Django文档阅读-Day3

Django文档阅读-Day3

目录

• Django文档阅读-Day3
  • Writing your first Django app, part 3
    • Overview
    • Writing more views
    • Writing views that actually do something
    • A shortcut: render()
    • Raising a 404 error
    • A shortcut: get_object_or_404()
    • Use the template system
    • Removing hardcoded URLs in templates
    • Namespacing URL names
  • Writing your first Django app, part 4
    • Write a minimal form
  • 今日收获单词

Writing your first Django app, part 3

Overview

A view is a “type” of Web page in your Django application that generally serves a specific function and has a specific template. For example, in a blog application, you might have the following views:

• Blog homepage – displays the latest few entries.
• Entry “detail” page – permalink page for a single entry.
• Year-based archive page – displays all months with entries in the given year.
• Month-based archive page – displays all days with entries in the given month.
• Day-based archive page – displays all entries in the given day.
• Comment action – handles posting comments to a given entry.

In our poll application, we’ll have the following four views:

• Question “index” page – displays the latest few questions.
• Question “detail” page – displays a question text, with no results but with a form to vote.
• Question “results” page – displays results for a particular question.
• Vote action – handles voting for a particular choice in a particular question.

In Django, web pages and other content are delivered by views. Each view is represented by a Python function (or method, in the case of class-based views). Django will choose a view by examining the URL that’s requested (to be precise, the part of the URL after the domain name).

Now in your time on the web you may have come across such beauties as “ME2/Sites/dirmod.asp?sid=&type=gen&mod=Core+Pages&gid=A6CD4967199A42D9B65B1B”. You will be pleased to know that Django allows us much more elegant URL patterns than that.

A URL pattern is the general form of a URL - for example: /newsarchive/<year>/<month>/.

To get from a URL to a view, Django uses what are known as ‘URLconfs’. A URLconf maps URL patterns to views.

This tutorial provides basic instruction in the use of URLconfs, and you can refer to URL dispatcher for more information.

一句话概括：Django通过路由映射找到视图函数，处理业务。

Writing more views

Now let’s add a few more views to polls/views.py. These views are slightly different, because they take an argument:

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)

Wire these new views into the polls.urls module by adding the following path() calls:

from django.urls import path

from . import views

urlpatterns = [
    # ex: /polls/
    path('', views.index, name='index'),
    # ex: /polls/5/
    path('<int:question_id>/', views.detail, name='detail'),
    # ex: /polls/5/results/
    path('<int:question_id>/results/', views.results, name='results'),
    # ex: /polls/5/vote/
    path('<int:question_id>/vote/', views.vote, name='vote'),
]

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.

When somebody requests a page from your website – say, “/polls/34/”, Django will load the mysite.urls Python module because it’s pointed to by the ROOT_URLCONF setting. It finds the variable named urlpatterns and traverses the patterns in order. After finding the match at 'polls/', it strips off the matching text ("polls/") and sends the remaining text – "34/" – to the ‘polls.urls’ URLconf for further processing. There it matches '<int:question_id>/', resulting in a call to the detail() view like so:

detail(request=<HttpRequest object>, question_id=34)

The question_id=34 part comes from <int:question_id>. Using angle brackets “captures” part of the URL and sends it as a keyword argument to the view function. The :question_id> part of the string defines the name that will be used to identify the matched pattern, and the <int: part is a converter that determines what patterns should match this part of the URL path.

There’s no need to add URL cruft such as .html – unless you want to, in which case you can do something like this:

path('polls/latest.html', views.index),

But, don’t do that. It’s silly.

哈哈，看来将Django团队很排挤在url中增加后缀名

Writing views that actually do something

Each view is responsible for doing one of two things: returning an HttpResponse object containing the content for the requested page, or raising an exception such as 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 HttpResponse. Or an exception.

