feature built into Python that allows you to keep a separate directory of installed packages for each of your projects so that they don’t interfere with each other.
It’s a good idea to keep all your virtual environments in one place, for example in
.virtualenvs/ in your home directory.
Create a new virtual environment by running:
$ python3 -m venv ~/.virtualenvs/djangodev
The path is where the new environment will be saved on your computer.
The final step in setting up your virtual environment is to activate it:
$ source ~/.virtualenvs/djangodev/bin/activate
source command is not available, you can try using a dot instead:
$ . ~/.virtualenvs/djangodev/bin/activate
You have to activate the virtual environment whenever you open a new terminal window. virtualenvwrapper is a useful tool for making this more convenient.
For Windows users
To activate your virtual environment on Windows, run:
or you can install a Windows version of virtualenvwrapper and then use:
...\> workon djangodev
The name of the currently activated virtual environment is displayed on the command line to help you keep track of which one you are using. Anything you install through
pip while this name is displayed will be installed in that virtual environment, isolated from other environments and system-wide packages.
Go ahead and install the previously cloned copy of Django:
$ python -m pip install -e /path/to/your/local/clone/django/
The installed version of Django is now pointing at your local copy by installing in editable mode. You will immediately see any changes you make to it, which is of great help when writing your first patch.
It may be helpful to test your local changes with a Django project. First you have to create a new virtual environment, install the previously cloned local copy of Django in editable mode, and create a new Django project outside of your local copy of Django. You will immediately see any changes you make to Django in your new project, which is of great help when writing your first patch.
Running Django’s test suite for the first time
When contributing to Django it’s very important that your code changes don’t introduce bugs into other areas of Django. One way to check that Django still works after you make your changes is by running Django’s test suite. If all the tests still pass, then you can be reasonably sure that your changes work and haven’t broken other parts Django. If you’ve never run Django’s test suite before, it’s a good idea to run it once beforehand to get familiar with its output.
Before running the test suite, install its dependencies by
cd-ing into the Django
tests/ directory and then running:
$ python -m pip install -r requirements/py3.txt
If you encounter an error during the installation, your system might be missing a dependency for one or more of the Python packages. Consult the failing package’s documentation or search the Web with the error message that you encounter.
Now we are ready to run the test suite. If you’re using GNU/Linux, macOS, or some other flavor of Unix, run:
Now sit back and relax. Django’s entire test suite has thousands of tests, and it takes at least a few minutes run, depending on the speed of your computer.
While Django’s test suite is running, you’ll see a stream of characters representing the status of each test as it completes.
E indicates that an error was raised during a test, and
F indicates that a test’s assertions failed. Both of these are considered to be test failures. Meanwhile,
s indicated expected failures and skipped tests, respectively. Dots indicate passing tests.
Skipped tests are typically due to missing external libraries required to run the test; see Running all the tests for a list of dependencies and be sure to install any for tests related to the changes you are making (we won’t need any for this tutorial). Some tests are specific to a particular database backend and will be skipped if not testing with that backend. SQLite is the database backend for the default settings. To run the tests using a different backend, see Using another settings module.
Once the tests complete, you should be greeted with a message informing you whether the test suite passed or failed. Since you haven’t yet made any changes to Django’s code, the entire test suite should pass. If you get failures or errors make sure you’ve followed all of the previous steps properly. See Running the unit tests for more information.
Note that the latest Django master may not always be stable. When developing against master, you can check Django’s continuous integration builds to determine if the failures are specific to your machine or if they are also present in Django’s official builds. If you click to view a particular build, you can view the “Configuration Matrix” which shows failures broken down by Python version and database backend.
For this tutorial and the ticket we’re working on, testing against SQLite is sufficient, however, it’s possible (and sometimes necessary) to run the tests using a different database.
Working on a feature
For this tutorial, we’ll work on a “fake ticket” as a case study. Here are the imaginary details:
Ticket #99999 – Allow making toast
We’ll now implement this feature and associated tests.
