Django入门笔记【三】

入门笔记翻译整理自：https://docs.djangoproject.com/en/1.8/

*该笔记将使用一个关于投票网络应用（poll application）的例子来阐述Django的用法。

Public interface - views.

1. 定义

视图(View)是Django应用中用于实现某一特定功能的Web页面。比如，一个博客可以有博客展示页面，博客创建页面，评论页面。

2. 视图示例

写入代码

#polls/views.py

from django.http import HttpResponse

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

为了调用视图，我们需要将它映射到一个URL上，因此在polls目录下创建urls.py，最终目录结构如图所示

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

在polls/urls.py中添加以下代码

# polls/urls.py

from django.conf.urls import url

from . import views

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

将根目录中的URLconf定位至polls/urls中，使用include()方法：

# 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/', include(admin.site.urls)),
]

现在可以通过http://localhost:8000/polls/进入polls应用的视图。

url()方法有四个参数，必填两个：regex, view；选填两个：kwargs, name。其中，regex对模式(pattern)进行匹配，不区分GET和POST；完成匹配后，Django会调用view函数，并将HttpRequest和其他参数传入view函数中；kwargs将关键字参数传入view函数中；name可以对URL 命名

3. 更多视图示例

向polls/views.py中添加更多视图，和前面不同，这些视图需要接收参数

# polls/views.py

def detail(request, question_id):
    return HttpResponse("You're looking at question %s." % question_id)

def results(request, question_id):
    response = "You're looking at the results of question %s."
    return HttpResponse(response % question_id)

def vote(request, question_id):
    return HttpResponse("You're voting on question %s." % question_id)

将这些视图映射到polls.urls

# polls/urls.py

from django.conf.urls import url

from . import views

urlpatterns = [
    # ex: /polls/
    url(r'^$', views.index, name='index'),
    # ex: /polls/5/
    url(r'^(?P<question_id>[0-9]+)/$', views.detail, name='detail'),
    # ex: /polls/5/results/
    url(r'^(?P<question_id>[0-9]+)/results/$', views.results, name='results'),
    # ex: /polls/5/vote/
    url(r'^(?P<question_id>[0-9]+)/vote/$', views.vote, name='vote'),
]

Django实现视图的逻辑如下，以用户访问"/polls/34/"为例：

首先，Django寻找'^polls/'的相应匹配；

然后，Django会剥离匹配的字符串"polls/"，同时将剩下的文本"34/"发送至'polls.urls'；

最后，在'polls.urls'的URLconf中进一步匹配r'^(?P<question_id>[0-9]+)/$'，并调用detail()视图。

4. 视图再举例

每一个视图都必须完成两个事件之一：返回HttpResponse，或者Http404。其余取决于应用需求。

# polls/views.py

from django.http import HttpResponse

from .models import Question

def index(request):
    latest_question_list = Question.objects.order_by('-pub_date')[:5]
    output = ', '.join([p.question_text for p in latest_question_list])
    return HttpResponse(output)
# Leave the rest of the views (detail, results, vote) unchanged

但是这里有一个问题，视图页面是硬编码的（hard-coded）。推荐将模板和Python代码独立。

首先，创建polls/templates目录。Django会在该目录下寻找模板。TEMPLATES设置中的默认后台DjangoTemplates会在每个INSTALLED_APP下寻找templates子目录。

然后，创建polls/templates/polls/index.html。这里创建第二个polls子目录的目的在于，区分不同应用下相同名称的index.html文件。

接着，修改index.html文件

# polls/templates/polls/index.html

{% if latest_question_list %}
    <ul>
    {% for question in latest_question_list %}
        <li><a href="/polls/{{ question.id }}/">{{ question.question_text }}</a></li>
    {% endfor %}
    </ul>
{% else %}
    <p>No polls are available.</p>
{% endif %}

最后，修改polls/views.py文件

# polls/views.py

from django.http import HttpResponse
from django.template import RequestContext, loader

from .models import Question


def index(request):
    latest_question_list = Question.objects.order_by('-pub_date')[:5]
    template = loader.get_template('polls/index.html')
    context = RequestContext(request, {
        'latest_question_list': latest_question_list,
    })
    return HttpResponse(template.render(context))

该代码加载polls/index.html，并将context传递给它。Context是映射模板变量到Python对象的字典。

render()方法可以简化代码

# polls/views.py

from django.shortcuts import render

from .models import Question


def index(request):
    latest_question_list = Question.objects.order_by('-pub_date')[:5]
    context = {'latest_question_list': latest_question_list}
    return render(request, 'polls/index.html', context)

5. 抛出404错误

通过detail视图来示范实现

# polls/views.py

from django.http import Http404
from django.shortcuts import render

from .models import Question
# ...
def detail(request, question_id):
    try:
        question = Question.objects.get(pk=question_id)
    except Question.DoesNotExist:
        raise Http404("Question does not exist")
    return render(request, 'polls/detail.html', {'question': question})

同时对polls/detail.html进行修改

# polls/templates/polls/detail.html

{{ question }}

使用get_object_or_404()进行代码简化

#polls/views.py

from django.shortcuts import get_object_or_404, render

from .models import Question
# ...
def detail(request, question_id):
    question = get_object_or_404(Question, pk=question_id)
    return render(request, 'polls/detail.html', {'question': question})

 6. 使用模板系统

考虑context变量，对polls/detail.html进行修改

# polls/templates/polls/detail.html

<h1>{{ question.question_text }}</h1>
<ul>
{% for choice in question.choice_set.all %}
    <li>{{ choice.choice_text }}</li>
{% endfor %}
</ul>

7. 移除模板中的硬编码URL

比如polls/index.html，原代码不利于修改：

<li><a href="/polls/{{ question.id }}/">{{ question.question_text }}</a></li>

改进为：

<li><a href="{% url 'detail' question.id %}">{{ question.question_text }}</a></li>

这样改进过后，对URL的修改会变得很容易，比如，把以下代码

...
# the 'name' value as called by the {% url %} template tag
url(r'^(?P<question_id>[0-9]+)/$', views.detail, name='detail'),
...

改为

...
# added the word 'specifics'
url(r'^specifics/(?P<question_id>[0-9]+)/$', views.detail, name='detail'),
...

8. URL命名空间

一个项目里往往有很多应用，如果两个应用都有叫做detail的视图，Django会怎么区分它们的URL名称呢？答案是mysite/urls.py中可以添加命名空间到URLconf。比如：

# mysite/urls.py

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

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

同时将polls.index.html从

# polls/templates/polls/index.html

<li><a href="{% url 'detail' question.id %}">{{ question.question_text }}</a></li>

修改为

# polls/templates/polls/index.html

<li><a href="{% url 'polls:detail' question.id %}">{{ question.question_text }}</a></li>

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