# Django 2.2.9.dev20191202104323 文档

# 模板

作为一个Web框架，Django需要一种动态生成HTML的便捷方法。最常用的方法依赖于模板。模板包含所需HTML输出的静态部分以及描述动态内容将被插入的一些特殊语法。有关创建带有模板的HTML页面的示例，请参阅:doc:`Tutorial 3</intro/tutorial03>

Django项目可以配置一个或多个模板引擎（或者如果不使用模板，甚至为零）。Django后端内置一个自己的模板系统，创造性地称为Django template language（DTL），和一个流行的替代品JICAN2*。后端也可以使用第三方提供其他可用的模板语言。

Django定义了一个标准的API，用于加载和渲染模板，而不用考虑后端的模板系统。加载包括查找给定标识符的模板并对其进行预处理，通常将其编译的结果保存在内存中。渲染工具将上下文数据插入模板并返回结果字符串。

Doc：Django template language </ref/templates/language>是Django自己的模板系统。直到Django 1.8，它是唯一可用的内置选项。这是一个很好的模板库，即使它是相当僵硬和使用时带有它自己特质。如果您没有紧迫的理由需要去选择另一个后端，则应该使用DTL，特别是如果您正在编写可插入的应用程序并打算分发模板。在 Django's contrib apps 中的有些模板，比如:doc:`django.contrib.admin </ref/contrib/admin/index>，使用DTL。

由于历史原因，模板引擎的通用支持和Django模板语言的实现都存在于``django.template`` 模块的命名空间中。

警告

模板系统使用不可信的模板作者的模板是不安全的。例如，一个站点不应该允许它的用户提供他们自己的模板，因为模板作者可以做一些事情，比如执行XSS攻击和拿到包含敏感信息的模板变量的访问权。

## 模板引擎的支持

### 配置

Templates engines are configured with the `TEMPLATES` setting. It's a list of configurations, one for each engine. The default value is empty. The `settings.py` generated by the `startproject` command defines a more useful value:

    TEMPLATES = [
        {
            'BACKEND': 'django.template.backends.django.DjangoTemplates',
            'DIRS': [],
            'APP_DIRS': True,
            'OPTIONS': {
                # ... some options here ...
            },
        },
    ]

`BACKEND` is a dotted Python path to a template engine class implementing Django's template backend API. The built-in backends are `django.template.backends.django.DjangoTemplates` and `django.template.backends.jinja2.Jinja2`.

Since most engines load templates from files, the top-level configuration for each engine contains two common settings:

- `DIRS` defines a list of directories where the engine should look for template source files, in search order.
- `APP_DIRS` tells whether the engine should look for templates inside installed applications. Each backend defines a conventional name for the subdirectory inside applications where its templates should be stored.

While uncommon, it's possible to configure several instances of the same backend with different options. In that case you should define a unique `NAME` for each engine.

`OPTIONS` contains backend-specific settings.

### 用法

The `django.template.loader` module defines two functions to load templates.

`get_template`(template_name, using=None)

  This function loads the template with the given name and returns a `Template` object.

  The exact type of the return value depends on the backend that loaded the template. Each backend has its own `Template` class.

  `get_template()` tries each template engine in order until one succeeds. If the template cannot be found, it raises `TemplateDoesNotExist`. If the template is found but contains invalid syntax, it raises `TemplateSyntaxError`.

  How templates are searched and loaded depends on each engine's backend and configuration.

  If you want to restrict the search to a particular template engine, pass the engine's `NAME` in the `using` argument.

`select_template`(template_name_list, using=None)

  `select_template()` is just like `get_template()`, except it takes a list of template names. It tries each name in order and returns the first template that exists.

If loading a template fails, the following two exceptions, defined in `django.template`, may be raised:

exception `TemplateDoesNotExist`(msg, tried=None, backend=None, chain=None)

  This exception is raised when a template cannot be found. It accepts the following optional arguments for populating the template postmortem on the debug page:

  `backend`
    The template backend instance from which the exception originated.
  `tried`
    A list of sources that were tried when finding the template. This is formatted as a list of tuples containing `(origin, status)`, where `origin` is an origin-like object and `status` is a string with the reason the template wasn't found.
  `chain`
    A list of intermediate `TemplateDoesNotExist` exceptions raised when trying to load a template. This is used by functions, such as `get_template()`, that try to load a given template from multiple engines.

exception `TemplateSyntaxError`(msg)

  This exception is raised when a template was found but contains errors.

