Code


Version 6 (modified by sangho, 4 years ago) (diff)

--

.. _intro-tutorial03:

===================================== 세번째 : views 작성하기 =====================================

이 페이지는 "views"라는 public interface에 대해 중점을 두고 Web-poll 어플리케이션을 다룹니다.

개념 ========== 뷰(view)는 장고 어플리케이션에서 구체적인 기능을 수행하고, 특정한 템플릿을 가지고 있기도 한 웹페이지의 "형태"중 한가지 입니다. 예를들어, 만약 당신이 블로그 어플리케이션을 만든다면, 아래와 같은 뷰를 사용해야 할 것입니다.

  • 블로그 홈페이지 -- 최근에 올라온 글 몇개를 보여줍니다.
  • 글 "자세히" 페이지 -- 한 개의 글에 대한 바로 가기 페이지
  • 1년 기준 문서 페이지 -- 1년 동안의 모든 글을 보여줍니다.
  • 한달 기준 문서 페이지 -- 한달 동안의 모든 글을 보여줍니다.
  • 하루 기준 문서 페이지 -- 하루 동안의 모든 글을 보여줍니다.
  • 댓글 처리 -- 글에 대한 댓글을 작성하는 작업을 합니다.

우리의 poll 어플리케이션에서는 아래의 네가지 뷰를 쓸 것 입니다.

  • Poll "문서" 페이지 -- 최근 몇개의 poll들을 보여줍니다.
  • Poll "자세히" 페이지 -- poll의 제목과, 투표를 하기위한 폼을 보여줍니다. (결과는 없음)
  • Poll "결과" 페이지 -- 특정한 poll의 결과를 보여줍니다.
  • 투표 처리 -- 특정한 poll에 대한 특정한 선택에 대한 투표를 처리합니다.

장고에서 각각의 뷰는 간단한 파이썬 함수로 만들 수 있습니다.

URL 디자인 하기 ================ 뷰를 작성하기 위한 첫 번째 단계는 URLconf라는 파이썬 모듈을 작성해서 URL 구조를 설계하는 것입니다. URLconf는 특정 URL을 그에 주어진 파이썬 코드로 연결시킵니다.

유저가 장고기반 페이지를 요청하면 시스템은 Python dotted 문법의 문자열을 포함한 :setting:ROOT_URLCONF 세팅을 불러들이고, 아래와 같은 튜플 형태인 urlpatterns 라는 모듈레벨의 변수를 찾습니다.::

(정규표현식, 파이썬 콜백 함수, [, 부가적인 사전자료형])

장고는 요청된 URL을 나타내는 정규표현식이 나올 때 까지 첫 번째 정규표현식부터 아래로 쭉 훑습니다.

적합한 정규표현식을 찾았다면, 장고는 :class:~django.http.HttpRequest를 첫번째 인자로 하는 파이썬 콜백 함수를 호출합니다. 정규표현식에 의해 "걸려진" 값들도 키워드 인자로 전달되고, 위의 사전자료형 (세번째로 받은 튜플값)에서도 인자가 전달됩니다.

:class:~django.http.HttpRequest에 대한 더 많은정보는 :ref:ref-request-response이곳에서, URLconfs에 관한 정보는 :ref:topics-http-urls이 곳을 참고 하세요

만약 여러분이 튜토리얼1 에서 django-admin.py startproject mysite 커맨드를 실행하셨다면 URLconf가 mysite/urls.py에 기본적으로 생겼을 것이고, :setting:ROOT_URLCONF세팅또한 setting.py에 생겼을 것입니다.::

ROOT_URLCONF = 'mysite.urls'

이제 예를들기위해 mysite/urls.py 를 수정해보겠습니다. 대략 아래와 같이 생겼습니다::

from django.conf.urls.defaults import *

from django.contrib import admin admin.autodiscover()

urlpatterns = patterns(,

(r'polls/$', 'mysite.polls.views.index'), (r'polls/(?P<poll_id>\d+)/$', 'mysite.polls.views.detail'), (r'polls/(?P<poll_id>\d+)/results/$', 'mysite.polls.views.results'), (r'polls/(?P<poll_id>\d+)/vote/$', 'mysite.polls.views.vote'), (r'admin/', include(admin.site.urls)),

)

This is worth a review. When somebody requests a page from your Web site -- say, "/polls/23/", Django will load this Python module, because it's pointed to by the :setting:ROOT_URLCONF setting. It finds the variable named urlpatterns and traverses the regular expressions in order. When it finds a regular expression that matches -- r'polls/(?P<poll_id>\d+)/$' -- it loads the function detail() from mysite/polls/views.py. Finally, it calls that detail() function like so::

detail(request=<HttpRequest object>, poll_id='23')

The poll_id='23' part comes from (?P<poll_id>\d+). Using parenthesis around a pattern "captures" the text matched by that pattern and sends it as an argument to the view function; the ?P<poll_id> defines the name that will be used to identify the matched pattern; and \d+ is a regular expression to match a sequence of digits (i.e., a number).