Because it’s convenient, let’s use Django’s own database API, which we covered in Tutorial 2. Here’s one stab at a new index() view, which displays the latest 5 poll questions in the system, separated by commas, according to publication date:

from django.http import HttpResponse

from .models import Question

def index(request):
    latest_question_list = Question.objects.order_by('-pub_date')[:5]
    output = ', '.join([q.question_text for q in latest_question_list])
    return HttpResponse(output)

# Leave the rest of the views (detail, results, vote) unchanged

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 by creating a template that the view can use.

First, create a directory called templates in your polls directory. Django will look for templates in there.

Your project’s TEMPLATES setting describes how Django will load and render templates. The default settings file configures a DjangoTemplates backend whose APP_DIRS option is set to True. By convention DjangoTemplates looks for a “templates” subdirectory in each of the INSTALLED_APPS.

Within the templates directory you have just created, create another directory called polls, and within that create a file called index.html. In other words, your template should be at polls/templates/polls/index.html. Because of how the app_directories template loader works as described above, you can refer to this template within Django as polls/index.html.

Template namespacing

Now we might be able to get away with putting our templates directly in polls/templates (rather than creating another polls subdirectory), but it would actually be a bad idea. Django will choose the first template it finds whose name matches, and if you had a template with the same name in a different application, Django would be unable to distinguish between them. We need to be able to point Django at the right one, and the best way to ensure this is by namespacing them. That is, by putting those templates inside another directory named for the application itself.

虽然我们现在可以将模板文件直接放在polls/templates文件夹中(而不是在再建立一个polls子文件夹)，但是这样做肯定不好。Django将要选择第一个匹配的模板文件，如果你有一个模板文件正好和另一个应用中的某个模板文件重名，Django没有办法区分它们。我们可以帮助Django区分正确的模板，最简单的方法就是放入它们各自的命名空间，也就是吧这些模板放入一个和自身应用重名的子文件夹里。

思考：
直观上看通过polls/index.html及“应用名/模板名"就可以区分不同应用的模板呀，为什么Django会混淆呢。哈哈哈，猜测：Django在生命周期中会预先加载所有应用的模板文件，为他们创建索引。这样所有的模板文件都在一起。就会出现重名的情况。所以要在templates文件夹中再建立一个与app同名的子文件夹。

Put the following code in that template:

{% 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 %}

Now let’s update our index view in polls/views.py to use the template:

from django.http import HttpResponse #http响应在http中
from django.template import loader #html渲染相关操作在templates中

from .models import Question

def index(request):
    latest_question_list = Question.objects.order_by('-pub_date')[:5]
    template = loader.get_template('polls/index.html') #先取得模板对象
    #构建上下文
    context = {
        'latest_question_list': latest_question_list,
    }
    return HttpResponse(template.render(context, request)) #通过render函数渲染

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.

Load the page by pointing your browser at “/polls/”, and you should see a bulleted-list containing the “What’s up” question from Tutorial 2. The link points to the question’s detail page.

A shortcut: render()

It’s a very common idiom to load a template, fill a context and return an 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 #原来shortcuts库是Django给我们的语法糖啊。

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)

Note that once we’ve done this in all these views, we no longer need to import loader and HttpResponse (you’ll want to keep HttpResponse if you still have the stub methods for detail, results, and vote).

The render() function takes the request object as its first argument, a template name as its second argument and a dictionary as its optional third argument. It returns an HttpResponse object of the given template rendered with the given context.

render函数本质也是返回一个HttpResponse，不要忘了文档多次强调视图始终希望返回一个HttpResponse

对象或者一个异常比如Http404

Raising a 404 error

Now, let’s tackle the question detail view – the page that displays the question text for a given poll. Here’s the view:

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})

The new concept here: The view raises the Http404 exception if a question 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, a file containing just:

polls/templates/polls/detail.html

{{ question }}

will get you started for now.

A shortcut: get_object_or_404()

It’s a very common idiom to use get() and raise Http404 if the object doesn’t exist. Django provides a shortcut. Here’s the detail() view, rewritten:

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})

The 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 get() function of the model’s manager. It raises Http404 if the object doesn’t exist.

