独自のテンプレートタグとフィルタ

Django のテンプレート言語は、アプリケーションのプレゼンテーションロジックのニーズに対応するように設計された、多種多様な 埋め込みタグやフィルタ を搭載しています。それにもかかわらず、あなた自身がテンプレート構成要素のコア・セットでカバーされていない機能を必要とするかもしれません。そのときは Python を使用し、カスタムタグやフィルタを定義することによって、テンプレートエンジンを拡張して、{% load %} タグを使用してからテンプレートにそれらの機能を利用することができます。

コードのレイアウト

独自のテンプレートタグやフィルタを指定するための最も一般的な場所は、Django のアプリケーションの内部です。それらが既存のアプリに関連するものである場合は、この場所にバンドルするのが最適です; それ以外の場合は、新しいアプリケーションに追加されてしまいます。Django のアプリケーションが INSTALLED_APPS に追加されると、以下に記載された従来の場所に定義したタグは、自動的にテンプレート内に読み込むことが可能になります。

アプリケーションは、"models.py" や "views.py" などと同じレベルに "templatetags" ディレクトリを含むべきです。まだ存在していない場合は、ディレクトリが Python パッケージとして扱われるようにするため、"__init__.py" を忘れないでください。

Webサーバが自動的にリスタートしない場合

` templatetags` モジュールを追加した後、あなたはテンプレートでタグやフィルタを使用する前に、サーバーを再起動する必要があります。

カスタムタグやフィルタは templatetags ディレクトリ内のモジュールにあります。モジュールファイルの名前は、あとでタグをロードして使うので、別のアプリでカスタムタグやフィルタと衝突しない名前を選択するように心がけてください。

例えば、カスタムタグやフィルタが "poll_extras.py" というファイルに入っている場合、あなたのアプリケーションのレイアウトは以下のようになります:

polls/
    __init__.py
    models.py
    templatetags/
        __init__.py
        poll_extras.py
    views.py

そしてあなたはテンプレートの中で次の手順を使用します。

{% load poll_extras %}

カスタムタグを含むアプリケーションは、{% load %} タグを機能させるために INSTALLED_APPS 内に記述される必要があります。これは、セキュリティ機能です: 毎回の Django のインストールでこれらへのアクセスを有効化することなく、単一のホストマシン上の多数のテンプレートライブラリに対して Python のコードをホストできるようにします。

どれだけ多くのモジュールを ``templatetags``パッケージに入れても制限はありません。ただ、{% load %} 文は、アプリケーションの名前ではなく、与えられた Python のモジュール名に対してタグ/フィルタをロードすることに注意してください。

有効なタグライブラリにするため、モジュールは、register という名前のモジュールレベルの変数を含む必要があります。これは、すべてのタグとフィルタが登録されている template.Library のインスタンスです。そのため、あなたのモジュールの上部に、次のコードを記述してください:

from django import template

register = template.Library()

あるいは、テンプレートタグのモジュールは、DjangoTemplates への 'libraries' 引数を通じて登録することもできます。テンプレートタグをロードするときに、テンプレートタグのモジュール名とは異なるラベルを使用したい場合に便利です。また、アプリケーションをインストールせずに、タグを登録できるようになります。

背景

ものすごい量の例になるため、Django のデフォルトのフィルタやタグのソースコードを読んでみてください。それらは django/template/defaultfilters.pydjango/template/defaulttags.py それぞれの中にあります。

load タグの詳細については、ドキュメントを参照してください。

独自のテンプレートフィルタを記述する

独自のフィルタは、1 つか 2 つの引数を取る、単なる Python の関数です:

  • 変数の値 (インプット) -- 文字列とは限りません。
  • 引数の値 -- デフォルト値を持つことも、完全に省略することもできます。

例えば、フィルタ {{ var|foo:"bar" }} の中で、フィルタ foo は変数 var と引数 "bar" を渡されます。

テンプレート言語は例外処理を提供しないので、テンプレートフィルタから生成された例外はすべてサーバーエラーとして公開されます。したがって、フィルタ関数は、返すべき妥当なフォールバック値がある場合には例外を発生させないようにする必要があります。テンプレートの明確なバグを表すインプットの場合、例外を発生させる方が、バグを隠すサイレントな失敗よりも適切でしょう。

以下はフィルタ定義の例です:

def cut(value, arg):
    """Removes all values of arg from the given string"""
    return value.replace(arg, '')

そして、以下はフィルタがどのように使われるかの例です:

{{ somevariable|cut:"0" }}

ほとんどのフィルタは引数を取りません。この場合、関数関数の引数を省略してください。例:

def lower(value): # Only one argument.
    """Converts a string into all lowercase"""
    return value.lower()

独自のフィルタを登録する

django.template.Library.filter()

フィルタ定義を書き終わったら、Django のテンプレート言語で使用できるようにするため、Library のインスタンスに登録する必要があります。

register.filter('cut', cut)
register.filter('lower', lower)

Library.filter() メソッドは 2 つの引数を取ります:

  1. フィルタの名前 -- 文字列です。
  2. 編集用の関数 -- Python の関数です (文字列としての関数名ではありません)。

代わりに register.filter() をデコレータとして使用できます。

@register.filter(name='cut')
def cut(value, arg):
    return value.replace(arg, '')

@register.filter
def lower(value):
    return value.lower()

name 引数を省略した場合、上記の 2 番目の例と同じように、Django はフィルタ名として関数の名前を利用します。

最後に、register.filter() は 3 つのキーワード引数 (is_safeneeds_autoescapeexpects_localtime) を受け入れます。これらの引数は、後述の フィルタと自動エスケープフィルタとタイムゾーン の中で説明されています。

文字列を要するテンプレートフィルタ

django.template.defaultfilters.stringfilter()

第 1 引数として文字数を要求するだけのテンプレートフィルタを記述している場合、デコレータ stringfilter を使う必要があります。これは、関数に渡される前にオブジェクトを文字列に変換します。

from django import template
from django.template.defaultfilters import stringfilter

register = template.Library()

@register.filter
@stringfilter
def lower(value):
    return value.lower()

この方法では、このフィルタに整数を渡すことができます。(整数には lower()``メソッドがないので) `` AttributeError が発生しません。

フィルタと自動エスケープ

When writing a custom filter, give some thought to how the filter will interact with Django's auto-escaping behavior. Note that two types of strings can be passed around inside the template code:

  • Raw strings are the native Python strings. On output, they're escaped if auto-escaping is in effect and presented unchanged, otherwise.

  • Safe strings are strings that have been marked safe from further escaping at output time. Any necessary escaping has already been done. They're commonly used for output that contains raw HTML that is intended to be interpreted as-is on the client side.

    Internally, these strings are of type SafeText. You can test for them using code like:

    from django.utils.safestring import SafeText
    
    if isinstance(value, SafeText):
        # Do something with the "safe" string.
        ...
    

Template filter code falls into one of two situations:

  1. Your filter does not introduce any HTML-unsafe characters (<, >, ', " or &) into the result that were not already present. In this case, you can let Django take care of all the auto-escaping handling for you. All you need to do is set the is_safe flag to True when you register your filter function, like so:

    @register.filter(is_safe=True)
    def myfilter(value):
        return value
    

    This flag tells Django that if a "safe" string is passed into your filter, the result will still be "safe" and if a non-safe string is passed in, Django will automatically escape it, if necessary.

    You can think of this as meaning "this filter is safe -- it doesn't introduce any possibility of unsafe HTML."

    The reason is_safe is necessary is because there are plenty of normal string operations that will turn a SafeData object back into a normal str object and, rather than try to catch them all, which would be very difficult, Django repairs the damage after the filter has completed.

    For example, suppose you have a filter that adds the string xx to the end of any input. Since this introduces no dangerous HTML characters to the result (aside from any that were already present), you should mark your filter with is_safe:

    @register.filter(is_safe=True)
    def add_xx(value):
        return '%sxx' % value
    

    When this filter is used in a template where auto-escaping is enabled, Django will escape the output whenever the input is not already marked as "safe".

    By default, is_safe is False, and you can omit it from any filters where it isn't required.

    Be careful when deciding if your filter really does leave safe strings as safe. If you're removing characters, you might inadvertently leave unbalanced HTML tags or entities in the result. For example, removing a > from the input might turn <a> into <a, which would need to be escaped on output to avoid causing problems. Similarly, removing a semicolon (;) can turn &amp; into &amp, which is no longer a valid entity and thus needs further escaping. Most cases won't be nearly this tricky, but keep an eye out for any problems like that when reviewing your code.

    Marking a filter is_safe will coerce the filter's return value to a string. If your filter should return a boolean or other non-string value, marking it is_safe will probably have unintended consequences (such as converting a boolean False to the string 'False').