`Template` objects returned by `get_template()` and `select_template()` must provide a `render()` method with the following signature:

`Template.``render`(context=None, request=None)

  Renders this template with a given context.

  If `context` is provided, it must be a `dict`. If it isn't provided, the engine will render the template with an empty context.

  If `request` is provided, it must be an `HttpRequest`. Then the engine must make it, as well as the CSRF token, available in the template. How this is achieved is up to each backend.

Here's an example of the search algorithm. For this example the `TEMPLATES` setting is:

    TEMPLATES = [
        {
            'BACKEND': 'django.template.backends.django.DjangoTemplates',
            'DIRS': [
                '/home/html/example.com',
                '/home/html/default',
            ],
        },
        {
            'BACKEND': 'django.template.backends.jinja2.Jinja2',
            'DIRS': [
                '/home/html/jinja2',
            ],
        },
    ]

If you call `get_template('story_detail.html')`, here are the files Django will look for, in order:

- `/home/html/example.com/story_detail.html` (`'django'` engine)
- `/home/html/default/story_detail.html` (`'django'` engine)
- `/home/html/jinja2/story_detail.html` (`'jinja2'` engine)

If you call `select_template(['story_253_detail.html', 'story_detail.html'])`, here's what Django will look for:

- `/home/html/example.com/story_253_detail.html` (`'django'` engine)
- `/home/html/default/story_253_detail.html` (`'django'` engine)
- `/home/html/jinja2/story_253_detail.html` (`'jinja2'` engine)
- `/home/html/example.com/story_detail.html` (`'django'` engine)
- `/home/html/default/story_detail.html` (`'django'` engine)
- `/home/html/jinja2/story_detail.html` (`'jinja2'` engine)

When Django finds a template that exists, it stops looking.

Tip

You can use `select_template()` for flexible template loading. For example, if you've written a news story and want some stories to have custom templates, use something like `select_template(['story_%s_detail.html' % story.id, 'story_detail.html'])`. That'll allow you to use a custom template for an individual story, with a fallback template for stories that don't have custom templates.

It's possible -- and preferable -- to organize templates in subdirectories inside each directory containing templates. The convention is to make a subdirectory for each Django app, with subdirectories within those subdirectories as needed.

Do this for your own sanity. Storing all templates in the root level of a single directory gets messy.

To load a template that's within a subdirectory, just use a slash, like so:

    get_template('news/story_detail.html')

Using the same `TEMPLATES` option as above, this will attempt to load the following templates:

- `/home/html/example.com/news/story_detail.html` (`'django'` engine)
- `/home/html/default/news/story_detail.html` (`'django'` engine)
- `/home/html/jinja2/news/story_detail.html` (`'jinja2'` engine)

此外，为了减少加载和渲染模板的重复性，Django 提供了一个自动处理的快捷函数。

`render_to_string`(template_name, context=None, request=None, using=None)

  `render_to_string()` 加载一个模板 `get_template()` ，并立即调用它的 `render()` 方法。它需要下面的参数。

  `template_name`
    加载和呈现模板的名称。如果是模板名称列表，Django 使用 `select_template()` ，而不是 `get_template()` 找到模板。
  `context`
    `dict` 用作模板的渲染上下文。
  `request`
    可选项 `HttpRequest` 在模板的渲染过程中可用。
  `using`
    可选的模板引擎 `NAME`。对模板的搜索将限于该引擎。

  使用实例：

      from django.template.loader import render_to_string
      rendered = render_to_string('my_template.html', {'foo': 'bar'})

还可以参看 `render()` 快捷函数，它调用 `render_to_string()` ，并将结果提供给 `HttpResponse` ，适合从视图返回。

最后，您可以直接使用配置好的引擎：

`engines`

  模板引擎可在 `django.template.engines` 中使用：

      from django.template import engines

      django_engine = engines['django']
      template = django_engine.from_string("Hello {{ name }}!")

  在这个例子中，查找关键字“django”是引擎的 `NAME`。

### 内置后端

class `DjangoTemplates`

设置 `BACKEND` 为 `'django.template.backends.django.DjangoTemplates'`，以配置 Django 模板引擎。

When `APP_DIRS` is `True`, `DjangoTemplates` engines look for templates in the `templates` subdirectory of installed applications. This generic name was kept for backwards-compatibility.