Because the URL patterns are regular expressions, there really is no limit on what you can do with them. And there's no need to add URL cruft such as .php -- unless you have a sick sense of humor, in which case you can do something like this::

(r'polls/latest\.php$', 'mysite.polls.views.index'),

But, don't do that. It's silly.

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

If you need help with regular expressions, see Wikipedia's entry_ and the Python documentation_. Also, the O'Reilly book "Mastering Regular Expressions" by Jeffrey Friedl is fantastic.

Finally, a performance note: these regular expressions are compiled the first time the URLconf module is loaded. They're super fast.

.. _Wikipedia's entry: http://en.wikipedia.org/wiki/Regular_expression .. _Python documentation: http://docs.python.org/library/re.html

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

Well, we haven't created any views yet -- we just have the URLconf. But let's make sure Django is following the URLconf properly.

Fire up the Django development Web server:

.. code-block:: bash

python manage.py runserver

Now go to "http://localhost:8000/polls/" on your domain in your Web browser. You should get a pleasantly-colored error page with the following message::

ViewDoesNotExist at /polls/

Tried index in module mysite.polls.views. Error was: 'module' object has no attribute 'index'

This error happened because you haven't written a function index() in the module mysite/polls/views.py.

Try "/polls/23/", "/polls/23/results/" and "/polls/23/vote/". The error messages tell you which view Django tried (and failed to find, because you haven't written any views yet).

Time to write the first view. Open the file mysite/polls/views.py and put the following Python code in it::

from django.http import HttpResponse

def index(request):

return HttpResponse("Hello, world. You're at the poll index.")

This is the simplest view possible. Go to "/polls/" in your browser, and you should see your text.

Now lets add a few more views. These views are slightly different, because they take an argument (which, remember, is passed in from whatever was captured by the regular expression in the URLconf)::

def detail(request, poll_id):

return HttpResponse("You're looking at poll %s." % poll_id)

def results(request, poll_id):

return HttpResponse("You're looking at the results of poll %s." % poll_id)

def vote(request, poll_id):

return HttpResponse("You're voting on poll %s." % poll_id)

Take a look in your browser, at "/polls/34/". It'll run the detail() method and display whatever ID you provide in the URL. Try "/polls/34/results/" and "/polls/34/vote/" too -- these will display the placeholder results and voting pages.

Write views that actually do something ======================================

Each view is responsible for doing one of two things: Returning an :class:~django.http.HttpResponse object containing the content for the requested page, or raising an exception such as :exc:~django.http.Http404. The rest is up to you.

Your view can read records from a database, or not. It can use a template system such as Django's -- or a third-party Python template system -- or not. It can generate a PDF file, output XML, create a ZIP file on the fly, anything you want, using whatever Python libraries you want.

All Django wants is that :class:~django.http.HttpResponse. Or an exception.

Because it's convenient, let's use Django's own database API, which we covered in :ref:Tutorial 1 <intro-tutorial01>. Here's one stab at the index() view, which displays the latest 5 poll questions in the system, separated by commas, according to publication date::

from mysite.polls.models import Poll from django.http import HttpResponse

def index(request):

latest_poll_list = Poll.objects.all().order_by('-pub_date')[:5] output = ', '.join([p.question for p in latest_poll_list]) return HttpResponse(output)

There's a problem here, though: The page's design is hard-coded in the view. If you want to change the way the page looks, you'll have to edit this Python code. So let's use Django's template system to separate the design from Python::

from django.template import Context, loader from mysite.polls.models import Poll from django.http import HttpResponse

def index(request):

latest_poll_list = Poll.objects.all().order_by('-pub_date')[:5] t = loader.get_template('polls/index.html') c = Context({

'latest_poll_list': latest_poll_list,

}) return HttpResponse(t.render(c))

That code loads the template called "polls/index.html" and passes it a context. The context is a dictionary mapping template variable names to Python objects.