  2. Alternatively, your filter code can manually take care of any necessary escaping. This is necessary when you're introducing new HTML markup into the result. You want to mark the output as safe from further escaping so that your HTML markup isn't escaped further, so you'll need to handle the input yourself.

    To mark the output as a safe string, use django.utils.safestring.mark_safe().

    Be careful, though. You need to do more than just mark the output as safe. You need to ensure it really is safe, and what you do depends on whether auto-escaping is in effect. The idea is to write filters that can operate in templates where auto-escaping is either on or off in order to make things easier for your template authors.

    In order for your filter to know the current auto-escaping state, set the needs_autoescape flag to True when you register your filter function. (If you don't specify this flag, it defaults to False). This flag tells Django that your filter function wants to be passed an extra keyword argument, called autoescape, that is True if auto-escaping is in effect and False otherwise. It is recommended to set the default of the autoescape parameter to True, so that if you call the function from Python code it will have escaping enabled by default.

    For example, let's write a filter that emphasizes the first character of a string:

    from django import template
    from django.utils.html import conditional_escape
    from django.utils.safestring import mark_safe
    
    register = template.Library()
    
    @register.filter(needs_autoescape=True)
    def initial_letter_filter(text, autoescape=True):
        first, other = text[0], text[1:]
        if autoescape:
            esc = conditional_escape
        else:
            esc = lambda x: x
        result = '<strong>%s</strong>%s' % (esc(first), esc(other))
        return mark_safe(result)
    

    The needs_autoescape flag and the autoescape keyword argument mean that our function will know whether automatic escaping is in effect when the filter is called. We use autoescape to decide whether the input data needs to be passed through django.utils.html.conditional_escape or not. (In the latter case, we just use the identity function as the "escape" function.) The conditional_escape() function is like escape() except it only escapes input that is not a SafeData instance. If a SafeData instance is passed to conditional_escape(), the data is returned unchanged.

    Finally, in the above example, we remember to mark the result as safe so that our HTML is inserted directly into the template without further escaping.

    There's no need to worry about the is_safe flag in this case (although including it wouldn't hurt anything). Whenever you manually handle the auto-escaping issues and return a safe string, the is_safe flag won't change anything either way.

警告

Avoiding XSS vulnerabilities when reusing built-in filters

Django's built-in filters have autoescape=True by default in order to get the proper autoescaping behavior and avoid a cross-site script vulnerability.

In older versions of Django, be careful when reusing Django's built-in filters as autoescape defaults to None. You'll need to pass autoescape=True to get autoescaping.

For example, if you wanted to write a custom filter called urlize_and_linebreaks that combined the urlize and linebreaksbr filters, the filter would look like:

from django.template.defaultfilters import linebreaksbr, urlize

@register.filter(needs_autoescape=True)
def urlize_and_linebreaks(text, autoescape=True):
    return linebreaksbr(
        urlize(text, autoescape=autoescape),
        autoescape=autoescape
    )

Then:

{{ comment|urlize_and_linebreaks }}

would be equivalent to:

{{ comment|urlize|linebreaksbr }}

Filters and time zones

If you write a custom filter that operates on datetime objects, you'll usually register it with the expects_localtime flag set to True:

@register.filter(expects_localtime=True)
def businesshours(value):
    try:
        return 9 <= value.hour < 17
    except AttributeError:
        return ''

When this flag is set, if the first argument to your filter is a time zone aware datetime, Django will convert it to the current time zone before passing it to your filter when appropriate, according to rules for time zones conversions in templates.

独自のテンプレートタグを記述する

タグはあらゆることができるため、フィルタより複雑です。Django は、ほとんどのタイプのタグを簡単に書けるように、多数のショートカットを提供しています。まず最初にこうしたショートカットを見てから、ショートカットでは機能が不足している場合にゼロからタグを書く方法を説明します。

シンプルなタグ

django.template.Library.simple_tag()

多くのテンプレートタグは、文字列やテンプレート変数などの引数 -- 文字列やテンプレートの変数 -- を取り、入力引数と外部情報のみに基づいて処理を行った後、結果を返します。 たとえば、current_time タグはフォーマット文字列を受け入れ、その時刻を適切な文字列フォーマットとして返します。

これらのタイプのタグの作成を容易にするため、Django はヘルパー関数 simple_tag を提供しています。 この関数は django.template.Library のメソッドで、任意の数の引数を受け取る関数を取り、それを render 関数と上記で説明した他の必要なビットにラップし、そしてテンプレートシステムに登録します。

私たちの current_time 関数は、以下のように書くことができます:

import datetime
from django import template

register = template.Library()

@register.simple_tag
def current_time(format_string):
    return datetime.datetime.now().strftime(format_string)

simple_tag ヘルパー関数について、注意すべきことがいくつかあります:

  • Checking for the required number of arguments, etc., has already been done by the time our function is called, so we don't need to do that.
  • The quotes around the argument (if any) have already been stripped away, so we just receive a plain string.
  • If the argument was a template variable, our function is passed the current value of the variable, not the variable itself.

