=============================================
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 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. Python 2.6 added support for user libraries, so if
   you are using an older version this won't work, but Django 1.5 requires
   Python 2.6 or newer anyway.

   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).

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 ``python setup.py install --user`` at the shell prompt.  If you're
   using Windows, start up a command shell and run the command
   ``setup.py install --user``.

   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?)::

    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 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
<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.
