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326 lines
13 KiB
Text
=============================================
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Advanced tutorial: How to write reusable apps
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=============================================
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This advanced tutorial begins where :doc:`Tutorial 7 </intro/tutorial07>`
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left off. We'll be turning our Web-poll into a standalone Python package
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you can reuse in new projects and share with other people.
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If you haven't recently completed Tutorials 1–7, we encourage you to review
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these so that your example project matches the one described below.
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Reusability matters
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===================
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It's a lot of work to design, build, test and maintain a web application. Many
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Python and Django projects share common problems. Wouldn't it be great if we
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could save some of this repeated work?
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Reusability is the way of life in Python. `The Python Package Index (PyPI)
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<https://pypi.python.org/pypi>`_ has a vast range of packages you can use in
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your own Python programs. Check out `Django Packages
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<https://www.djangopackages.com>`_ for existing reusable apps you could
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incorporate in your project. Django itself is also just a Python package. This
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means that you can take existing Python packages or Django apps and compose
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them into your own web project. You only need to write the parts that make
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your project unique.
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Let's say you were starting a new project that needed a polls app like the one
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we've been working on. How do you make this app reusable? Luckily, you're well
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on the way already. In :doc:`Tutorial 3 </intro/tutorial03>`, we saw how we
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could decouple polls from the project-level URLconf using an ``include``.
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In this tutorial, we'll take further steps to make the app easy to use in new
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projects and ready to publish for others to install and use.
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.. admonition:: Package? App?
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A Python :term:`package` provides a way of grouping related Python code for
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easy reuse. A package contains one or more files of Python code (also known
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as "modules").
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A package can be imported with ``import foo.bar`` or ``from foo import
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bar``. For a directory (like ``polls``) to form a package, it must contain
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a special file ``__init__.py``, even if this file is empty.
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A Django *application* is just a Python package that is specifically
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intended for use in a Django project. An application may use common Django
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conventions, such as having ``models``, ``tests``, ``urls``, and ``views``
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submodules.
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Later on we use the term *packaging* to describe the process of making a
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Python package easy for others to install. It can be a little confusing, we
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know.
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Your project and your reusable app
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==================================
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After the previous tutorials, our project should look like this::
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mysite/
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manage.py
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mysite/
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__init__.py
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settings.py
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urls.py
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wsgi.py
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polls/
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__init__.py
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admin.py
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migrations/
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__init__.py
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0001_initial.py
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models.py
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static/
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polls/
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images/
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background.gif
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style.css
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templates/
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polls/
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detail.html
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index.html
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results.html
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tests.py
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urls.py
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views.py
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templates/
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admin/
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base_site.html
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You created ``mysite/templates`` in :doc:`Tutorial 7 </intro/tutorial07>`,
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and ``polls/templates`` in :doc:`Tutorial 3 </intro/tutorial03>`. Now perhaps
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it is clearer why we chose to have separate template directories for the
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project and application: everything that is part of the polls application is in
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``polls``. It makes the application self-contained and easier to drop into a
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new project.
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The ``polls`` directory could now be copied into a new Django project and
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immediately reused. It's not quite ready to be published though. For that, we
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need to package the app to make it easy for others to install.
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.. _installing-reusable-apps-prerequisites:
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Installing some prerequisites
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=============================
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The current state of Python packaging is a bit muddled with various tools. For
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this tutorial, we're going to use setuptools_ to build our package. It's the
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recommended packaging tool (merged with the ``distribute`` fork). We'll also be
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using `pip`_ to install and uninstall it. You should install these
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two packages now. If you need help, you can refer to :ref:`how to install
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Django with pip<installing-official-release>`. You can install ``setuptools``
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the same way.
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.. _setuptools: https://pypi.python.org/pypi/setuptools
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.. _pip: https://pypi.python.org/pypi/pip
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Packaging your app
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==================
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Python *packaging* refers to preparing your app in a specific format that can
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be easily installed and used. Django itself is packaged very much like
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this. For a small app like polls, this process isn't too difficult.
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1. First, create a parent directory for ``polls``, outside of your Django
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project. Call this directory ``django-polls``.
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.. admonition:: Choosing a name for your app
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When choosing a name for your package, check resources like PyPI to avoid
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naming conflicts with existing packages. It's often useful to prepend
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``django-`` to your module name when creating a package to distribute.
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This helps others looking for Django apps identify your app as Django
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specific.
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Application labels (that is, the final part of the dotted path to
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application packages) *must* be unique in :setting:`INSTALLED_APPS`.
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Avoid using the same label as any of the Django :doc:`contrib packages
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</ref/contrib/index>`, for example ``auth``, ``admin``, or
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``messages``.
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2. Move the ``polls`` directory into the ``django-polls`` directory.
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3. Create a file ``django-polls/README.rst`` with the following contents:
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.. snippet::
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:filename: django-polls/README.rst
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=====
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Polls
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=====
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Polls is a simple Django app to conduct Web-based polls. For each
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question, visitors can choose between a fixed number of answers.
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Detailed documentation is in the "docs" directory.
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Quick start
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-----------
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1. Add "polls" to your INSTALLED_APPS setting like this::
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INSTALLED_APPS = [
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...
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'polls',
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]
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2. Include the polls URLconf in your project urls.py like this::
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url(r'^polls/', include('polls.urls')),
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3. Run `python manage.py migrate` to create the polls models.
