=====================================
Writing your first Django app, part 1
=====================================

Let's learn by example.

Throughout this tutorial, we'll walk you through the creation of a basic
poll application.

It'll consist of two parts:

* A public site that lets people view polls and vote in them.
* An admin site that lets you add, change, and delete polls.

We'll assume you have :doc:`Django installed </intro/install>` already. You can
tell Django is installed and which version by running the following command
in a shell prompt (indicated by the $ prefix):

.. code-block:: console

    $ python -m django --version

If Django is installed, you should see the version of your installation. If it
isn't, you'll get an error telling "No module named django".

This tutorial is written for Django |version| and Python 3.4 or later. If the
Django version doesn't match, you can refer to the tutorial for your version
of Django by using the version switcher at the bottom right corner of this
page, or update Django to the newest version. If you are still using Python
2.7, you will need to adjust the code samples slightly, as described in
comments.

See :doc:`How to install Django </topics/install>` for advice on how to remove
older versions of Django and install a newer one.

.. admonition:: Where to get help:

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

Creating a project
==================

If this is your first time using Django, you'll have to take care of some
initial setup. Namely, you'll need to auto-generate some code that establishes a
Django :term:`project` -- a collection of settings for an instance of Django,
including database configuration, Django-specific options and
application-specific settings.

From the command line, ``cd`` into a directory where you'd like to store your
code, then run the following command:

.. code-block:: console

   $ django-admin startproject mysite

This will create a ``mysite`` directory in your current directory. If it didn't
work, see :ref:`troubleshooting-django-admin`.

.. note::

    You'll need to avoid naming projects after built-in Python or Django
    components. In particular, this means you should avoid using names like
    ``django`` (which will conflict with Django itself) or ``test`` (which
    conflicts with a built-in Python package).

.. admonition:: Where should this code live?

    If your background is in plain old PHP (with no use of modern frameworks),
    you're probably used to putting code under the Web server's document root
    (in a place such as ``/var/www``). With Django, you don't do that. It's
    not a good idea to put any of this Python code within your Web server's
    document root, because it risks the possibility that people may be able
    to view your code over the Web. That's not good for security.

    Put your code in some directory **outside** of the document root, such as
    :file:`/home/mycode`.

Let's look at what :djadmin:`startproject` created::

    mysite/
        manage.py
        mysite/
            __init__.py
            settings.py
            urls.py
            wsgi.py

These files are:

* The outer :file:`mysite/` root directory is just a container for your
  project. Its name doesn't matter to Django; you can rename it to anything
  you like.

* :file:`manage.py`: A command-line utility that lets you interact with this
  Django project in various ways. You can read all the details about
  :file:`manage.py` in :doc:`/ref/django-admin`.

* The inner :file:`mysite/` directory is the actual Python package for your
  project. Its name is the Python package name you'll need to use to import
  anything inside it (e.g. ``mysite.urls``).

* :file:`mysite/__init__.py`: An empty file that tells Python that this
  directory should be considered a Python package. If you're a Python beginner,
  read :ref:`more about packages <tut-packages>` in the official Python docs.

* :file:`mysite/settings.py`: Settings/configuration for this Django
  project.  :doc:`/topics/settings` will tell you all about how settings
  work.

* :file:`mysite/urls.py`: The URL declarations for this Django project; a
  "table of contents" of your Django-powered site. You can read more about
  URLs in :doc:`/topics/http/urls`.

* :file:`mysite/wsgi.py`: An entry-point for WSGI-compatible web servers to
  serve your project. See :doc:`/howto/deployment/wsgi/index` for more details.

The development server
======================

Let's verify your Django project works. Change into the outer :file:`mysite` directory, if
you haven't already, and run the following commands:

.. code-block:: console

   $ python manage.py runserver

You'll see the following output on the command line:

.. parsed-literal::

    Performing system checks...

    System check identified no issues (0 silenced).

    You have unapplied migrations; your app may not work properly until they are applied.
    Run 'python manage.py migrate' to apply them.

    |today| - 15:50:53
    Django version |version|, using settings 'mysite.settings'
    Starting development server at http://127.0.0.1:8000/
    Quit the server with CONTROL-C.

