===================================
Writing your first patch for Django
===================================

Introduction
============

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 really pretty simple.
We'll walk you through the entire process, so you can learn by example.

Who's this tutorial for?
------------------------

.. seealso::

    If you are looking for a reference on how to submit patches, see the
    :doc:`/internals/contributing/writing-code/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 :doc:`writing your first Django app</intro/tutorial01>`.
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.

.. admonition:: Where to get help:

    If you're having trouble going through this tutorial, please post a message
    to |django-developers| or drop by `#django-dev on irc.freenode.net`__ to
    chat with other Django users who might be able to help.

__ http://www.diveintopython3.net/
__ irc://irc.freenode.net/django-dev

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.
* How to download a development copy of Django.
* Running Django's test suite.
* Writing a test for your patch.
* Writing the code for your patch.
* Testing your patch.
* Generating a patch file for your changes.
* Where to look for more information.

Once you're done with the tutorial, you can look through the rest of
:doc:`Django's documentation on contributing</internals/contributing/index>`.
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.

.. admonition:: Python 3 required!

    This tutorial assumes you are using Python 3. Get the latest version at
    `Python's download page <https://www.python.org/download/>`_ or with your
    operating system's package manager.

.. admonition:: For Windows users

    When installing Python on Windows, make sure you check the option "Add
    python.exe to Path", so that it is always available on the command line.

Code of Conduct
===============

As a contributor, you can help us keep the Django community open and inclusive.
Please read and follow our `Code of Conduct <https://www.djangoproject.com/conduct/>`_.

Installing Git
==============

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`__.

.. admonition:: For Windows users

    When installing Git on Windows, it is recommended that you pick the
    "Git Bash" option so that Git runs in its own shell. This tutorial assumes
    that's how you have installed it.

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.

__ http://git-scm.com/download

Getting a copy of Django's development version
==============================================

The first step to contributing to Django is to get a copy of the source code.
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:

.. code-block:: console

    $ git clone https://github.com/django/django.git

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* (or virtualenv) 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 virtualenvs in one place, for example in
``.virtualenvs/`` in your home directory. Create it if it doesn't exist yet:

.. code-block:: console

    $ mkdir ~/.virtualenvs

Now create a new virtualenv by running:

.. code-block:: console

    $ python3 -m venv ~/.virtualenvs/djangodev

The path is where the new environment will be saved on your computer.

.. admonition:: For Windows users

    Using the built-in ``venv`` module will not work if you are also using the
    Git Bash shell on Windows, since activation scripts are only created for the
    system shell (``.bat``) and PowerShell (``.ps1``). Use the ``virtualenv``
    package instead:

    .. code-block:: none

        $ pip install virtualenv
        $ virtualenv ~/.virtualenvs/djangodev

.. admonition:: For Ubuntu users

    On some versions of Ubuntu the above command might fail. Use the
    ``virtualenv`` package instead, first making sure you have ``pip3``:

    .. code-block:: console

        $ sudo apt-get install python3-pip
        $ # Prefix the next command with sudo if it gives a permission denied error
        $ pip3 install virtualenv
        $ virtualenv --python=`which python3` ~/.virtualenvs/djangodev

The final step in setting up your virtualenv is to activate it:

.. code-block:: console

    $ source ~/.virtualenvs/djangodev/bin/activate

If the ``source`` command is not available, you can try using a dot instead:

.. code-block:: console

    $ . ~/.virtualenvs/djangodev/bin/activate

.. admonition:: For Windows users

    To activate your virtualenv on Windows, run:

    .. code-block:: none

        $ source ~/virtualenvs/djangodev/Scripts/activate

You have to activate the virtualenv whenever you open a new terminal window.
virtualenvwrapper__ is a useful tool for making this more convenient.

__ https://virtualenvwrapper.readthedocs.io/en/latest/

Anything you install through ``pip`` from now on will be installed in your new
virtualenv, isolated from other environments and system-wide packages. Also, the
name of the currently activated virtualenv is displayed on the command line to
help you keep track of which one you are using. Go ahead and install the
previously cloned copy of Django:

.. code-block:: console

    $ pip install -e /path/to/your/local/clone/django/

The installed version of Django is now pointing at your local copy. You will
immediately see any changes you make to it, which is of great help when writing
your first patch.

Rolling back to a previous revision of Django
=============================================

For this tutorial, we'll be using ticket :ticket:`24788` as a case study, so
we'll rewind Django's version history in git to before that ticket's patch was
applied. This will allow us to go through all of the steps involved in writing
that patch from scratch, including running Django's test suite.

