Django Book 翻译

Chapter 9: Generic Views
------------------------

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Here again is a recurring theme of this book: at its worst, Web development is
boring and monotonous. So far, weve covered how Django tries to take away some
of that monotony at the model and template layers, but Web developers also
experience this boredom at the view level.

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Djangos *generic views* were developed to ease that pain. They take certain
common idioms and patterns found in view development and abstract them so that
you can quickly write common views of data without having to write too much
code. In fact, nearly every view example in the preceding chapters could be
rewritten with the help of generic views.

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Chapter 8 touched briefly on how youd go about making a view generic. To
review, we can recognize certain common tasks, like displaying a list of
objects, and write code that displays a list of *any* object. Then the model in
question can be passed as an extra argument to the URLconf.

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Django ships with generic views to do the following:

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*   Perform common simple tasks: redirect to a different page and render a
    given template.

* 完成常用的简单任务：重定向到另一个页面以及渲染一个指定的模板。

*   Display list and detail pages for a single object. The ``event_list`` and
    ``entry_list`` views from Chapter 8 are examples of list views. A single
    event page is an example of what we call a detail view.

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*   Present date-based objects in year/month/day archive pages, associated
    detail, and latest pages. The Django Weblogs
    (`http://www.djangoproject.com/weblog/`_) year, month, and day archives are
    built with these, as would be a typical newspapers archives.

*   呈现基于日期的数据的年/月/日归档页面，关联的详情页面，最新页面。Django Weblogs
    (`http://www.djangoproject.com/weblog/`_)的年、月、日的归档就是使用通用视图
    架构的，就像是典型的新闻报纸归档。

*   Allow users to create, update, and delete objects with or without
    authorization.

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Taken together, these views provide easy interfaces to perform the most common
tasks developers encounter.

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Using Generic Views
```````````````````

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All of these views are used by creating configuration dictionaries in your
URLconf files and passing those dictionaries as the third member of the URLconf
tuple for a given pattern.

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For example, heres a simple URLconf you could use to present a static about
page:

例如，下面是一个呈现静态“关于”页面的URLconf：

Though this might seem a bit magical at first glance look, a view with no code!
, its actually exactly the same as the examples in Chapter 8: the
``direct_to_template`` view simply grabs information from the extra-parameters
dictionary and uses that information when rendering the view.

一眼看上去似乎有点不可思议，不需要编写代码的视图！它和第八章中的例子完全一样：
``direct_to_template`` 视图从参数中获取渲染视图所需的相关信息。

Because this generic view and all the others is a regular view functions like
any other, we can reuse it inside our own views. As an example, lets extend our
about example to map URLs of the form ``/about/<whatever>/`` to statically
rendered ``about/<whatever>.html`` . Well do this by first modifying the
URLconf to point to a view function:

因为通用视图都是标准的视图函数，我们可以在我们自己的视图中重用它。例如，我们扩展
about例子把映射的URL从 ``/about/<whatever>/`` 到一个静态渲染 ``about/<whatever>.html`` 。
我们首先修改URL配置到新的视图函数：

Next, well write the ``about_pages`` view:

接下来，我们编写 ``about_pages`` 视图的代码：

Here were treating ``direct_to_template`` like any other function. Since it
returns an ``HttpResponse`` , we can simply return it as-is. The only slightly
tricky business here is dealing with missing templates. We dont want a
nonexistent template to cause a server error, so we catch
``TemplateDoesNotExist`` exceptions and return 404 errors instead.

在这里我们象使用其他函数一样使用 ``direct_to_template`` 。因为它返回一个 ``HttpResponse``
对象，我们只需要简单的返回它就好了。有一个稍微复杂的地方，要处理没有找到模板的情况。
我们不希望一个不存在的模板引发服务器错误，所以我们捕捉 ``TempalteDoesNotExist`` 异常
并返回404错误。

Is There a Security Vulnerability Here?

这里有没有安全性问题？

Sharp-eyed readers may have noticed a possible security hole: were constructing
the template name using interpolated content from the browser
(``template="about/%s.html" % page`` ). At first glance, this looks like a
classic *directory traversal* vulnerability (discussed in detail in Chapter
19). But is it really?

眼尖的读者可能已经注意到一个可能的安全漏洞：我们直接使用从客户端浏览器来的数据构造
模板名称(``template="about/%s.html" % page`` )。乍看起来，这像是一个经典的
*目录遍历（directory traversal）* 攻击（详情请看第十九章）。事实真是这样吗？

Not exactly. Yes, a maliciously crafted value of ``page`` could cause directory
traversal, but although ``page`` *is* taken from the request URL, not every
value will be accepted. They key is in the URLconf: were using the regular
expression ``\w+`` to match the ``page`` part of the URL, and ``\w`` only
accepts letters and numbers. Thus, any malicious characters (dots and slashes,
here) will be rejected by the URL resolver before they reach the view itself.

完全不是。是的，一个恶意的 ``page`` 值可以导致目录跨越，但是尽管 ``page`` *是* 从
请求的URL中获取的，并不是所有的值都被接受。这就是URL配置的关键所在：我们使用正则表达式
``\w+`` 来从URL里匹配 ``page`` ，而 ``\w`` 只接受字符和数字。因此，任何恶意的字符
（例如在这里是点 ``.`` 和正斜线 ``/`` ）将在URL解析时被拒绝，根本不会传递给视图函数。

Generic Views of Objects
````````````````````````

