[1.5.X] Fixed #16671 - Added a tutorial on reuseable apps

    Christian Metts

    michal@plovarna.cz

    Slawek Mikula <slawek dot mikula at gmail dot com>

    Katie Miller <katie@sub50.com>

    Shawn Milochik <shawn@milochik.com>

    mitakummaa@gmail.com

    Taylor Mitchell <taylor.mitchell@gmail.com>

    Johan C. Stöver <johan@nilling.nl>

    Nowell Strite <http://nowell.strite.org/>

    Thomas Stromberg <tstromberg@google.com>

    Ben Sturmfels <ben@sturm.com.au>

    Travis Swicegood <travis@domain51.com>

    Pascal Varet

    SuperJared

  :doc:`Part 3 <intro/tutorial03>` |

  :doc:`Part 4 <intro/tutorial04>`

* **Advanced Tutorials:**

  :doc:`How to write reusable apps <intro/reusable-apps>`

The model layer

===============

   tutorial02

   tutorial03

   tutorial04

   reusable-apps

   whatsnext

.. seealso::

=============================================

Advanced tutorial: How to write reusable apps

=============================================

This advanced tutorial begins where :doc:`Tutorial 4 </intro/tutorial04>` 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 1–4, 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)

<http://guide.python-distribute.org/contributing.html#pypi-info>`_ has a vast

range of packages you can use in your own Python programs. Check out `Django

Packages <http://www.djangopackages.com>`_ 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 3 </intro/tutorial03>`, 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 `package <http://docs.python.org/tutorial/modules.html#packages>`_

    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 *app* is just a Python package that is specifically intended for

    use in a Django project. An app may also use common Django conventions,

    such as having a ``models.py`` file.

    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.

Completing 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/

            admin.py

            __init__.py

            models.py

            tests.py

            urls.py

            views.py

You also have a directory somewhere called ``mytemplates`` which you created in

:doc:`Tutorial 2 </intro/tutorial02>`. You specified its location in the

TEMPLATE_DIRS setting. This directory should look like this::

    mytemplates/

        admin/

            base_site.html

        polls/

            detail.html

            index.html

            results.html

The polls app is already a Python package, thanks to the ``polls/__init__.py``

file. That's a great start, but we can't just pick up this package and drop it

into a new project. The polls templates are currently stored in the

project-wide ``mytemplates`` directory. To make the app self-contained, it

should also contain the necessary templates.

Inside the ``polls`` app, create a new ``templates`` directory. Now move the

``polls`` template directory from ``mytemplates`` into the new

``templates``. Your project should now look like this::

    mysite/

        manage.py

        mysite/

            __init__.py

            settings.py

            urls.py

            wsgi.py

        polls/

            admin.py

            __init__.py

            models.py

            templates/

                polls/

                    detail.html

                    index.html

                    results.html

            tests.py

            urls.py

            views.py

Your project-wide templates directory should now look like this::

    mytemplates/

        admin/

            base_site.html

Looking good! Now would be a good time to confirm that your polls application

still works correctly.  How does Django know how to find the new location of

the polls templates even though we didn't modify :setting:`TEMPLATE_DIRS`?

Django has a :setting:`TEMPLATE_LOADERS` setting which contains a list

of callables that know how to import templates from various sources.  One of

the defaults is :class:`django.template.loaders.app_directories.Loader` which

looks for a "templates" subdirectory in each of the :setting:`INSTALLED_APPS`.

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.

.. admonition:: Why nested?

   Why create a ``polls`` directory under ``templates`` when we're

   already inside the polls app? This directory is needed to avoid conflicts in

   Django's ``app_directories`` template loader.  For example, if two

   apps had a template called ``base.html``, without the extra directory it

   wouldn't be possible to distinguish between the two. It's a good convention

   to use the name of your app for this directory.

.. _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 distribute_ to build our package. It's a

community-maintained fork of the older ``setuptools`` project. We'll also be

using `pip`_ to uninstall it after we're finished. 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 ``distribute``

the same way.

.. _distribute: http://pypi.python.org/pypi/distribute

.. _pip: http://pypi.python.org/pypi/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.

1. 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.

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

3. Create a file ``django-polls/README.txt`` with the following contents::

    =====

    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::

          url(r'^polls/', include('polls.urls')),

    3. Run `python manage.py syncdb` 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.

4. 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.