**Keep in mind that while we'll be using an older revision of Django's trunk
for the purposes of the tutorial below, you should always use the current
development revision of Django when working on your own patch for a ticket!**

.. note::

    The patch for this ticket was written by Paweł Marczewski, and it was
    applied to Django as `commit 4df7e8483b2679fc1cba3410f08960bac6f51115`__.
    Consequently, we'll be using the revision of Django just prior to that,
    `commit 4ccfc4439a7add24f8db4ef3960d02ef8ae09887`__.

__ https://github.com/django/django/commit/4df7e8483b2679fc1cba3410f08960bac6f51115
__ https://github.com/django/django/commit/4ccfc4439a7add24f8db4ef3960d02ef8ae09887

Navigate into Django's root directory (that's the one that contains ``django``,
``docs``, ``tests``, ``AUTHORS``, etc.). You can then check out the older
revision of Django that we'll be using in the tutorial below:

.. code-block:: console

    $ git checkout 4ccfc4439a7add24f8db4ef3960d02ef8ae09887

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
haven't completely broken Django. If you've never run Django's test suite
before, it's a good idea to run it once beforehand just to get familiar with
what its output is supposed to look like.

Before running the test suite, install its dependencies by first ``cd``-ing
into the Django ``tests/`` directory and then running:

.. code-block:: console

    $ pip install -r requirements/py3.txt

Now we are ready to run the test suite. If you're using GNU/Linux, Mac OS X or
some other flavor of Unix, run:

.. code-block:: console

    $ ./runtests.py

Now sit back and relax. Django's entire test suite has over 9,600 different
tests, so it can take anywhere from 5 to 15 minutes to 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's run. ``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, ``x`` and ``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 :ref:`running-unit-tests-dependencies` 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).

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
:ref:`running-unit-tests` for more information. If you're using Python 3.5+,
there will be a couple failures related to deprecation warnings that you can
ignore. These failures have since been fixed in Django.

Note that the latest Django trunk may not always be stable. When developing
against trunk, 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.

__ http://djangoci.com

.. note::

    For this tutorial and the ticket we're working on, testing against SQLite
    is sufficient, however, it's possible (and sometimes necessary) to
    :ref:`run the tests using a different database
    <running-unit-tests-settings>`.

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.

__ https://en.wikipedia.org/wiki/Test-driven_development

Writing some tests for ticket #24788
------------------------------------

Ticket :ticket:`24788` proposes a small feature addition: the ability to
specify the class level attribute ``prefix`` on Form classes, so that::

    […] forms which ship with apps could effectively namespace themselves such
    that N overlapping form fields could be POSTed at once and resolved to the
    correct form.

In order to resolve this ticket, we'll add a ``prefix`` attribute to the
``BaseForm`` class. When creating instances of this class, passing a prefix to
the ``__init__()`` method will still set that prefix on the created instance.
But not passing a prefix (or passing ``None``) will use the class-level prefix.
Before we make those changes though, we're going to write a couple tests to
verify that our modification functions correctly and continues to function
correctly in the future.

Navigate to Django's ``tests/forms_tests/tests/`` folder and open the
``test_forms.py`` file. Add the following code on line 1674 right before the
``test_forms_with_null_boolean`` function::

    def test_class_prefix(self):
        # Prefix can be also specified at the class level.
        class Person(Form):
            first_name = CharField()
            prefix = 'foo'

        p = Person()
        self.assertEqual(p.prefix, 'foo')

        p = Person(prefix='bar')
        self.assertEqual(p.prefix, 'bar')

This new test checks that setting a class level prefix works as expected, and
that passing a ``prefix`` parameter when creating an instance still works too.

.. admonition:: 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 :doc:`/topics/testing/overview`.
    * 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 :mod:`unittest` documentation.

__ http://www.diveintopython.net/unit_testing/index.html

Running your new test
---------------------

Remember that we haven't actually made any modifications to ``BaseForm`` yet,
so our tests are going to fail. Let's run all the tests in the ``forms_tests``
folder to make sure that's really what happens. From the command line, ``cd``
into the Django ``tests/`` directory and run:

.. code-block:: console

    $ ./runtests.py forms_tests

If the tests ran correctly, you should see one failure corresponding to the test
method we added. 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 class.

Writing the code for your ticket
================================

Next we'll be adding the functionality described in ticket :ticket:`24788` to
Django.