Unlike other tag utilities, simple_tag passes its output through conditional_escape() if the template context is in autoescape mode, to ensure correct HTML and protect you from XSS vulnerabilities.

If additional escaping is not desired, you will need to use mark_safe() if you are absolutely sure that your code does not contain XSS vulnerabilities. For building small HTML snippets, use of format_html() instead of mark_safe() is strongly recommended.

If your template tag needs to access the current context, you can use the takes_context argument when registering your tag:

@register.simple_tag(takes_context=True)
def current_time(context, format_string):
    timezone = context['timezone']
    return your_get_current_time_method(timezone, format_string)

Note that the first argument must be called context.

For more information on how the takes_context option works, see the section on inclusion tags.

If you need to rename your tag, you can provide a custom name for it:

register.simple_tag(lambda x: x - 1, name='minusone')

@register.simple_tag(name='minustwo')
def some_function(value):
    return value - 2

simple_tag functions may accept any number of positional or keyword arguments. For example:

@register.simple_tag
def my_tag(a, b, *args, **kwargs):
    warning = kwargs['warning']
    profile = kwargs['profile']
    ...
    return ...

Then in the template any number of arguments, separated by spaces, may be passed to the template tag. Like in Python, the values for keyword arguments are set using the equal sign ("=") and must be provided after the positional arguments. For example:

{% my_tag 123 "abcd" book.title warning=message|lower profile=user.profile %}

It's possible to store the tag results in a template variable rather than directly outputting it. This is done by using the as argument followed by the variable name. Doing so enables you to output the content yourself where you see fit:

{% current_time "%Y-%m-%d %I:%M %p" as the_time %}
<p>The time is {{ the_time }}.</p>

Inclusion tags

django.template.Library.inclusion_tag()

Another common type of template tag is the type that displays some data by rendering another template. For example, Django's admin interface uses custom template tags to display the buttons along the bottom of the "add/change" form pages. Those buttons always look the same, but the link targets change depending on the object being edited -- so they're a perfect case for using a small template that is filled with details from the current object. (In the admin's case, this is the submit_row tag.)

These sorts of tags are called "inclusion tags".

Writing inclusion tags is probably best demonstrated by example. Let's write a tag that outputs a list of choices for a given Poll object, such as was created in the tutorials. We'll use the tag like this:

{% show_results poll %}

...and the output will be something like this:

<ul>
  <li>First choice</li>
  <li>Second choice</li>
  <li>Third choice</li>
</ul>

First, define the function that takes the argument and produces a dictionary of data for the result. The important point here is we only need to return a dictionary, not anything more complex. This will be used as a template context for the template fragment. Example:

def show_results(poll):
    choices = poll.choice_set.all()
    return {'choices': choices}

Next, create the template used to render the tag's output. This template is a fixed feature of the tag: the tag writer specifies it, not the template designer. Following our example, the template is very simple:

<ul>
{% for choice in choices %}
    <li> {{ choice }} </li>
{% endfor %}
</ul>

Now, create and register the inclusion tag by calling the inclusion_tag() method on a Library object. Following our example, if the above template is in a file called results.html in a directory that's searched by the template loader, we'd register the tag like this:

# Here, register is a django.template.Library instance, as before
@register.inclusion_tag('results.html')
def show_results(poll):
    ...

Alternatively it is possible to register the inclusion tag using a django.template.Template instance:

from django.template.loader import get_template
t = get_template('results.html')
register.inclusion_tag(t)(show_results)

...when first creating the function.

Sometimes, your inclusion tags might require a large number of arguments, making it a pain for template authors to pass in all the arguments and remember their order. To solve this, Django provides a takes_context option for inclusion tags. If you specify takes_context in creating a template tag, the tag will have no required arguments, and the underlying Python function will have one argument -- the template context as of when the tag was called.