Philosophy

Why do we use a helper function get_object_or_404() instead of automatically catching the ObjectDoesNotExist exceptions at a higher level, or having the model API raise Http404 instead of 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. Some controlled coupling is introduced in the django.shortcuts module.

为什么我们使用辅助函数呢，而不是自己捕捉错误呢？还有为什么模型不直接抛出错误，而是抛出Http404错误呢？因为这样做会增加模型与视图层的耦合度。指导Django的设计思想之一是保证松耦合。一些受控的耦合将会被被包含在shortcuts库中。

There’s also a get_list_or_404() function, which works just as get_object_or_404() – except using filter() instead of get(). It raises Http404 if the list is empty.

Use the template system

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

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

The template system uses dot-lookup syntax to access variable attributes. In the example of {{ question.question_text }}, first Django does a dictionary lookup on the object question. Failing that, it tries an attribute lookup – which works, in this case. If attribute lookup had failed, it would’ve tried a list-index lookup.

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

See the template guide for more about templates.

Removing hardcoded URLs in templates

Remember, when we wrote the link to a question in the polls/index.html template, the link was partially hardcoded like this:

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

The problem with this hardcoded, tightly-coupled approach is that it becomes challenging to change URLs on projects with a lot of templates. However, since you defined the name argument in the path() functions in the polls.urls module, you can remove a reliance on specific URL paths defined in your url configurations by using the {% url %} template tag:

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

The way this works is by looking up the URL definition as specified in the polls.urls module. You can see exactly where the URL name of ‘detail’ is defined below:

...
# the 'name' value as called by the {% url %} template tag
path('<int:question_id>/', views.detail, name='detail'),
...

If you want to change the URL of the polls detail view to something else, perhaps to something like polls/specifics/12/ instead of doing it in the template (or templates) you would change it in polls/urls.py:

...
# added the word 'specifics'
path('specifics/<int:question_id>/', views.detail, name='detail'),
...

Namespacing URL names

The tutorial project has just one app, polls. In real Django projects, there might be five, ten, twenty apps or more. How does Django differentiate the URL names between them? For example, the polls app has a detail view, and so might an app on the same project that is for a blog. How does one make it so that Django knows which app view to create for a url when using the {% url %} template tag?

The answer is to add namespaces to your URLconf. In the polls/urls.py file, go ahead and add an app_name to set the application namespace:

from django.urls import path

from . import views

#创建app命名空间
app_name = 'polls'
urlpatterns = [
    path('', views.index, name='index'),
    path('<int:question_id>/', views.detail, name='detail'),
    path('<int:question_id>/results/', views.results, name='results'),
    path('<int:question_id>/vote/', views.vote, name='vote'),
]

Now change your polls/index.html template from:

polls/templates/polls/index.html

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

to point at the namespaced detail view:

polls/templates/polls/index.html

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

When you’re comfortable with writing views, read part 4 of this tutorial to learn the basics about form processing and generic views.

Writing your first Django app, part 4

This tutorial begins where Tutorial 3 left off. We’re continuing the Web-poll application and will focus on form processing and cutting down our code.

Write a minimal form

Let’s update our poll detail template (“polls/detail.html”) from the last tutorial, so that the template contains an HTML <form> element:

<h1>{{ question.question_text }}</h1>

{% if error_message %}<p><strong>{{ error_message }}</strong></p>{% endif %}

<form action="{% url 'polls:vote' question.id %}" method="post">
{% csrf_token %}
{% for choice in question.choice_set.all %}
    <input type="radio" name="choice" id="choice{{ forloop.counter }}" value="{{ choice.id }}">
    <label for="choice{{ forloop.counter }}">{{ choice.choice_text }}</label><br>
{% endfor %}
<input type="submit" value="Vote">
</form>

A quick rundown:

• The above template displays a radio button for each question choice. The value of each radio button is the associated question choice’s ID. The name of each radio button is "choice". That means, when somebody selects one of the radio buttons and submits the form, it’ll send the POST data choice=# where # is the ID of the selected choice. This is the basic concept of HTML forms.