`DjangoTemplates` engines accept the following `OPTIONS`:

- `'autoescape'`: a boolean that controls whether HTML autoescaping is enabled.

  It defaults to `True`.

  警告

  Only set it to `False` if you're rendering non-HTML templates!

- `'context_processors'`: a list of dotted Python paths to callables that are used to populate the context when a template is rendered with a request. These callables take a request object as their argument and return a `dict` of items to be merged into the context.

  It defaults to an empty list.

  See `RequestContext` for more information.

- `'debug'`: a boolean that turns on/off template debug mode. If it is `True`, the fancy error page will display a detailed report for any exception raised during template rendering. This report contains the relevant snippet of the template with the appropriate line highlighted.

  It defaults to the value of the `DEBUG` setting.

- `'loaders'`: a list of dotted Python paths to template loader classes. Each `Loader` class knows how to import templates from a particular source. Optionally, a tuple can be used instead of a string. The first item in the tuple should be the `Loader` class name, and subsequent items are passed to the `Loader` during initialization.

  The default depends on the values of `DIRS` and `APP_DIRS`.

  See Loader types for details.

- `'string_if_invalid'`: the output, as a string, that the template system should use for invalid (e.g. misspelled) variables.

  It defaults to an empty string.

  See How invalid variables are handled for details.

- `'file_charset'`: the charset used to read template files on disk.

  It defaults to `'utf-8'`.

- `'libraries'`: A dictionary of labels and dotted Python paths of template tag modules to register with the template engine. This can be used to add new libraries or provide alternate labels for existing ones. For example:

      OPTIONS={
          'libraries': {
              'myapp_tags': 'path.to.myapp.tags',
              'admin.urls': 'django.contrib.admin.templatetags.admin_urls',
          },
      }

  Libraries can be loaded by passing the corresponding dictionary key to the `{% load %}` tag.

- `'builtins'`: A list of dotted Python paths of template tag modules to add to built-ins. For example:

      OPTIONS={
          'builtins': ['myapp.builtins'],
      }

  Tags and filters from built-in libraries can be used without first calling the `{% load %}` tag.

class `Jinja2`

Requires Jinja2 to be installed:

/ 

    $ pip install Jinja2

    ...\> pip install Jinja2

Set `BACKEND` to `'django.template.backends.jinja2.Jinja2'` to configure a Jinja2 engine.

When `APP_DIRS` is `True`, `Jinja2` engines look for templates in the `jinja2` subdirectory of installed applications.

The most important entry in `OPTIONS` is `'environment'`. It's a dotted Python path to a callable returning a Jinja2 environment. It defaults to `'jinja2.Environment'`. Django invokes that callable and passes other options as keyword arguments. Furthermore, Django adds defaults that differ from Jinja2's for a few options:

- `'autoescape'`: `True`
- `'loader'`: a loader configured for `DIRS` and `APP_DIRS`
- `'auto_reload'`: `settings.DEBUG`
- `'undefined'`: `DebugUndefined if settings.DEBUG else Undefined`

`Jinja2` engines also accept the following `OPTIONS`:

- `'context_processors'`: a list of dotted Python paths to callables that are used to populate the context when a template is rendered with a request. These callables take a request object as their argument and return a `dict` of items to be merged into the context.

  It defaults to an empty list.

  Using context processors with Jinja2 templates is discouraged.

  Context processors are useful with Django templates because Django templates don't support calling functions with arguments. Since Jinja2 doesn't have that limitation, it's recommended to put the function that you would use as a context processor in the global variables available to the template using `jinja2.Environment` as described below. You can then call that function in the template:

      {{ function(request) }}

  Some Django templates context processors return a fixed value. For Jinja2 templates, this layer of indirection isn't necessary since you can add constants directly in `jinja2.Environment`.

  The original use case for adding context processors for Jinja2 involved:

  - Making an expensive computation that depends on the request.
  - Needing the result in every template.
  - Using the result multiple times in each template.

  Unless all of these conditions are met, passing a function to the template is simpler and more in line with the design of Jinja2.

The default configuration is purposefully kept to a minimum. If a template is rendered with a request (e.g. when using `render()`), the `Jinja2` backend adds the globals `request`, `csrf_input`, and `csrf_token` to the context. Apart from that, this backend doesn't create a Django-flavored environment. It doesn't know about Django filters and tags. In order to use Django-specific APIs, you must configure them into the environment.