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4. Start the development server and visit http://127.0.0.1:8000/admin/
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to create a poll (you'll need the Admin app enabled).
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5. Visit http://127.0.0.1:8000/polls/ to participate in the poll.
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4. Create a ``django-polls/LICENSE`` file. Choosing a license is beyond the
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scope of this tutorial, but suffice it to say that code released publicly
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without a license is *useless*. Django and many Django-compatible apps are
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distributed under the BSD license; however, you're free to pick your own
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license. Just be aware that your licensing choice will affect who is able
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to use your code.
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5. Next we'll create a ``setup.py`` file which provides details about how to
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build and install the app. A full explanation of this file is beyond the
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scope of this tutorial, but the `setuptools docs
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<https://setuptools.readthedocs.io/en/latest/>`_ have a good
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explanation. Create a file ``django-polls/setup.py`` with the following
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contents:
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.. snippet::
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:filename: django-polls/setup.py
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import os
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from setuptools import find_packages, setup
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with open(os.path.join(os.path.dirname(__file__), 'README.rst')) as readme:
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README = readme.read()
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# allow setup.py to be run from any path
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os.chdir(os.path.normpath(os.path.join(os.path.abspath(__file__), os.pardir)))
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setup(
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name='django-polls',
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version='0.1',
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packages=find_packages(),
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include_package_data=True,
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license='BSD License', # example license
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description='A simple Django app to conduct Web-based polls.',
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long_description=README,
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url='https://www.example.com/',
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author='Your Name',
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author_email='yourname@example.com',
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classifiers=[
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'Environment :: Web Environment',
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'Framework :: Django',
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'Framework :: Django :: X.Y', # replace "X.Y" as appropriate
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'Intended Audience :: Developers',
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'License :: OSI Approved :: BSD License', # example license
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'Operating System :: OS Independent',
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'Programming Language :: Python',
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# Replace these appropriately if you are stuck on Python 2.
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'Programming Language :: Python :: 3',
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'Programming Language :: Python :: 3.4',
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'Programming Language :: Python :: 3.5',
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'Topic :: Internet :: WWW/HTTP',
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'Topic :: Internet :: WWW/HTTP :: Dynamic Content',
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],
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)
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6. Only Python modules and packages are included in the package by default. To
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include additional files, we'll need to create a ``MANIFEST.in`` file. The
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setuptools docs referred to in the previous step discuss this file in more
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details. To include the templates, the ``README.rst`` and our ``LICENSE``
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file, create a file ``django-polls/MANIFEST.in`` with the following
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contents:
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.. snippet::
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:filename: django-polls/MANIFEST.in
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include LICENSE
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include README.rst
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recursive-include polls/static *
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recursive-include polls/templates *
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7. It's optional, but recommended, to include detailed documentation with your
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app. Create an empty directory ``django-polls/docs`` for future
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documentation. Add an additional line to ``django-polls/MANIFEST.in``::
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recursive-include docs *
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Note that the ``docs`` directory won't be included in your package unless
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you add some files to it. Many Django apps also provide their documentation
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online through sites like `readthedocs.org <https://readthedocs.org>`_.
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8. Try building your package with ``python setup.py sdist`` (run from inside
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``django-polls``). This creates a directory called ``dist`` and builds your
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new package, ``django-polls-0.1.tar.gz``.
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For more information on packaging, see Python's `Tutorial on Packaging and
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Distributing Projects <https://packaging.python.org/en/latest/distributing.html>`_.
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Using your own package
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======================
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Since we moved the ``polls`` directory out of the project, it's no longer
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working. We'll now fix this by installing our new ``django-polls`` package.
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.. admonition:: Installing as a user library
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The following steps install ``django-polls`` as a user library. Per-user
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installs have a lot of advantages over installing the package system-wide,
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such as being usable on systems where you don't have administrator access
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as well as preventing the package from affecting system services and other
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users of the machine.
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Note that per-user installations can still affect the behavior of system
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tools that run as that user, so ``virtualenv`` is a more robust solution
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(see below).
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1. To install the package, use pip (you already :ref:`installed it
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<installing-reusable-apps-prerequisites>`, right?)::
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pip install --user django-polls/dist/django-polls-0.1.tar.gz
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2. With luck, your Django project should now work correctly again. Run the
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server again to confirm this.
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3. To uninstall the package, use pip::
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pip uninstall django-polls
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.. _pip: https://pypi.python.org/pypi/pip
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Publishing your app
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===================
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Now that we've packaged and tested ``django-polls``, it's ready to share with
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the world! If this wasn't just an example, you could now:
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* Email the package to a friend.
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* Upload the package on your website.
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* Post the package on a public repository, such as `the Python Package Index
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(PyPI)`_. `packaging.python.org <https://packaging.python.org>`_ has `a good
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tutorial <https://packaging.python.org/en/latest/distributing.html#uploading-your-project-to-pypi>`_
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for doing this.
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Installing Python packages with virtualenv
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==========================================
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Earlier, we installed the polls app as a user library. This has some
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disadvantages:
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* Modifying the user libraries can affect other Python software on your system.
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* You won't be able to run multiple versions of this package (or others with
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the same name).
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Typically, these situations only arise once you're maintaining several Django
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projects. When they do, the best solution is to use `virtualenv
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<http://www.virtualenv.org/>`_. This tool allows you to maintain multiple
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isolated Python environments, each with its own copy of the libraries and
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package namespace.
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