对象的通用视图
````````````````````````

The ``direct_to_template`` certainly is useful, but Djangos generic views
really shine when it comes to presenting views on your database content.
Because its such a common task, Django comes with a handful of built-in generic
views that make generating list and detail views of objects incredibly easy.

``direct_to_template`` 毫无疑问是非常有用的，但Django通用视图最有用的是在呈现
数据库中的数据。因为这个应用实在太普遍了，Django带有很多内建的通用视图来帮助你很容易
的生成对象的列表和明细视图。

Lets take a look at one of these generic views: the object list view. Well be
using this ``Publisher`` object from Chapter 5:

让我们先看看其中的一个通用视图：对象列表视图。我们使用第五章中的 ``Publisher`` 
来举例：

To build a list page of all books, wed use a URLconf along these lines:

要为所有的书籍创建一个列表页面，我们使用下面的URL配置：

Thats all the Python code we need to write. We still need to write a template,
however. We could explicitly tell the ``object_list`` view which template to
use by including a ``template_name`` key in the extra arguments dictionary, but
in the absence of an explicit template Django will infer one from the objects
name. In this case, the inferred template will be
``"books/publisher_list.html"`` the books part comes from the name of the app
that defines the model, while the publisher bit is just the lowercased version
of the models name.

这就是所要编写的所有Python代码。当然，我们还需要编写一个模板。我们可以通过在额外参数
字典里包含 ``template_name`` 来清楚的告诉 ``object_list`` 视图使用哪个模板，但是
由于Django在不给定模板的时候会用对象的名称推导出一个。在这个例子中，这个推导出的模板名称
将是 ``"books/publisher_list.html"`` ，其中books部分是定义这个模型的app的名称，
publisher部分是这个模型名称的小写。

This template will be rendered against a context containing a variable called
``object_list`` that contains all the book objects. A very simple template
might look like the following:

这个模板将按照 context 中包含的变量 ``object_list`` 来渲染，这个变量包含所有的书籍对象。
一个非常简单的模板看起来象下面这样：

Thats really all there is to it. All the cool features of generic views come
from changing the info dictionary passed to the generic view. Appendix D
documents all the generic views and all their options in detail; the rest of
this chapter will consider some of the common ways you might customize and
extend generic views.

这就是所有要做的事。要使用通用视图酷酷的特性只需要修改参数字典并传递给通用视图函数。
附录D是通用视图的完全参考资料；本章接下来的章节将讲到自定义和扩展通用视图的一些方法。

Extending Generic Views
```````````````````````