For example, you can create `myproject/jinja2.py` with this content:

    from django.templatetags.static import static
    from django.urls import reverse

    from jinja2 import Environment

    def environment(**options):
        env = Environment(**options)
        env.globals.update({
            'static': static,
            'url': reverse,
        })
        return env

and set the `'environment'` option to `'myproject.jinja2.environment'`.

Then you could use the following constructs in Jinja2 templates:

    <img src="{{ static('path/to/company-logo.png') }}" alt="Company Logo">

    <a href="{{ url('admin:index') }}">Administration</a>

The concepts of tags and filters exist both in the Django template language and in Jinja2 but they're used differently. Since Jinja2 supports passing arguments to callables in templates, many features that require a template tag or filter in Django templates can be achieved simply by calling a function in Jinja2 templates, as shown in the example above. Jinja2's global namespace removes the need for template context processors. The Django template language doesn't have an equivalent of Jinja2 tests.

### Custom backends

Here's how to implement a custom template backend in order to use another template system. A template backend is a class that inherits `django.template.backends.base.BaseEngine`. It must implement `get_template()` and optionally `from_string()`. Here's an example for a fictional `foobar` template library:

    from django.template import TemplateDoesNotExist, TemplateSyntaxError
    from django.template.backends.base import BaseEngine
    from django.template.backends.utils import csrf_input_lazy, csrf_token_lazy

    import foobar

    class FooBar(BaseEngine):

        # Name of the subdirectory containing the templates for this engine
        # inside an installed application.
        app_dirname = 'foobar'

        def __init__(self, params):
            params = params.copy()
            options = params.pop('OPTIONS').copy()
            super().__init__(params)

            self.engine = foobar.Engine(**options)

        def from_string(self, template_code):
            try:
                return Template(self.engine.from_string(template_code))
            except foobar.TemplateCompilationFailed as exc:
                raise TemplateSyntaxError(exc.args)

        def get_template(self, template_name):
            try:
                return Template(self.engine.get_template(template_name))
            except foobar.TemplateNotFound as exc:
                raise TemplateDoesNotExist(exc.args, backend=self)
            except foobar.TemplateCompilationFailed as exc:
                raise TemplateSyntaxError(exc.args)

    class Template:

        def __init__(self, template):
            self.template = template

        def render(self, context=None, request=None):
            if context is None:
                context = {}
            if request is not None:
                context['request'] = request
                context['csrf_input'] = csrf_input_lazy(request)
                context['csrf_token'] = csrf_token_lazy(request)
            return self.template.render(context)

See DEP 182 for more information.

### Debug integration for custom engines

The Django debug page has hooks to provide detailed information when a template error arises. Custom template engines can use these hooks to enhance the traceback information that appears to users. The following hooks are available:

#### Template postmortem

The postmortem appears when `TemplateDoesNotExist` is raised. It lists the template engines and loaders that were used when trying to find a given template. For example, if two Django engines are configured, the postmortem will appear like:

Custom engines can populate the postmortem by passing the `backend` and `tried` arguments when raising `TemplateDoesNotExist`. Backends that use the postmortem should specify an origin on the template object.

#### Contextual line information

If an error happens during template parsing or rendering, Django can display the line the error happened on. For example:

Custom engines can populate this information by setting a `template_debug` attribute on exceptions raised during parsing and rendering. This attribute is a `dict` with the following values:

- `'name'`: The name of the template in which the exception occurred.
- `'message'`: The exception message.
- `'source_lines'`: The lines before, after, and including the line the exception occurred on. This is for context, so it shouldn't contain more than 20 lines or so.
- `'line'`: The line number on which the exception occurred.
- `'before'`: The content on the error line before the token that raised the error.
- `'during'`: The token that raised the error.
- `'after'`: The content on the error line after the token that raised the error.
- `'total'`: The number of lines in `source_lines`.
- `'top'`: The line number where `source_lines` starts.
- `'bottom'`: The line number where `source_lines` ends.