Reload the page. Now you'll see an error::

TemplateDoesNotExist at /polls/ polls/index.html

Ah. There's no template yet. First, create a directory, somewhere on your filesystem, whose contents Django can access. (Django runs as whatever user your server runs.) Don't put them under your document root, though. You probably shouldn't make them public, just for security's sake. Then edit :setting:TEMPLATE_DIRS in your settings.py to tell Django where it can find templates -- just as you did in the "Customize the admin look and feel" section of Tutorial 2.

When you've done that, create a directory polls in your template directory. Within that, create a file called index.html. Note that our loader.get_template('polls/index.html') code from above maps to "[template_directory]/polls/index.html" on the filesystem.

Put the following code in that template:

.. code-block:: html+django

{% if latest_poll_list %}

<ul> {% for poll in latest_poll_list %}

<li>{{ poll.question }}</li>

{% endfor %} </ul>

{% else %}

<p>No polls are available.</p>

{% endif %}

Load the page in your Web browser, and you should see a bulleted-list containing the "What's up" poll from Tutorial 1.

A shortcut: render_to_response()


It's a very common idiom to load a template, fill a context and return an :class:~django.http.HttpResponse object with the result of the rendered template. Django provides a shortcut. Here's the full index() view, rewritten::

from django.shortcuts import render_to_response from mysite.polls.models import Poll

def index(request):

latest_poll_list = Poll.objects.all().order_by('-pub_date')[:5] return render_to_response('polls/index.html', {'latest_poll_list': latest_poll_list})

Note that once we've done this in all these views, we no longer need to import :mod:~django.template.loader, :class:~django.template.Context and :class:~django.http.HttpResponse.

The :func:~django.shortcuts.render_to_response function takes a template name as its first argument and a dictionary as its optional second argument. It returns an :class:~django.http.HttpResponse object of the given template rendered with the given context.

Raising 404 ===========

Now, let's tackle the poll detail view -- the page that displays the question for a given poll. Here's the view::

from django.http import Http404 # ... def detail(request, poll_id):

try:

p = Poll.objects.get(pk=poll_id)

except Poll.DoesNotExist:

raise Http404

return render_to_response('polls/detail.html', {'poll': p})

The new concept here: The view raises the :exc:~django.http.Http404 exception if a poll with the requested ID doesn't exist.

We'll discuss what you could put in that polls/detail.html template a bit later, but if you'd like to quickly get the above example working, just::

{{ poll }}

will get you started for now.

A shortcut: get_object_or_404()


It's a very common idiom to use :meth:~django.db.models.QuerySet.get and raise :exc:~django.http.Http404 if the object doesn't exist. Django provides a shortcut. Here's the detail() view, rewritten::

from django.shortcuts import render_to_response, get_object_or_404 # ... def detail(request, poll_id):

p = get_object_or_404(Poll, pk=poll_id) return render_to_response('polls/detail.html', {'poll': p})

The :func:~django.shortcuts.get_object_or_404 function takes a Django model as its first argument and an arbitrary number of keyword arguments, which it passes to the module's :meth:~django.db.models.QuerySet.get function. It raises :exc:~django.http.Http404 if the object doesn't exist.

.. admonition:: Philosophy

Why do we use a helper function :func:~django.shortcuts.get_object_or_404 instead of automatically catching the :exc:~django.core.exceptions.ObjectDoesNotExist exceptions at a higher level, or having the model API raise :exc:~django.http.Http404 instead of :exc:~django.core.exceptions.ObjectDoesNotExist?

Because that would couple the model layer to the view layer. One of the foremost design goals of Django is to maintain loose coupling.

There's also a :func:~django.shortcuts.get_list_or_404 function, which works just as :func:~django.shortcuts.get_object_or_404 -- except using :meth:~django.db.models.QuerySet.filter instead of :meth:~django.db.models.QuerySet.get. It raises :exc:~django.http.Http404 if the list is empty.

Write a 404 (page not found) view =================================

When you raise :exc:~django.http.Http404 from within a view, Django will load a special view devoted to handling 404 errors. It finds it by looking for the variable handler404, which is a string in Python dotted syntax -- the same format the normal URLconf callbacks use. A 404 view itself has nothing special: It's just a normal view.

You normally won't have to bother with writing 404 views. By default, URLconfs have the following line up top::

from django.conf.urls.defaults import *

That takes care of setting handler404 in the current module. As you can see in django/conf/urls/defaults.py, handler404 is set to :func:django.views.defaults.page_not_found by default.