扩展通用视图
```````````````````````

Theres no question that using generic views can speed up development
substantially. In most projects, however, there comes a moment when the generic
views no longer suffice. Indeed, the most common question asked by new Django
developers is how to make generic views handle a wider array of situations.

毫无疑问，使用通用视图可以充分加快开发速度。然而，在多数的工程中，也会出现通用视图不能
满足需求的情况。实际上，刚接触Django的开发者最常见的问题就是怎样使用通用视图来处理更多的情况。

Luckily, in nearly every one of these cases, there are ways to simply extend
generic views to handle a larger array of use cases. These situations usually
fall into a handful of patterns dealt with in the sections that follow.

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Making Friendly Template Contexts
'''''''''''''''''''''''''''''''''

制作友好的模板Context
'''''''''''''''''''''''''''''''''

You might have noticed that sample publisher list template stores all the books
in a variable named ``object_list`` . While this works just fine, it isnt all
that friendly to template authors: they have to just know that theyre dealing
with books here. A better name for that variable would be ``publisher_list`` ;
that variables content is pretty obvious.

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We can change the name of that variable easily with the
``template_object_name`` argument:

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Providing a useful ``template_object_name`` is always a good idea. Your
coworkers who design templates will thank you.

使用有用的 ``template_object_name`` 总是个好想法。你的设计模板的合作伙伴会感谢你的。

Adding Extra Context
''''''''''''''''''''

添加额外的Context
''''''''''''''''''''

Often you simply need to present some extra information beyond that provided by
the generic view. For example, think of showing a list of all the other
publishers on each publisher detail page. The ``object_detail`` generic view
provides the publisher to the context, but it seems theres no way to get a list
of *all* publishers in that template.

你常常需要呈现比通用视图提供的更多的额外信息。例如，考虑一下在每个出版商页面实现所有其他
出版商列表。 ``object_detail`` 通用视图提供了出版商到context，但是看起来没有办法在模板中
获取 *所有* 出版商列表。

But there is: all generic views take an extra optional parameter,
``extra_context`` . This is a dictionary of extra objects that will be added to
the templates context. So, to provide the list of all publishers on the detail
detail view, wed use an info dict like this:

这是解决方法：所有的通用视图都有一个额外的可选参数 ``extra_context`` 。这个参数是一个字典数据类型，包含要添加到模板的context中的额外的对象。所以要提供所有的出版商明细给视图，我们就用这样的info字典：

This would populate a ``{{ book_list }}`` variable in the template context.
This pattern can be used to pass any information down into the template for the
generic view. Its very handy.

这样就把一个 ``{{ book_list }}`` 变量放到模板的context中。这个方法可以用来传递任意数据
到通用视图模板中去，非常方便。

However, theres actually a subtle bug here can you spot it?

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The problem has to do with when the queries in ``extra_context`` are evaluated.
Because this example puts ``Publisher.objects.all()`` in the URLconf, it will
be evaluated only once (when the URLconf is first loaded). Once you add or
remove publishers, youll notice that the generic view doesnt reflect those
changes until you reload the Web server (see Caching and QuerySets in Appendix
C for more information about when QuerySets are cached and evaluated).

我们现在来看一下， ``extra_context`` 里包含数据库查询的问题。因为在这个例子中，我们把 ``Publisher.objects.all()`` 放在URLconf中，它只会执行一次（当URLconf第一次加载的时候）。当你添加或删除出版商，你会发现在重启Web服务器之前，通用视图不会反映出这些修改的（有关QuerySet何时被缓存和赋值的更多信息请参考附录C中“缓存与查询集”一节）。

Note

备注

This problem doesnt apply to the ``queryset`` generic view argument. Since
Django knows that particular QuerySet should *never* be cached, the generic
view takes care of clearing the cache when each view is rendered.

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The solution is to use a callback in ``extra_context`` instead of a value. Any
callable (i.e., a function) thats passed to ``extra_context`` will be evaluated
when the view is rendered (instead of only once). You could do this with an
explicitly defined function:

解决这个问题的办法是在 ``extra_context`` 中用一个回调（callback）来
代替使用一个变量。任何可以调用的对象（例如一个函数）在传递给 ``extra_context`` 后都会在每次视图渲染前执行（而不是只执行一次）。
你可以象这样定义一个函数：

or you could use a less obvious but shorter version that relies on the fact
that ``Publisher.objects.all`` is itself a callable:

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Notice the lack of parentheses after ``Book.objects.all`` ; this references the
function without actually calling it (which the generic view will do later).

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Viewing Subsets of Objects
''''''''''''''''''''''''''

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Now lets take a closer look at this ``queryset`` key weve been using all along.
Most generic views take one of these ``queryset`` arguments its how the view
knows which set of objects to display (see Selecting Objects in Chapter 5 for
an introduction to QuerySets, and see Appendix C for the complete details).

现在让我们来仔细看看这个 ``queryset`` 。大多数通用视图有一个 ``queryset`` 
参数，这个参数告诉视图要显示对象的集合 （有关QuerySet的解释请看第五章的
“选择对象”章节，详细资料请参看附录C）。

To pick a simple example, we might want to order a list of books by publication
date, with the most recent first:

