author | Lennard de Rijk <ljvderijk@gmail.com> |
Mon, 13 Jul 2009 00:53:42 +0200 | |
changeset 2616 | 9321e100a7fb |
parent 109 | 620f9b141567 |
permissions | -rw-r--r-- |
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================== |
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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. Please see our more-detailed Django documentation_ when you're ready |
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to start a project. |
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.. _documentation: ../ |
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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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The data-model syntax offers many rich ways of representing your models -- so |
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far, it's been solving two years' worth of database-schema problems. Here's a |
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quick example:: |
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class Reporter(models.Model): |
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full_name = models.CharField(maxlength=70) |
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def __str__(self): |
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return self.full_name |
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class Article(models.Model): |
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pub_date = models.DateTimeField() |
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headline = models.CharField(maxlength=200) |
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article = models.TextField() |
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reporter = models.ForeignKey(Reporter) |
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def __str__(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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manage.py syncdb |
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The ``syncdb`` command looks at all your available models and creates tables |
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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, Python API to access your data. The API |
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is created on the fly: No code generation necessary:: |
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>>> from mysite.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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[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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John Smith |
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>>> Reporter.objects.get(full_name__startswith='John') |
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John Smith |
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>>> Reporter.objects.get(full_name__contains='mith') |
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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 does not exist for {'id__exact': 2} |
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# Create an article. |
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>>> from datetime import datetime |
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>>> a = Article(pub_date=datetime.now(), headline='Django is cool', |
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... article='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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[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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[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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[Django is cool] |
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119 |
|
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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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123 |
|
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# Delete an object with delete(). |
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>>> r.delete() |
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126 |
|
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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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129 |
|
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Once your models are defined, Django can automatically create a professional, |
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production ready administrative interface -- a Web site that lets authenticated |
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users add, change and delete objects. It's as easy as adding a line of code to |
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your model classes:: |
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134 |
|
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class Article(models.Model): |
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pub_date = models.DateTimeField() |
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headline = models.CharField(maxlength=200) |
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article = models.TextField() |
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reporter = models.ForeignKey(Reporter) |
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class Admin: pass |
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141 |
|
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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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145 |
|
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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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149 |
|
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Design your URLs |
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================ |
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152 |
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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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156 |
|
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To design URLs for an app, you create a Python module called a URLconf. A table |
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of contents for your app, it contains a simple mapping between URL patterns and |
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Python callback functions. URLconfs also serve to decouple URLs from Python |
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160 |
code. |
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161 |
|
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162 |
Here's what a URLconf might look like for the ``Reporter``/``Article`` |
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163 |
example above:: |
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164 |
|
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165 |
from django.conf.urls.defaults import * |
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166 |
|
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167 |
urlpatterns = patterns('', |
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(r'^/articles/(\d{4})/$', 'mysite.views.year_archive'), |
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169 |
(r'^/articles/(\d{4})/(\d{2})/$', 'mysite.views.month_archive'), |
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(r'^/articles/(\d{4})/(\d{2})/(\d+)/$', 'mysite.views.article_detail'), |
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171 |
) |
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172 |
|
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173 |
The code above maps URLs, as simple regular expressions, to the location of |
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174 |
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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176 |
through each pattern, in order, and stops at the first one that matches the |
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177 |
requested URL. (If none of them matches, Django calls a special-case 404 view.) |
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178 |
This is blazingly fast, because the regular expressions are compiled at load |
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179 |
time. |
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180 |
|
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181 |
Once one of the regexes matches, Django imports and calls the given view, which |
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182 |
is a simple Python function. Each view gets passed a request object -- |
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183 |
which contains request metadata -- and the values captured in the regex. |
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184 |
|
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185 |
For example, if a user requested the URL "/articles/2005/05/39323/", Django |
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186 |
would call the function ``mysite.views.article_detail(request, |
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187 |
'2005', '05', '39323')``. |
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188 |
|
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189 |
Write your views |
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190 |
================ |
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191 |
|
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192 |
Each view is responsible for doing one of two things: Returning an |
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193 |
``HttpResponse`` object containing the content for the requested page, or |
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194 |
raising an exception such as ``Http404``. The rest is up to you. |
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195 |
|
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196 |
Generally, a view retrieves data according to the parameters, loads a template |
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197 |
and renders the template with the retrieved data. Here's an example view for |
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198 |
``year_archive`` from above:: |
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199 |
|
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200 |
def year_archive(request, year): |
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201 |
a_list = Article.objects.filter(pub_date__year=year) |
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202 |
return render_to_response('news/year_archive.html', {'year': year, 'article_list': a_list}) |
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203 |
|
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204 |
This example uses Django's template system, which has several powerful |
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205 |
features but strives to stay simple enough for non-programmers to use. |
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206 |
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207 |
Design your templates |
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208 |
===================== |
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209 |
|
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210 |
The code above loads the ``news/year_archive.html`` template. |
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211 |
|
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212 |
Django has a template search path, which allows you to minimize redundancy among |
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213 |
templates. In your Django settings, you specify a list of directories to check |
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214 |
for templates. If a template doesn't exist in the first directory, it checks the |
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215 |
second, and so on. |
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216 |
|
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217 |
Let's say the ``news/article_detail.html`` template was found. Here's what that |
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218 |
might look like:: |
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219 |
|
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220 |
{% extends "base.html" %} |
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221 |
|
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222 |
{% block title %}Articles for {{ year }}{% endblock %} |
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223 |
|
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224 |
{% block content %} |
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225 |
<h1>Articles for {{ year }}</h1> |
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226 |
|
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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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|
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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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|
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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; yes, there is one good idea |
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in PHP). |
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|
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You can chain together as many filters as you'd like. You can write custom |
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filters. You can write custom template tags, which run custom Python code behind |
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the scenes. |
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|
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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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|
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Here's what the "base.html" template might look like:: |
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|
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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="sitelogo.gif" alt="Logo" /> |
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{% block content %}{% endblock %} |
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</body> |
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</html> |
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|
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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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|
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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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|
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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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|
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This is just the surface |
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======================== |
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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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|
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* A caching framework that integrates with memcached or other backends. |
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* A syndication framework that makes creating RSS and Atom feeds as easy as |
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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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|
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The next obvious steps are for you to `download Django`_, read `the tutorial`_ |
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and join `the community`_. Thanks for your interest! |
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|
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.. _download Django: http://www.djangoproject.com/download/ |
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.. _the tutorial: http://www.djangoproject.com/documentation/tutorial1/ |
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.. _the community: http://www.djangoproject.com/community/ |