Writing the code for ticket #24788
----------------------------------

Navigate to the ``django/django/forms/`` folder and open the ``forms.py`` file.
Find the ``BaseForm`` class on line 72 and add the ``prefix`` class attribute
right after the ``field_order`` attribute::

    class BaseForm(object):
        # This is the main implementation of all the Form logic. Note that this
        # class is different than Form. See the comments by the Form class for
        # more information. Any improvements to the form API should be made to
        # *this* class, not to the Form class.
        field_order = None
        prefix = None

Verifying your test now passes
------------------------------

Once you're done modifying Django, we need to make sure that the tests we wrote
earlier pass, so we can see whether the code we wrote above is working
correctly. To run the tests in the ``forms_tests`` folder, ``cd`` into the
Django ``tests/`` directory and run:

.. code-block:: console

    $ ./runtests.py forms_tests

Oops, good thing we wrote those tests! You should still see one failure with
the following exception::

    AssertionError: None != 'foo'

We forgot to add the conditional statement in the ``__init__`` method. Go ahead
and change ``self.prefix = prefix`` that is now on line 87 of
``django/forms/forms.py``, adding a conditional statement::

    if prefix is not None:
        self.prefix = prefix

Re-run the tests and everything should pass. If it doesn't, make sure you
correctly modified the ``BaseForm`` class as shown above and copied the new test
correctly.

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 just 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.

To run the entire Django test suite, ``cd`` into the Django ``tests/``
directory and run:

.. code-block:: console

    $ ./runtests.py

As long as you don't see any failures, you're good to go.

Writing Documentation
=====================

This is a new feature, so it should be documented. Add the following section on
line 1068 (at the end of the file) of ``django/docs/ref/forms/api.txt``::

    The prefix can also be specified on the form class::

        >>> class PersonForm(forms.Form):
        ...     ...
        ...     prefix = 'person'

    .. versionadded:: 1.9

        The ability to specify ``prefix`` on the form class was added.

Since this new feature will be in an upcoming release it is also added to the
release notes for Django 1.9, on line 164 under the "Forms" section in the file
``docs/releases/1.9.txt``::

    * A form prefix can be specified inside a form class, not only when
      instantiating a form. See :ref:`form-prefix` for details.

For more information on writing documentation, including an explanation of what
the ``versionadded`` bit is all about, see
:doc:`/internals/contributing/writing-documentation`. That page also includes
an explanation of how to build a copy of the documentation locally, so you can
preview the HTML that will be generated.

Generating a patch for your changes
===================================

Now it's time to generate a patch file that can be uploaded to Trac or applied
to another copy of Django. To get a look at the content of your patch, run the
following command:

.. code-block:: console

    $ git diff

This will display the differences between your current copy of Django (with
your changes) and the revision that you initially checked out earlier in the
tutorial.

Once you're done looking at the patch, hit the ``q`` key to exit back to the
command line. If the patch's content looked okay, you can run the following
command to save the patch file to your current working directory:

.. code-block:: console

    $ git diff > 24788.diff

You should now have a file in the root Django directory called ``24788.diff``.
This patch file contains all your changes and should look this:

.. code-block:: diff

    diff --git a/django/forms/forms.py b/django/forms/forms.py
    index 509709f..d1370de 100644
    --- a/django/forms/forms.py
    +++ b/django/forms/forms.py
    @@ -75,6 +75,7 @@ class BaseForm(object):
         # information. Any improvements to the form API should be made to *this*
         # class, not to the Form class.
         field_order = None
    +    prefix = None

         def __init__(self, data=None, files=None, auto_id='id_%s', prefix=None,
                      initial=None, error_class=ErrorList, label_suffix=None,
    @@ -83,7 +84,8 @@ class BaseForm(object):
             self.data = data or {}
             self.files = files or {}
             self.auto_id = auto_id
    -        self.prefix = prefix
    +        if prefix is not None:
    +            self.prefix = prefix
             self.initial = initial or {}
             self.error_class = error_class
             # Translators: This is the default suffix added to form field labels
    diff --git a/docs/ref/forms/api.txt b/docs/ref/forms/api.txt
    index 3bc39cd..008170d 100644
    --- a/docs/ref/forms/api.txt
    +++ b/docs/ref/forms/api.txt
    @@ -1065,3 +1065,13 @@ You can put several Django forms inside one ``<form>`` tag. To give each
         >>> print(father.as_ul())
         <li><label for="id_father-first_name">First name:</label> <input type="text" name="father-first_name" id="id_father-first_name" /></li>
         <li><label for="id_father-last_name">Last name:</label> <input type="text" name="father-last_name" id="id_father-last_name" /></li>
    +
    +The prefix can also be specified on the form class::
    +
    +    >>> class PersonForm(forms.Form):
    +    ...     ...
    +    ...     prefix = 'person'
    +
    +.. versionadded:: 1.9
    +
    +    The ability to specify ``prefix`` on the form class was added.
    diff --git a/docs/releases/1.9.txt b/docs/releases/1.9.txt
    index 5b58f79..f9bb9de 100644
    --- a/docs/releases/1.9.txt
    +++ b/docs/releases/1.9.txt
    @@ -161,6 +161,9 @@ Forms
       :attr:`~django.forms.Form.field_order` attribute, the ``field_order``
       constructor argument , or the :meth:`~django.forms.Form.order_fields` method.