Given the above template error, `template_debug` would look like:

    {
        'name': '/path/to/template.html',
        'message': "Invalid block tag: 'syntax'",
        'source_lines': [
            (1, 'some\n'),
            (2, 'lines\n'),
            (3, 'before\n'),
            (4, 'Hello {% syntax error %} {{ world }}\n'),
            (5, 'some\n'),
            (6, 'lines\n'),
            (7, 'after\n'),
            (8, ''),
        ],
        'line': 4,
        'before': 'Hello ',
        'during': '{% syntax error %}',
        'after': ' {{ world }}\n',
        'total': 9,
        'bottom': 9,
        'top': 1,
    }

#### Origin API and 3rd-party integration

Django templates have an `Origin` object available through the `template.origin` attribute. This enables debug information to be displayed in the template postmortem, as well as in 3rd-party libraries, like the Django Debug Toolbar.

Custom engines can provide their own `template.origin` information by creating an object that specifies the following attributes:

- `'name'`: The full path to the template.
- `'template_name'`: The relative path to the template as passed into the the template loading methods.
- `'loader_name'`: An optional string identifying the function or class used to load the template, e.g. `django.template.loaders.filesystem.Loader`.

## The Django template language

### Syntax

About this section

This is an overview of the Django template language's syntax. For details see the language syntax reference.

A Django template is simply a text document or a Python string marked-up using the Django template language. Some constructs are recognized and interpreted by the template engine. The main ones are variables and tags.

A template is rendered with a context. Rendering replaces variables with their values, which are looked up in the context, and executes tags. Everything else is output as is.

The syntax of the Django template language involves four constructs.

#### 变量

A variable outputs a value from the context, which is a dict-like object mapping keys to values.

Variables are surrounded by `{{` and `}}` like this:

    My first name is {{ first_name }}. My last name is {{ last_name }}.

With a context of `{'first_name': 'John', 'last_name': 'Doe'}`, this template renders to:

    My first name is John. My last name is Doe.

Dictionary lookup, attribute lookup and list-index lookups are implemented with a dot notation:

    {{ my_dict.key }}
    {{ my_object.attribute }}
    {{ my_list.0 }}

If a variable resolves to a callable, the template system will call it with no arguments and use its result instead of the callable.

#### 标签(Tags)

Tags provide arbitrary logic in the rendering process.

This definition is deliberately vague. For example, a tag can output content, serve as a control structure e.g. an "if" statement or a "for" loop, grab content from a database, or even enable access to other template tags.

Tags are surrounded by `{%` and `%}` like this:

    {% csrf_token %}

Most tags accept arguments:

    {% cycle 'odd' 'even' %}

Some tags require beginning and ending tags:

    {% if user.is_authenticated %}Hello, {{ user.username }}.{% endif %}

A reference of built-in tags is available as well as instructions for writing custom tags.

#### 过滤器

Filters transform the values of variables and tag arguments.

They look like this:

    {{ django|title }}

With a context of `{'django': 'the web framework for perfectionists with deadlines'}`, this template renders to:

    The Web Framework For Perfectionists With Deadlines

Some filters take an argument:

    {{ my_date|date:"Y-m-d" }}

A reference of built-in filters is available as well as instructions for writing custom filters.

#### 注释(Comments)

Comments look like this:

    {# this won't be rendered #}

A `{% comment %}` tag provides multi-line comments.

### Components

About this section

This is an overview of the Django template language's APIs. For details see the API reference.

#### Engine

`django.template.Engine` encapsulates an instance of the Django template system. The main reason for instantiating an `Engine` directly is to use the Django template language outside of a Django project.

`django.template.backends.django.DjangoTemplates` is a thin wrapper adapting `django.template.Engine` to Django's template backend API.

#### Template

`django.template.Template` represents a compiled template. Templates are obtained with `Engine.get_template()` or `Engine.from_string()`

Likewise `django.template.backends.django.Template` is a thin wrapper adapting `django.template.Template` to the common template API.

#### Context

`django.template.Context` holds some metadata in addition to the context data. It is passed to `Template.render()` for rendering a template.

`django.template.RequestContext` is a subclass of `Context` that stores the current `HttpRequest` and runs template context processors.

The common API doesn't have an equivalent concept. Context data is passed in a plain `dict` and the current `HttpRequest` is passed separately if needed.

#### Loaders

Template loaders are responsible for locating templates, loading them, and returning `Template` objects.

Django provides several built-in template loaders and supports custom template loaders.

#### Context processors

Context processors are functions that receive the current `HttpRequest` as an argument and return a `dict` of data to be added to the rendering context.

Their main use is to add common data shared by all templates to the context without repeating code in every view.

Django provides many built-in context processors. Implementing a custom context processor is as simple as defining a function.