=============================================
Advanced tutorial: How to write reusable apps
=============================================

This advanced tutorial begins where :doc:`Tutorial 7 </intro/tutorial07>`
left off. We'll be turning our Web-poll into a standalone Python package
you can reuse in new projects and share with other people.

If you haven't recently completed Tutorials 1–7, we encourage you to review
these so that your example project matches the one described below.

Reusability matters
===================

It's a lot of work to design, build, test and maintain a web application. Many
Python and Django projects share common problems. Wouldn't it be great if we
could save some of this repeated work?

Reusability is the way of life in Python. `The Python Package Index (PyPI)
<https://pypi.org/>`_ has a vast range of packages you can use in your own
Python programs. Check out `Django Packages <https://djangopackages.org>`_ for
existing reusable apps you could incorporate in your project. Django itself is
also just a Python package. This means that you can take existing Python
packages or Django apps and compose them into your own web project. You only
need to write the parts that make your project unique.

Let's say you were starting a new project that needed a polls app like the one
we've been working on. How do you make this app reusable? Luckily, you're well
on the way already. In :doc:`Tutorial 1 </intro/tutorial01>`, we saw how we
could decouple polls from the project-level URLconf using an ``include``.
In this tutorial, we'll take further steps to make the app easy to use in new
projects and ready to publish for others to install and use.

.. admonition:: Package? App?

    A Python :term:`package` provides a way of grouping related Python code for
    easy reuse. A package contains one or more files of Python code (also known
    as "modules").

    A package can be imported with ``import foo.bar`` or ``from foo import
    bar``. For a directory (like ``polls``) to form a package, it must contain
    a special file ``__init__.py``, even if this file is empty.

    A Django *application* is just a Python package that is specifically
    intended for use in a Django project. An application may use common Django
    conventions, such as having ``models``, ``tests``, ``urls``, and ``views``
    submodules.

    Later on we use the term *packaging* to describe the process of making a
    Python package easy for others to install. It can be a little confusing, we
    know.

Your project and your reusable app
==================================

After the previous tutorials, our project should look like this::

    mysite/
        manage.py
        mysite/
            __init__.py
            settings.py
            urls.py
            wsgi.py
        polls/
            __init__.py
            admin.py
            migrations/
                __init__.py
                0001_initial.py
            models.py
            static/
                polls/
                    images/
                        background.gif
                    style.css
            templates/
                polls/
                    detail.html
                    index.html
                    results.html
            tests.py
            urls.py
            views.py
        templates/
            admin/
                base_site.html

You created ``mysite/templates`` in :doc:`Tutorial 7 </intro/tutorial07>`,
and ``polls/templates`` in :doc:`Tutorial 3 </intro/tutorial03>`. Now perhaps
it is clearer why we chose to have separate template directories for the
project and application: everything that is part of the polls application is in
``polls``. It makes the application self-contained and easier to drop into a
new project.

The ``polls`` directory could now be copied into a new Django project and
immediately reused. It's not quite ready to be published though. For that, we
need to package the app to make it easy for others to install.

.. _installing-reusable-apps-prerequisites:

Installing some prerequisites
=============================

The current state of Python packaging is a bit muddled with various tools. For
this tutorial, we're going to use setuptools_ to build our package. It's the
recommended packaging tool (merged with the ``distribute`` fork). We'll also be
using `pip`_ to install and uninstall it. You should install these
two packages now. If you need help, you can refer to :ref:`how to install
Django with pip<installing-official-release>`. You can install ``setuptools``
the same way.

.. _setuptools: https://pypi.org/project/setuptools/
.. _pip: https://pypi.org/project/pip/

Packaging your app
==================

Python *packaging* refers to preparing your app in a specific format that can
be easily installed and used. Django itself is packaged very much like
this. For a small app like polls, this process isn't too difficult.

#. First, create a parent directory for ``polls``, outside of your Django
   project. Call this directory ``django-polls``.

   .. admonition::  Choosing a name for your app

       When choosing a name for your package, check resources like PyPI to avoid
       naming conflicts with existing packages. It's often useful to prepend
       ``django-`` to your module name when creating a package to distribute.
       This helps others looking for Django apps identify your app as Django
       specific.

       Application labels (that is, the final part of the dotted path to
       application packages) *must* be unique in :setting:`INSTALLED_APPS`.
       Avoid using the same label as any of the Django :doc:`contrib packages
       </ref/contrib/index>`, for example ``auth``, ``admin``, or
       ``messages``.

#. Move the ``polls`` directory into the ``django-polls`` directory.

#. Create a file ``django-polls/README.rst`` with the following contents:

   .. code-block:: rst
       :caption: django-polls/README.rst

       =====
       Polls
       =====

       Polls is a simple Django app to conduct Web-based polls. For each
       question, visitors can choose between a fixed number of answers.

       Detailed documentation is in the "docs" directory.

       Quick start
       -----------

       1. Add "polls" to your INSTALLED_APPS setting like this::

           INSTALLED_APPS = [
               ...
               'polls',
           ]

       2. Include the polls URLconf in your project urls.py like this::

           path('polls/', include('polls.urls')),

       3. Run `python manage.py migrate` to create the polls models.

       4. Start the development server and visit http://127.0.0.1:8000/admin/
          to create a poll (you'll need the Admin app enabled).