Creating a branch for your patch
Before making any changes, create a new branch for the ticket:
$ git checkout -b ticket_99999
You can choose any name that you want for the branch, “ticket_99999” is an example. All changes made in this branch will be specific to the ticket and won’t affect the main copy of the code that we cloned earlier.
Writing some tests for your ticket
In most cases, for a patch to be accepted into Django it has to include tests. For bug fix patches, this means writing a regression test to ensure that the bug is never reintroduced into Django later on. A regression test should be written in such a way that it will fail while the bug still exists and pass once the bug has been fixed. For patches containing new features, you’ll need to include tests which ensure that the new features are working correctly. They too should fail when the new feature is not present, and then pass once it has been implemented.
A good way to do this is to write your new tests first, before making any changes to the code. This style of development is called test-driven development and can be applied to both entire projects and single patches. After writing your tests, you then run them to make sure that they do indeed fail (since you haven’t fixed that bug or added that feature yet). If your new tests don’t fail, you’ll need to fix them so that they do. After all, a regression test that passes regardless of whether a bug is present is not very helpful at preventing that bug from reoccurring down the road.
Now for our hands-on example.
Writing a test for ticket #99999
In order to resolve this ticket, we’ll add a
make_toast() function to the top-level
django module. First we are going to write a test that tries to use the function and check that its output looks correct.
Navigate to Django’s
tests/shortcuts/ folder and create a new file
test_make_toast.py. Add the following code:
from django.shortcuts import make_toast from django.test import SimpleTestCase class MakeToastTests(SimpleTestCase): def test_make_toast(self): self.assertEqual(make_toast(), 'toast')
This test checks that the
But this testing thing looks kinda hard…
If you’ve never had to deal with tests before, they can look a little hard to write at first glance. Fortunately, testing is a very big subject in computer programming, so there’s lots of information out there:
- A good first look at writing tests for Django can be found in the documentation on Writing and running tests.
- Dive Into Python (a free online book for beginning Python developers) includes a great introduction to Unit Testing.
After reading those, if you want something a little meatier to sink your teeth into, there’s always the Python
Running your new test
Since we haven’t made any modifications to
django.shortcuts yet, our test should fail. Let’s run all the tests in the
shortcuts folder to make sure that’s really what happens.
cd to the Django
tests/ directory and run:
$ ./runtests.py shortcuts
If the tests ran correctly, you should see one failure corresponding to the test method we added, with this error:
ImportError: cannot import name 'make_toast' from 'django.shortcuts'
If all of the tests passed, then you’ll want to make sure that you added the new test shown above to the appropriate folder and file name.
Writing the code for your ticket
Next we’ll be adding the
Navigate to the
django/ folder and open the
shortcuts.py file. At the bottom, add:
def make_toast(): return 'toast'
Now we need to make sure that the test we wrote earlier passes, so we can see whether the code we added is working correctly. Again, navigate to the Django
tests/ directory and run:
$ ./runtests.py shortcuts
Running Django’s test suite for the second time
Once you’ve verified that your patch and your test are working correctly, it’s a good idea to run the entire Django test suite to verify that your change hasn’t introduced any bugs into other areas of Django. While successfully passing the entire test suite doesn’t guarantee your code is bug free, it does help identify many bugs and regressions that might otherwise go unnoticed.
This is a new feature, so it should be documented. Open the file
docs/topics/http/shortcuts.txt and add the following at the end of the file:
``make_toast()`` ================ .. versionadded:: 2.2 Returns ``'toast'``.
Since this new feature will be in an upcoming release it is also added to the release notes for the next version of Django. Open the release notes for the latest version in
docs/releases/, which at time of writing is
2.2.txt. Add a note under the “Minor Features” header:
:mod:`django.shortcuts` ~ + +* The new :func:`django.shortcuts.make_toast` function returns ``'toast'``. + :mod:`django.contrib.admin`