.. note::
    Ignore the warning about unapplied database migrations for now; we'll deal
    with the database shortly.

You've started the Django development server, a lightweight Web server written
purely in Python. We've included this with Django so you can develop things
rapidly, without having to deal with configuring a production server -- such as
Apache -- until you're ready for production.

Now's a good time to note: **don't** use this server in anything resembling a
production environment. It's intended only for use while developing. (We're in
the business of making Web frameworks, not Web servers.)

Now that the server's running, visit http://127.0.0.1:8000/ with your Web
browser. You'll see a "Welcome to Django" page, in pleasant, light-blue pastel.
It worked!

.. admonition:: Changing the port

    By default, the :djadmin:`runserver` command starts the development server
    on the internal IP at port 8000.

    If you want to change the server's port, pass
    it as a command-line argument. For instance, this command starts the server
    on port 8080:

    .. code-block:: console

        $ python manage.py runserver 8080

    If you want to change the server's IP, pass it along with the port. For
    example, to listen on all available public IPs (which is useful if you are
    running Vagrant or want to show off your work on other computers on the
    network), use:

    .. code-block:: console

        $ python manage.py runserver 0:8000

    **0** is a shortcut for **0.0.0.0**. Full docs for the development server
    can be found in the :djadmin:`runserver` reference.

.. admonition:: Automatic reloading of :djadmin:`runserver`

    The development server automatically reloads Python code for each request
    as needed. You don't need to restart the server for code changes to take
    effect. However, some actions like adding files don't trigger a restart,
    so you'll have to restart the server in these cases.

Creating the Polls app
======================

Now that your environment -- a "project" -- is set up, you're set to start
doing work.

Each application you write in Django consists of a Python package that follows
a certain convention. Django comes with a utility that automatically generates
the basic directory structure of an app, so you can focus on writing code
rather than creating directories.

.. admonition:: Projects vs. apps

    What's the difference between a project and an app? An app is a Web
    application that does something -- e.g., a Weblog system, a database of
    public records or a simple poll app. A project is a collection of
    configuration and apps for a particular website. A project can contain
    multiple apps. An app can be in multiple projects.

Your apps can live anywhere on your :ref:`Python path <tut-searchpath>`. In
this tutorial, we'll create our poll app right next to your :file:`manage.py`
file so that it can be imported as its own top-level module, rather than a
submodule of ``mysite``.

To create your app, make sure you're in the same directory as :file:`manage.py`
and type this command:

.. code-block:: console

    $ python manage.py startapp polls

That'll create a directory :file:`polls`, which is laid out like this::

    polls/
        __init__.py
        admin.py
        apps.py
        migrations/
            __init__.py
        models.py
        tests.py
        views.py

This directory structure will house the poll application.

Write your first view
=====================

Let's write the first view. Open the file ``polls/views.py``
and put the following Python code in it:

.. snippet::
    :filename: polls/views.py

    from django.http import HttpResponse


    def index(request):
        return HttpResponse("Hello, world. You're at the polls index.")

This is the simplest view possible in Django. To call the view, we need to map
it to a URL - and for this we need a URLconf.

To create a URLconf in the polls directory, create a file called ``urls.py``.
Your app directory should now look like::

    polls/
        __init__.py
        admin.py
        apps.py
        migrations/
            __init__.py
        models.py
        tests.py
        urls.py
        views.py

In the ``polls/urls.py`` file include the following code:

.. snippet::
    :filename: polls/urls.py

    from django.conf.urls import url

    from . import views

    urlpatterns = [
        url(r'^$', views.index, name='index'),
    ]

The next step is to point the root URLconf at the ``polls.urls`` module. In
``mysite/urls.py``, add an import for ``django.conf.urls.include`` and insert
an :func:`~django.conf.urls.include` in the ``urlpatterns`` list, so you have:

.. snippet::
    :filename: mysite/urls.py

    from django.conf.urls import include, url
    from django.contrib import admin

    urlpatterns = [
        url(r'^polls/', include('polls.urls')),
        url(r'^admin/', admin.site.urls),
    ]

The :func:`~django.conf.urls.include` function allows referencing other
URLconfs. Note that the regular expressions for the
:func:`~django.conf.urls.include` function doesn't have a ``$`` (end-of-string
match character) but rather a trailing slash. Whenever Django encounters
:func:`~django.conf.urls.include`, it chops off whatever part of the URL
matched up to that point and sends the remaining string to the included URLconf
for further processing.