       5. Visit http://127.0.0.1:8000/polls/ to participate in the poll.

#. Create a ``django-polls/LICENSE`` file. Choosing a license is beyond the
   scope of this tutorial, but suffice it to say that code released publicly
   without a license is *useless*. Django and many Django-compatible apps are
   distributed under the BSD license; however, you're free to pick your own
   license. Just be aware that your licensing choice will affect who is able
   to use your code.

#. Next we'll create ``setup.cfg`` and ``setup.py`` files which detail how to
   build and install the app. A full explanation of these files is beyond the
   scope of this tutorial, but the `setuptools documentation
   <https://setuptools.readthedocs.io/en/latest/>`_ has a good explanation.
   Create the files ``django-polls/setup.cfg`` and ``django-polls/setup.py``
   with the following contents:

   .. code-block:: ini
        :caption: django-polls/setup.cfg

        [metadata]
        name = django-polls
        version = 0.1
        description = A Django app to conduct Web-based polls.
        long_description = file: README.rst
        url = https://www.example.com/
        author = Your Name
        author_email = yourname@example.com
        license = BSD-3-Clause  # Example license
        classifiers =
            Environment :: Web Environment
            Framework :: Django
            Framework :: Django :: X.Y  # Replace "X.Y" as appropriate
            Intended Audience :: Developers
            License :: OSI Approved :: BSD License
            Operating System :: OS Independent
            Programming Language :: Python
            Programming Language :: Python :: 3
            Programming Language :: Python :: 3 :: Only
            Programming Language :: Python :: 3.6
            Programming Language :: Python :: 3.7
            Programming Language :: Python :: 3.8
            Topic :: Internet :: WWW/HTTP
            Topic :: Internet :: WWW/HTTP :: Dynamic Content

        [options]
        include_package_data = true
        packages = find:

   .. code-block:: python
       :caption: django-polls/setup.py

        from setuptools import setup

        setup()

#. Only Python modules and packages are included in the package by default. To
   include additional files, we'll need to create a ``MANIFEST.in`` file. The
   setuptools docs referred to in the previous step discuss this file in more
   details. To include the templates, the ``README.rst`` and our ``LICENSE``
   file, create a file ``django-polls/MANIFEST.in`` with the following
   contents:

   .. code-block:: text
       :caption: django-polls/MANIFEST.in

       include LICENSE
       include README.rst
       recursive-include polls/static *
       recursive-include polls/templates *

#. It's optional, but recommended, to include detailed documentation with your
   app. Create an empty directory ``django-polls/docs`` for future
   documentation. Add an additional line to ``django-polls/MANIFEST.in``::

    recursive-include docs *

   Note that the ``docs`` directory won't be included in your package unless
   you add some files to it. Many Django apps also provide their documentation
   online through sites like `readthedocs.org <https://readthedocs.org>`_.

#. Try building your package with ``python setup.py sdist`` (run from inside
   ``django-polls``). This creates a directory called ``dist`` and builds your
   new package, ``django-polls-0.1.tar.gz``.

For more information on packaging, see Python's `Tutorial on Packaging and
Distributing Projects
<https://packaging.python.org/tutorials/packaging-projects/>`_.

Using your own package
======================

Since we moved the ``polls`` directory out of the project, it's no longer
working. We'll now fix this by installing our new ``django-polls`` package.

.. admonition:: Installing as a user library

   The following steps install ``django-polls`` as a user library. Per-user
   installs have a lot of advantages over installing the package system-wide,
   such as being usable on systems where you don't have administrator access
   as well as preventing the package from affecting system services and other
   users of the machine.

   Note that per-user installations can still affect the behavior of system
   tools that run as that user, so ``virtualenv`` is a more robust solution
   (see below).

#. To install the package, use pip (you already :ref:`installed it
   <installing-reusable-apps-prerequisites>`, right?)::

    pip install --user django-polls/dist/django-polls-0.1.tar.gz

#. With luck, your Django project should now work correctly again. Run the
   server again to confirm this.

#. To uninstall the package, use pip::

    pip uninstall django-polls

Publishing your app
===================

Now that we've packaged and tested ``django-polls``, it's ready to share with
the world! If this wasn't just an example, you could now:

* Email the package to a friend.

* Upload the package on your website.

* Post the package on a public repository, such as `the Python Package Index
  (PyPI)`_. `packaging.python.org <https://packaging.python.org>`_ has `a good
  tutorial <https://packaging.python.org/tutorials/packaging-projects/#uploading-the-distribution-archives>`_
  for doing this.

Installing Python packages with virtualenv
==========================================

Earlier, we installed the polls app as a user library. This has some
disadvantages:

* Modifying the user libraries can affect other Python software on your system.

* You won't be able to run multiple versions of this package (or others with
  the same name).

Typically, these situations only arise once you're maintaining several Django
projects. When they do, the best solution is to use `virtualenv
<https://virtualenv.pypa.io/>`_. This tool allows you to maintain multiple
isolated Python environments, each with its own copy of the libraries and
package namespace.
