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338 lines
12 KiB
Plaintext
==================
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Django at a glance
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==================
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Because Django was developed in a fast-paced newsroom environment, it was
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designed to make common Web-development tasks fast and easy. Here's an informal
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overview of how to write a database-driven Web app with Django.
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The goal of this document is to give you enough technical specifics to
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understand how Django works, but this isn't intended to be a tutorial or
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reference -- but we've got both! When you're ready to start a project, you can
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:doc:`start with the tutorial </intro/tutorial01>` or :doc:`dive right into more
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detailed documentation </topics/index>`.
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Design your model
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=================
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Although you can use Django without a database, it comes with an
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`object-relational mapper`_ in which you describe your database layout in Python
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code.
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.. _object-relational mapper: http://en.wikipedia.org/wiki/Object-relational_mapping
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The :doc:`data-model syntax </topics/db/models>` offers many rich ways of
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representing your models -- so far, it's been solving two years' worth of
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database-schema problems. Here's a quick example, which might be saved in
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the file ``mysite/news/models.py``::
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from django.db import models
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class Reporter(models.Model):
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full_name = models.CharField(max_length=70)
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def __unicode__(self):
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return self.full_name
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class Article(models.Model):
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pub_date = models.DateField()
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headline = models.CharField(max_length=200)
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content = models.TextField()
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reporter = models.ForeignKey(Reporter)
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def __unicode__(self):
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return self.headline
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Install it
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==========
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Next, run the Django command-line utility to create the database tables
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automatically:
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.. code-block:: bash
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manage.py syncdb
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The :djadmin:`syncdb` command looks at all your available models and creates
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tables in your database for whichever tables don't already exist.
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Enjoy the free API
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==================
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With that, you've got a free, and rich, :doc:`Python API </topics/db/queries>`
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to access your data. The API is created on the fly, no code generation
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necessary:
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.. code-block:: python
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# Import the models we created from our "news" app
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>>> from news.models import Reporter, Article
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# No reporters are in the system yet.
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>>> Reporter.objects.all()
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[]
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# Create a new Reporter.
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>>> r = Reporter(full_name='John Smith')
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# Save the object into the database. You have to call save() explicitly.
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>>> r.save()
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# Now it has an ID.
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>>> r.id
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1
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# Now the new reporter is in the database.
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>>> Reporter.objects.all()
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[<Reporter: John Smith>]
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# Fields are represented as attributes on the Python object.
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>>> r.full_name
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'John Smith'
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# Django provides a rich database lookup API.
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>>> Reporter.objects.get(id=1)
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<Reporter: John Smith>
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>>> Reporter.objects.get(full_name__startswith='John')
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<Reporter: John Smith>
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>>> Reporter.objects.get(full_name__contains='mith')
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<Reporter: John Smith>
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>>> Reporter.objects.get(id=2)
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Traceback (most recent call last):
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...
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DoesNotExist: Reporter matching query does not exist. Lookup parameters were {'id': 2}
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# Create an article.
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>>> from datetime import date
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>>> a = Article(pub_date=date.today(), headline='Django is cool',
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... content='Yeah.', reporter=r)
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>>> a.save()
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# Now the article is in the database.
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>>> Article.objects.all()
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[<Article: Django is cool>]
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# Article objects get API access to related Reporter objects.
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>>> r = a.reporter
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>>> r.full_name
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'John Smith'
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# And vice versa: Reporter objects get API access to Article objects.
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>>> r.article_set.all()
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[<Article: Django is cool>]
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# The API follows relationships as far as you need, performing efficient
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# JOINs for you behind the scenes.
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# This finds all articles by a reporter whose name starts with "John".
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>>> Article.objects.filter(reporter__full_name__startswith="John")
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[<Article: Django is cool>]
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# Change an object by altering its attributes and calling save().
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>>> r.full_name = 'Billy Goat'
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>>> r.save()
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# Delete an object with delete().
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>>> r.delete()
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A dynamic admin interface: it's not just scaffolding -- it's the whole house
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============================================================================
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Once your models are defined, Django can automatically create a professional,
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production ready :doc:`administrative interface </ref/contrib/admin/index>` --
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a Web site that lets authenticated users add, change and delete objects. It's
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as easy as registering your model in the admin site::
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# In models.py...
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from django.db import models
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class Article(models.Model):
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pub_date = models.DateField()
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headline = models.CharField(max_length=200)
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content = models.TextField()
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reporter = models.ForeignKey(Reporter)
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# In admin.py in the same directory...
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import models
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from django.contrib import admin
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admin.site.register(models.Article)
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The philosophy here is that your site is edited by a staff, or a client, or
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maybe just you -- and you don't want to have to deal with creating backend
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interfaces just to manage content.
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One typical workflow in creating Django apps is to create models and get the
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admin sites up and running as fast as possible, so your staff (or clients) can
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start populating data. Then, develop the way data is presented to the public.
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Design your URLs
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================
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A clean, elegant URL scheme is an important detail in a high-quality Web
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application. Django encourages beautiful URL design and doesn't put any cruft
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in URLs, like ``.php`` or ``.asp``.
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To design URLs for an app, you create a Python module called a :doc:`URLconf
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</topics/http/urls>`. A table of contents for your app, it contains a simple
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mapping between URL patterns and Python callback functions. URLconfs also serve
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to decouple URLs from Python code.