• We set the form’s action to {% url 'polls:vote' question.id %}, and we set method="post". Using method="post" (as opposed to method="get") is very important, because the act of submitting this form will alter data server-side. Whenever you create a form that alters data server-side, use method="post". This tip isn’t specific to Django; it’s good Web development practice in general.

• forloop.counter indicates how many times the for tag has gone through its loop

• Since we’re creating a POST form (which can have the effect of modifying data), we need to worry about Cross Site Request Forgeries. Thankfully, you don’t have to worry too hard, because Django comes with a helpful system for protecting against it. In short, all POST forms that are targeted at internal URLs should use the {% csrf_token %} template tag.

Now, let’s create a Django view that handles the submitted data and does something with it. Remember, in Tutorial 3, we created a URLconf for the polls application that includes this line:

path('<int:question_id>/vote/', views.vote, name='vote'),

We also created a dummy implementation of the vote() function. Let’s create a real version. Add the following to polls/views.py:

from django.http import HttpResponse, HttpResponseRedirect
from django.shortcuts import get_object_or_404, render
from django.urls import reverse

from .models import Choice, Question
# ...
def vote(request, question_id):
    question = get_object_or_404(Question, pk=question_id)
    try:
        selected_choice = question.choice_set.get(pk=request.POST['choice'])
    except (KeyError, Choice.DoesNotExist):
        # Redisplay the question voting form.
        return render(request, 'polls/detail.html', {
            'question': question,
            'error_message': "You didn't select a choice.",
        })
    else:
        selected_choice.votes += 1
        selected_choice.save()
        # Always return an HttpResponseRedirect after successfully dealing
        # with POST data. This prevents data from being posted twice if a
        # user hits the Back button.
        return HttpResponseRedirect(reverse('polls:results', args=(question.id,)))

This code includes a few things we haven’t covered yet in this tutorial:

• request.POST is a dictionary-like object that lets you access submitted data by key name. In this case, request.POST['choice'] returns the ID of the selected choice, as a string. request.POST values are always strings.

  Note that Django also provides request.GET for accessing GET data in the same way – but we’re explicitly using request.POST in our code, to ensure that data is only altered via a POST call.

• request.POST['choice'] will raise KeyError if choice wasn’t provided in POST data. The above code checks for KeyError and redisplays the question form with an error message if choice isn’t given.

• After incrementing the choice count, the code returns an HttpResponseRedirect rather than a normal HttpResponse. HttpResponseRedirect takes a single argument: the URL to which the user will be redirected (see the following point for how we construct the URL in this case).

  As the Python comment above points out, you should always return an HttpResponseRedirect after successfully dealing with POST data. This tip isn’t specific to Django; it’s good Web development practice in general.

• We are using the reverse() function in the HttpResponseRedirect constructor in this example. This function helps avoid having to hardcode a URL in the view function. It is given the name of the view that we want to pass control to and the variable portion of the URL pattern that points to that view. In this case, using the URLconf we set up in Tutorial 3, this reverse() call will return a string like

  '/polls/3/results/'
  

where the 3 is the value of question.id. This redirected URL will then call the 'results' view to display the final page.

今日收获单词

specific  /spəˈsɪfɪk/  特殊的，特定的
permalink 永久链接
permanent 永久的
archive 把...存档 档案文件
entry 条目，入口，进入
action 功能
come across 偶遇，无意中发现
precise 精确的
wire 打电报，给...装电线
wire into 把...装进
traverse 遍历，穿过，横贯
angle 角度，视角
bracket 支架，把...归为同一类，括号
angle bracket 尖括号
cruft 令人讨厌的东西
stab 尝试，刺，戳
the rest of 其余的
get away with 侥幸做成，侥幸惩罚
bulleted-list 无序列表
idiom 习语，土话
stub 存根，烟蒂
tackle 处理，应对，与人交涉
hardcoded 硬编码
reliance 信赖，信心
differentiate 区分，区别
rundown 概要，纲要
server-side 服务器端
dummy 虚拟的，人体模型，假的，蠢得
portion 部分