5. Next we'll create a ``setup.py`` file which provides details about how to

build and install the app.  A full explanation of this file is beyond the

scope of this tutorial, but the `distribute docs

<http://packages.python.org/distribute/setuptools.html>`_ have a good explanation.

Create a file ``django-polls/setup.py`` with the following contents::

    import os

    from setuptools import setup

    README = open(os.path.join(os.path.dirname(__file__), 'README.txt')).read()

    # allow setup.py to be run from any path

    os.chdir(os.path.normpath(os.path.join(os.path.abspath(__file__), os.pardir)))

    setup(

        name = 'django-polls',

        version = '0.1',

        packages = ['polls'],

        include_package_data = True,

        license = 'BSD License', # example license

        description = 'A simple Django app to conduct Web-based polls.',

        long_description = README,

        url = 'http://www.example.com/',

        author = 'Your Name',

        author_email = 'yourname@example.com',

        classifiers = [

            'Environment :: Web Environment',

            'Framework :: Django',

            'Intended Audience :: Developers',

            'License :: OSI Approved :: BSD License', # example license

            'Operating System :: OS Independent',

            'Programming Language :: Python',

            'Programming Language :: Python :: 2.6',

            'Programming Language :: Python :: 2.7',

            'Topic :: Internet :: WWW/HTTP',

            'Topic :: Internet :: WWW/HTTP :: Dynamic Content',

        ],

    )

.. admonition:: I thought you said we were going to use ``distribute``?

    Distribute is a drop-in replacement for ``setuptools``. Even though we

    appear to import from ``setuptools``, since we have ``distribute``

    installed, it will override the import.

6. 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

   distribute docs referred to in the previous step discuss this file in more

   details. To include the templates and our LICENSE file, create a file

   ``django-polls/MANIFEST.in`` with the following contents::

    include LICENSE

    recursive-include polls/templates *

7. 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 <http://readthedocs.org>`_.

8. 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 `The Hitchhiker's Guide to Packaging

<http://guide.python-distribute.org/quickstart.html>`_.

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 system library

   The following steps install ``django-polls`` as a system library. In

   general, it's best to avoid messing with your system libraries to avoid

   breaking things. For this simple example though, the risk is low and it will

   help with understanding packaging. We'll explain how to uninstall in

   step 4.

   For experienced users, a neater way to manage your packages is to use

   "virtualenv" (see below).

1. Inside ``django-polls/dist``, untar the new package

   ``django-polls-0.1.tar.gz`` (e.g. ``tar xzvf django-polls-0.1.tar.gz``). If

   you're using Windows, you can download the command-line tool bsdtar_ to do

   this, or you can use a GUI-based tool such as 7-zip_.

2. Change into the directory created in step 1 (e.g. ``cd django-polls-0.1``).

3. If you're using GNU/Linux, Mac OS X or some other flavor of Unix, enter the

   command ``sudo python setup.py install`` at the shell prompt.  If you're

   using Windows, start up a command shell with administrator privileges and

   run the command ``setup.py install``.

   With luck, your Django project should now work correctly again. Run the

   server again to confirm this.

4. To uninstall the package, use pip (you already :ref:`installed it

   <installing-reusable-apps-prerequisites>`, right?)::

    sudo pip uninstall django-polls

.. _bsdtar: http://gnuwin32.sourceforge.net/packages/bsdtar.htm

.. _7-zip: http://www.7-zip.org/

.. _pip: http://pypi.python.org/pypi/pip

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 Web site.

* Post the package on a public repository, such as `The Python Package Index

  (PyPI) <http://guide.python-distribute.org/contributing.html#pypi-info>`_.

For more information on PyPI, see the `Quickstart

<http://guide.python-distribute.org/quickstart.html#register-your-package-with-the-python-package-index-pypi>`_

section of The Hitchhiker's Guide to Packaging. One detail this guide mentions

is choosing the license under which your code is distributed.

Installing Python packages with virtualenv

==========================================

Earlier, we installed the polls app as a system library. This has some

disadvantages:

* Modifying the system 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

<http://www.virtualenv.org/>`_. This tool allows you to maintain multiple

isolated Python environments, each with its own copy of the libraries and

package namespace.

containing the "What's up" poll from Tutorial 1. The link points to the poll's

detail page.

.. admonition:: Organizing Templates

    Rather than one big templates directory, you can also store templates

    within each app. We'll discuss this in more detail in the :doc:`reusable

    apps tutorial</intro/reusable-apps>`.

A shortcut: :func:`~django.shortcuts.render`

--------------------------------------------