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Here's what a URLconf might look like for the ``Reporter``/``Article``
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example above::
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from django.conf.urls import patterns
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urlpatterns = patterns('',
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(r'^articles/(\d{4})/$', 'news.views.year_archive'),
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(r'^articles/(\d{4})/(\d{2})/$', 'news.views.month_archive'),
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(r'^articles/(\d{4})/(\d{2})/(\d+)/$', 'news.views.article_detail'),
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)
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The code above maps URLs, as simple `regular expressions`_, to the location of
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Python callback functions ("views"). The regular expressions use parenthesis to
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"capture" values from the URLs. When a user requests a page, Django runs
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through each pattern, in order, and stops at the first one that matches the
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requested URL. (If none of them matches, Django calls a special-case 404 view.)
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This is blazingly fast, because the regular expressions are compiled at load
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time.
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.. _regular expressions: http://docs.python.org/2/howto/regex.html
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Once one of the regexes matches, Django imports and calls the given view, which
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is a simple Python function. Each view gets passed a request object --
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which contains request metadata -- and the values captured in the regex.
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For example, if a user requested the URL "/articles/2005/05/39323/", Django
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would call the function ``news.views.article_detail(request,
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'2005', '05', '39323')``.
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Write your views
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================
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Each view is responsible for doing one of two things: Returning an
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:class:`~django.http.HttpResponse` object containing the content for the
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requested page, or raising an exception such as :class:`~django.http.Http404`.
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The rest is up to you.
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Generally, a view retrieves data according to the parameters, loads a template
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and renders the template with the retrieved data. Here's an example view for
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``year_archive`` from above::
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from django.shortcuts import render_to_response
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def year_archive(request, year):
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a_list = Article.objects.filter(pub_date__year=year)
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return render_to_response('news/year_archive.html', {'year': year, 'article_list': a_list})
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This example uses Django's :doc:`template system </topics/templates>`, which has
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several powerful features but strives to stay simple enough for non-programmers
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to use.
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Design your templates
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=====================
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The code above loads the ``news/year_archive.html`` template.
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Django has a template search path, which allows you to minimize redundancy among
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templates. In your Django settings, you specify a list of directories to check
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for templates with :setting:`TEMPLATE_DIRS`. If a template doesn't exist in the
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first directory, it checks the second, and so on.
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Let's say the ``news/year_archive.html`` template was found. Here's what that
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might look like:
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.. code-block:: html+django
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{% extends "base.html" %}
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{% block title %}Articles for {{ year }}{% endblock %}
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{% block content %}
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<h1>Articles for {{ year }}</h1>
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{% for article in article_list %}
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<p>{{ article.headline }}</p>
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<p>By {{ article.reporter.full_name }}</p>
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<p>Published {{ article.pub_date|date:"F j, Y" }}</p>
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{% endfor %}
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{% endblock %}
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Variables are surrounded by double-curly braces. ``{{ article.headline }}``
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means "Output the value of the article's headline attribute." But dots aren't
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used only for attribute lookup: They also can do dictionary-key lookup, index
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lookup and function calls.
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Note ``{{ article.pub_date|date:"F j, Y" }}`` uses a Unix-style "pipe" (the "|"
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character). This is called a template filter, and it's a way to filter the value
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of a variable. In this case, the date filter formats a Python datetime object in
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the given format (as found in PHP's date function).
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You can chain together as many filters as you'd like. You can write :ref:`custom
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template filters <howto-writing-custom-template-filters>`. You can write
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:doc:`custom template tags </howto/custom-template-tags>`, which run custom
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Python code behind the scenes.
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Finally, Django uses the concept of "template inheritance": That's what the
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``{% extends "base.html" %}`` does. It means "First load the template called
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'base', which has defined a bunch of blocks, and fill the blocks with the
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following blocks." In short, that lets you dramatically cut down on redundancy
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in templates: each template has to define only what's unique to that template.
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Here's what the "base.html" template, including the use of :doc:`static files
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</howto/static-files/index>`, might look like:
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.. code-block:: html+django
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{% load staticfiles %}
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<html>
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<head>
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<title>{% block title %}{% endblock %}</title>
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</head>
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<body>
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<img src="{% static "images/sitelogo.png" %}" alt="Logo" />
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{% block content %}{% endblock %}
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</body>
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</html>
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Simplistically, it defines the look-and-feel of the site (with the site's logo),
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and provides "holes" for child templates to fill. This makes a site redesign as
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easy as changing a single file -- the base template.
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It also lets you create multiple versions of a site, with different base
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templates, while reusing child templates. Django's creators have used this
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technique to create strikingly different cell-phone editions of sites -- simply
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by creating a new base template.
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Note that you don't have to use Django's template system if you prefer another
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system. While Django's template system is particularly well-integrated with
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Django's model layer, nothing forces you to use it. For that matter, you don't
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have to use Django's database API, either. You can use another database
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abstraction layer, you can read XML files, you can read files off disk, or
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anything you want. Each piece of Django -- models, views, templates -- is
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decoupled from the next.
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This is just the surface
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========================
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This has been only a quick overview of Django's functionality. Some more useful
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features:
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* A :doc:`caching framework </topics/cache>` that integrates with memcached
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or other backends.
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* A :doc:`syndication framework </ref/contrib/syndication>` that makes
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creating RSS and Atom feeds as easy as writing a small Python class.
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* More sexy automatically-generated admin features -- this overview barely
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scratched the surface.
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The next obvious steps are for you to `download Django`_, read :doc:`the
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tutorial </intro/tutorial01>` and join `the community`_. Thanks for your
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interest!
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.. _download Django: https://www.djangoproject.com/download/
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.. _the community: https://www.djangoproject.com/community/
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