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Thats a pretty simple example, but it illustrates the idea nicely. Of course,
youll usually want to do more than just reorder objects. If you want to present
a list of books by a particular publisher, you can use the same technique:

这是一个相当简单的例子，但是很说明问题。当然，你通常还想做比重新排序更多的事。
如果你想要呈现某个特定出版商出版的所有书籍列表，你可以使用同样的技术：

Notice that along with a filtered ``queryset`` , were also using a custom
template name. If we didnt, the generic view would use the same template as the
vanilla object list, which might not be what we want.

注意 在使用一个过滤的 ``queryset`` 的同时，我们还使用一个自定义的模板名称。
如果我们不这么做，通用视图就会用以前的模板，这可能不是我们想要的结果。

Also notice that this isnt a very elegant way of doing publisher-specific
books. If we want to add another publisher page, wed need another handful of
lines in the URLconf, and more than a few publishers would get unreasonable.
Well deal with this problem in the next section.

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Note

备注

If you get a 404 when requesting ``/books/apress/`` , check to ensure you
actually have a Publisher with the name Apress Publishing. Generic views have
an ``allow_empty`` parameter for this case. See Appendix D for more details.

如果你在请求 ``/books/apress/`` 时出现404错误，请检查以确保你的数据库中出版商
中有名为Apress Publishing的记录。通用视图有一个 ``allow_empty`` 参数可以
用来处理这个情况，详情请看附录D。

Complex Filtering with Wrapper Functions
''''''''''''''''''''''''''''''''''''''''

用函数包装来处理复杂的数据过滤
''''''''''''''''''''''''''''''''''''''''

Another common need is to filter down the objects given in a list page by some
key in the URL. Earlier we hard-coded the publishers name in the URLconf, but
what if we wanted to write a view that displayed all the books by some
arbitrary publisher? We can wrap the ``object_list`` generic view to avoid
writing a lot of code by hand. As usual, well start by writing a URLconf:

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Next, well write the ``books_by_publisher`` view itself:

接下来，我们写 ``books_by_publisher`` 这个视图：（上面的代码中正则表达式有误，在 ``w`` 前要加反斜線）

This works because theres really nothing special about generic views theyre
just Python functions. Like any view function, generic views expect a certain
set of arguments and return ``HttpResponse`` objects. Thus, its incredibly easy
to wrap a small function around a generic view that does additional work before
(or after; see the next section) handing things off to the generic view.

这是因为通用视图就是Python函数。和其他的视图函数一样，通用视图也是接受一些
参数并返回 ``HttpResponse`` 对象。因此，通过包装通用视图函数可以做更多的事。

Note

注意

Notice that in the preceding example we passed the current publisher being
displayed in the ``extra_context`` . This is usually a good idea in wrappers of
this nature; it lets the template know which parent object is currently being
browsed.

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Performing Extra Work
'''''''''''''''''''''

处理额外工作
'''''''''''''''''''''

The last common pattern well look at involves doing some extra work before or
after calling the generic view.

我们再来看看最后一个常用模式：在调用通用视图前后做些额外工作。

Imagine we had a ``last_accessed`` field on our ``Author`` object that we were
using to keep track of the last time anybody looked at that author. The generic
``object_detail`` view, of course, wouldnt know anything about this field, but
once again we could easily write a custom view to keep that field updated.

想象一下我们在 ``Author`` 对象里有一个 ``last_accessed`` 字段，我们用这个字段来更正对author的最近访问时间。当然通用视图 ``object_detail`` 并不能处理
这个问题，我们可以很容易的写一个自定义的视图来更新这个字段。

First, wed need to add an author detail bit in the URLconf to point to a custom
view:

首先，我们需要在URL配置里设置指向到新的自定义视图：

Then wed write our wrapper function:

接下来写包装函数：

Note

注意

This code wont actually work unless you add a ``last_accessed`` field to your
``Author`` model and create a ``books/author_detail.html`` template.

除非你添加 ``last_accessed`` 字段到你的 ``Author`` 模型并创建
``books/author_detail.html`` 模板，否则这段代码不能真正工作。

We can use a similar idiom to alter the response returned by the generic view.
If we wanted to provide a downloadable plain-text version of the list of
authors, we could use a view like this:

我们可以用同样的方法修改通用视图的返回值。如果我们想要提供一个供下载用的
纯文本版本的author列表，我们可以用下面这个视图：

This works because the generic views return simple ``HttpResponse`` objects
that can be treated like dictionaries to set HTTP headers. This
``Content-Disposition`` business, by the way, instructs the browser to download
and save the page instead of displaying it in the browser.

这个方法之所以工作是因为通用视图返回的 ``HttpResponse`` 对象可以象一个字典
一样的设置HTTP的头部。随便说一下，这个 ``Content-Disposition`` 的含义是
告诉浏览器下载并保存这个页面，而不是在浏览器中显示它。

Whats Next?
```````````

下一步
```````````

In this chapter we looked at only a couple of the generic views Django ships
with, but the general ideas presented here should apply pretty closely to any
generic view. Appendix D covers all the available views in detail, and its
recommended reading if you want to get the most out of this powerful feature.

在这一章我们只讲了Django带的通用视图其中一部分，不过这些方法也适用于其他的
通用视图。有关更详细的内容，请看附录D。

In the next chapter we delve deep into the inner workings of Djangos templates,
showing all the cool ways they can be extended. Until now, weve treated the
template engine as a mostly static tool you can use to render your content.

在下一章中我们将深入到Django模板系统的内部去，展示所有扩展它的酷方法。目前
为止，我们还只是把模板引擎当作一个渲染内容的静态工具。