Four more things to note about 404 views:

  • If :setting:DEBUG is set to True (in your settings module) then your 404 view will never be used (and thus the 404.html template will never be rendered) because the traceback will be displayed instead.
  • The 404 view is also called if Django doesn't find a match after checking every regular expression in the URLconf.
  • If you don't define your own 404 view -- and simply use the default, which is recommended -- you still have one obligation: To create a 404.html template in the root of your template directory. The default 404 view will use that template for all 404 errors.
  • If :setting:DEBUG is set to False (in your settings module) and if you didn't create a 404.html file, an Http500 is raised instead. So remember to create a 404.html.

Write a 500 (server error) view ===============================

Similarly, URLconfs may define a handler500, which points to a view to call in case of server errors. Server errors happen when you have runtime errors in view code.

Use the template system =======================

Back to the detail() view for our poll application. Given the context variable poll, here's what the "polls/detail.html" template might look like:

.. code-block:: html+django

<h1>{{ poll.question }}</h1> <ul> {% for choice in poll.choice_set.all %}

<li>{{ choice.choice }}</li>

{% endfor %} </ul>

The template system uses dot-lookup syntax to access variable attributes. In the example of {{ poll.question }}, first Django does a dictionary lookup on the object poll. Failing that, it tries attribute lookup -- which works, in this case. If attribute lookup had failed, it would've tried calling the method question() on the poll object.

Method-calling happens in the {% for %} loop: poll.choice_set.all is interpreted as the Python code poll.choice_set.all(), which returns an iterable of Choice objects and is suitable for use in the {% for %} tag.

See the :ref:template guide <topics-templates> for more about templates.

Simplifying the URLconfs ========================

Take some time to play around with the views and template system. As you edit the URLconf, you may notice there's a fair bit of redundancy in it::

urlpatterns = patterns(,

(r'polls/$', 'mysite.polls.views.index'), (r'polls/(?P<poll_id>\d+)/$', 'mysite.polls.views.detail'), (r'polls/(?P<poll_id>\d+)/results/$', 'mysite.polls.views.results'), (r'polls/(?P<poll_id>\d+)/vote/$', 'mysite.polls.views.vote'),

)

Namely, mysite.polls.views is in every callback.

Because this is a common case, the URLconf framework provides a shortcut for common prefixes. You can factor out the common prefixes and add them as the first argument to :func:~django.conf.urls.defaults.patterns, like so::

urlpatterns = patterns('mysite.polls.views',

(r'polls/$', 'index'), (r'polls/(?P<poll_id>\d+)/$', 'detail'), (r'polls/(?P<poll_id>\d+)/results/$', 'results'), (r'polls/(?P<poll_id>\d+)/vote/$', 'vote'),

)

This is functionally identical to the previous formatting. It's just a bit tidier.

Decoupling the URLconfs =======================

While we're at it, we should take the time to decouple our poll-app URLs from our Django project configuration. Django apps are meant to be pluggable -- that is, each particular app should be transferable to another Django installation with minimal fuss.

Our poll app is pretty decoupled at this point, thanks to the strict directory structure that python manage.py startapp created, but one part of it is coupled to the Django settings: The URLconf.

We've been editing the URLs in mysite/urls.py, but the URL design of an app is specific to the app, not to the Django installation -- so let's move the URLs within the app directory.

Copy the file mysite/urls.py to mysite/polls/urls.py. Then, change mysite/urls.py to remove the poll-specific URLs and insert an :func:~django.conf.urls.defaults.include::

# ... urlpatterns = patterns(,

(r'polls/', include('mysite.polls.urls')), # ...

:func:~django.conf.urls.defaults.include, simply, references another URLconf. Note that the regular expression doesn't have a $ (end-of-string match character) but has the trailing slash. Whenever Django encounters :func:~django.conf.urls.defaults.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.

Here's what happens if a user goes to "/polls/34/" in this system:

  • Django will find the match at 'polls/'
  • Then, Django will strip off the matching text ("polls/") and send the remaining text -- "34/" -- to the 'mysite.polls.urls' URLconf for further processing.

Now that we've decoupled that, we need to decouple the 'mysite.polls.urls' URLconf by removing the leading "polls/" from each line, and removing the lines registering the admin site::

urlpatterns = patterns('mysite.polls.views',

(r'$', 'index'), (r'(?P<poll_id>\d+)/$', 'detail'), (r'(?P<poll_id>\d+)/results/$', 'results'), (r'(?P<poll_id>\d+)/vote/$', 'vote'),

)

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

All the poll app cares about is its relative URLs, not its absolute URLs.

When you're comfortable with writing views, read :ref:`part 4 of this tutorial <intro-tutorial04>` to learn about simple form processing and generic views.

Attachments (1)

Download all attachments as: .zip