    +* A form prefix can be specified inside a form class, not only when
    +  instantiating a form. See :ref:`form-prefix` for details.
    +
     Generic Views
     ^^^^^^^^^^^^^

    diff --git a/tests/forms_tests/tests/test_forms.py b/tests/forms_tests/tests/test_forms.py
    index 690f205..e07fae2 100644
    --- a/tests/forms_tests/tests/test_forms.py
    +++ b/tests/forms_tests/tests/test_forms.py
    @@ -1671,6 +1671,18 @@ class FormsTestCase(SimpleTestCase):
             self.assertEqual(p.cleaned_data['last_name'], 'Lennon')
             self.assertEqual(p.cleaned_data['birthday'], datetime.date(1940, 10, 9))

    +    def test_class_prefix(self):
    +        # Prefix can be also specified at the class level.
    +        class Person(Form):
    +            first_name = CharField()
    +            prefix = 'foo'
    +
    +        p = Person()
    +        self.assertEqual(p.prefix, 'foo')
    +
    +        p = Person(prefix='bar')
    +        self.assertEqual(p.prefix, 'bar')
    +
         def test_forms_with_null_boolean(self):
             # NullBooleanField is a bit of a special case because its presentation (widget)
             # is different than its data. This is handled transparently, though.

So what do I do next?
=====================

Congratulations, you've generated your very first Django patch! Now that you've
got that under your belt, you can put those skills to good use by helping to
improve Django's codebase. Generating patches and attaching them to Trac
tickets is useful, however, since we are using git - adopting a more :doc:`git
oriented workflow </internals/contributing/writing-code/working-with-git>` is
recommended.

Since we never committed our changes locally, perform the following to get your
git branch back to a good starting point:

.. code-block:: console

    $ git reset --hard HEAD
    $ git checkout master

More information for new contributors
-------------------------------------

Before you get too into writing patches for Django, there's a little more
information on contributing that you should probably take a look at:

* You should make sure to read Django's documentation on
  :doc:`claiming tickets and submitting patches
  </internals/contributing/writing-code/submitting-patches>`.
  It covers Trac etiquette, how to claim tickets for yourself, expected
  coding style for patches, and many other important details.
* First time contributors should also read Django's :doc:`documentation
  for first time contributors</internals/contributing/new-contributors/>`.
  It has lots of good advice for those of us who are new to helping out
  with Django.
* After those, if you're still hungry for more information about
  contributing, you can always browse through the rest of
  :doc:`Django's documentation on contributing</internals/contributing/index>`.
  It contains a ton of useful information and should be your first source
  for answering any questions you might have.

Finding your first real ticket
------------------------------

Once you've looked through some of that information, you'll be ready to go out
and find a ticket of your own to write a patch for. Pay special attention to
tickets with the "easy pickings" criterion. These tickets are often much
simpler in nature and are great for first time contributors. Once you're
familiar with contributing to Django, you can move on to writing patches for
more difficult and complicated tickets.

If you just want to get started already (and nobody would blame you!), try
taking a look at the list of `easy tickets that need patches`__ and the
`easy tickets that have patches which need improvement`__. If you're familiar
with writing tests, you can also look at the list of
`easy tickets that need tests`__. Just remember to follow the guidelines about
claiming tickets that were mentioned in the link to Django's documentation on
:doc:`claiming tickets and submitting patches
</internals/contributing/writing-code/submitting-patches>`.

__ https://code.djangoproject.com/query?status=new&status=reopened&has_patch=0&easy=1&col=id&col=summary&col=status&col=owner&col=type&col=milestone&order=priority
__ https://code.djangoproject.com/query?status=new&status=reopened&needs_better_patch=1&easy=1&col=id&col=summary&col=status&col=owner&col=type&col=milestone&order=priority
__ https://code.djangoproject.com/query?status=new&status=reopened&needs_tests=1&easy=1&col=id&col=summary&col=status&col=owner&col=type&col=milestone&order=priority

What's next?
------------

After a ticket has a patch, it needs to be reviewed by a second set of eyes.
After uploading a patch or submitting a pull request, be sure to update the
ticket metadata by setting the flags on the ticket to say "has patch",
"doesn't need tests", etc, so others can find it for review. Contributing
doesn't necessarily always mean writing a patch from scratch. Reviewing
existing patches is also a very helpful contribution. See
:doc:`/internals/contributing/triaging-tickets` for details.