The idea behind :func:`~django.conf.urls.include` is to make it easy to
plug-and-play URLs. Since polls are in their own URLconf
(``polls/urls.py``), they can be placed under "/polls/", or under
"/fun_polls/", or under "/content/polls/", or any other path root, and the
app will still work.

.. admonition:: When to use :func:`~django.conf.urls.include()`

    You should always use ``include()`` when you include other URL patterns.
    ``admin.site.urls`` is the only exception to this.

.. admonition:: Doesn't match what you see?

    If you're seeing ``include(admin.site.urls)`` instead of just
    ``admin.site.urls``, you're probably using a version of Django that
    doesn't match this tutorial version.  You'll want to either switch to the
    older tutorial or the newer Django version.

You have now wired an ``index`` view into the URLconf. Lets verify it's
working, run the following command:

.. code-block:: console

   $ python manage.py runserver

Go to http://localhost:8000/polls/ in your browser, and you should see the
text "*Hello, world. You're at the polls index.*", which you defined in the
``index`` view.

The :func:`~django.conf.urls.url` function is passed four arguments, two
required: ``regex`` and ``view``, and two optional: ``kwargs``, and ``name``.
At this point, it's worth reviewing what these arguments are for.

:func:`~django.conf.urls.url` argument: regex
---------------------------------------------

The term "regex" is a commonly used short form meaning "regular expression",
which is a syntax for matching patterns in strings, or in this case, url
patterns. Django starts at the first regular expression and makes its way down
the list,  comparing the requested URL against each regular expression until it
finds one that matches.

Note that these regular expressions do not search GET and POST parameters, or
the domain name. For example, in a request to
``https://www.example.com/myapp/``, the URLconf will look for ``myapp/``. In a
request to ``https://www.example.com/myapp/?page=3``, the URLconf will also
look for ``myapp/``.

If you need help with regular expressions, see `Wikipedia's entry`_ and the
documentation of the :mod:`re` module. Also, the O'Reilly book "Mastering
Regular Expressions" by Jeffrey Friedl is fantastic. In practice, however,
you don't need to be an expert on regular expressions, as you really only need
to know how to capture simple patterns. In fact, complex regexes can have poor
lookup performance, so you probably shouldn't rely on the full power of regexes.

Finally, a performance note: these regular expressions are compiled the first
time the URLconf module is loaded. They're super fast (as long as the lookups
aren't too complex as noted above).

.. _Wikipedia's entry: https://en.wikipedia.org/wiki/Regular_expression

:func:`~django.conf.urls.url` argument: view
--------------------------------------------

When Django finds a regular expression match, Django calls the specified view
function, with an :class:`~django.http.HttpRequest` object as the first
argument and any “captured” values from the regular expression as other
arguments. If the regex uses simple captures, values are passed as positional
arguments; if it uses named captures, values are passed as keyword arguments.
We'll give an example of this in a bit.

:func:`~django.conf.urls.url` argument: kwargs
----------------------------------------------

Arbitrary keyword arguments can be passed in a dictionary to the target view. We
aren't going to use this feature of Django in the tutorial.

:func:`~django.conf.urls.url` argument: name
---------------------------------------------

Naming your URL lets you refer to it unambiguously from elsewhere in Django,
especially from within templates. This powerful feature allows you to make
global changes to the URL patterns of your project while only touching a single
file.

When you're comfortable with the basic request and response flow, read
:doc:`part 2 of this tutorial </intro/tutorial02>` to start working with the
database.