For example, say you're writing an inclusion tag that will always be used in a context that contains home_link and home_title variables that point back to the main page. Here's what the Python function would look like:

@register.inclusion_tag('link.html', takes_context=True)
def jump_link(context):
    return {
        'link': context['home_link'],
        'title': context['home_title'],
    }

Note that the first parameter to the function must be called context.

In that register.inclusion_tag() line, we specified takes_context=True and the name of the template. Here's what the template link.html might look like:

Jump directly to <a href="{{ link }}">{{ title }}</a>.

Then, any time you want to use that custom tag, load its library and call it without any arguments, like so:

{% jump_link %}

Note that when you're using takes_context=True, there's no need to pass arguments to the template tag. It automatically gets access to the context.

The takes_context parameter defaults to False. When it's set to True, the tag is passed the context object, as in this example. That's the only difference between this case and the previous inclusion_tag example.

inclusion_tag functions may accept any number of positional or keyword arguments. For example:

@register.inclusion_tag('my_template.html')
def my_tag(a, b, *args, **kwargs):
    warning = kwargs['warning']
    profile = kwargs['profile']
    ...
    return ...

Then in the template any number of arguments, separated by spaces, may be passed to the template tag. Like in Python, the values for keyword arguments are set using the equal sign ("=") and must be provided after the positional arguments. For example:

{% my_tag 123 "abcd" book.title warning=message|lower profile=user.profile %}

Advanced custom template tags

Sometimes the basic features for custom template tag creation aren't enough. Don't worry, Django gives you complete access to the internals required to build a template tag from the ground up.

A quick overview

The template system works in a two-step process: compiling and rendering. To define a custom template tag, you specify how the compilation works and how the rendering works.

When Django compiles a template, it splits the raw template text into ''nodes''. Each node is an instance of django.template.Node and has a render() method. A compiled template is, simply, a list of Node objects. When you call render() on a compiled template object, the template calls render() on each Node in its node list, with the given context. The results are all concatenated together to form the output of the template.

Thus, to define a custom template tag, you specify how the raw template tag is converted into a Node (the compilation function), and what the node's render() method does.

Writing the compilation function

For each template tag the template parser encounters, it calls a Python function with the tag contents and the parser object itself. This function is responsible for returning a Node instance based on the contents of the tag.

For example, let's write a full implementation of our simple template tag, {% current_time %}, that displays the current date/time, formatted according to a parameter given in the tag, in strftime() syntax. It's a good idea to decide the tag syntax before anything else. In our case, let's say the tag should be used like this:

<p>The time is {% current_time "%Y-%m-%d %I:%M %p" %}.</p>

The parser for this function should grab the parameter and create a Node object:

from django import template

def do_current_time(parser, token):
    try:
        # split_contents() knows not to split quoted strings.
        tag_name, format_string = token.split_contents()
    except ValueError:
        raise template.TemplateSyntaxError(
            "%r tag requires a single argument" % token.contents.split()[0]
        )
    if not (format_string[0] == format_string[-1] and format_string[0] in ('"', "'")):
        raise template.TemplateSyntaxError(
            "%r tag's argument should be in quotes" % tag_name
        )
    return CurrentTimeNode(format_string[1:-1])

Notes:

  • parser is the template parser object. We don't need it in this example.
  • token.contents is a string of the raw contents of the tag. In our example, it's 'current_time "%Y-%m-%d %I:%M %p"'.
  • The token.split_contents() method separates the arguments on spaces while keeping quoted strings together. The more straightforward token.contents.split() wouldn't be as robust, as it would naively split on all spaces, including those within quoted strings. It's a good idea to always use token.split_contents().
  • This function is responsible for raising django.template.TemplateSyntaxError, with helpful messages, for any syntax error.
  • The TemplateSyntaxError exceptions use the tag_name variable. Don't hard-code the tag's name in your error messages, because that couples the tag's name to your function. token.contents.split()[0] will ''always'' be the name of your tag -- even when the tag has no arguments.
  • The function returns a CurrentTimeNode with everything the node needs to know about this tag. In this case, it just passes the argument -- "%Y-%m-%d %I:%M %p". The leading and trailing quotes from the template tag are removed in format_string[1:-1].
  • The parsing is very low-level. The Django developers have experimented with writing small frameworks on top of this parsing system, using techniques such as EBNF grammars, but those experiments made the template engine too slow. It's low-level because that's fastest.

Writing the renderer

The second step in writing custom tags is to define a Node subclass that has a render() method.

Continuing the above example, we need to define CurrentTimeNode:

import datetime
from django import template

class CurrentTimeNode(template.Node):
    def __init__(self, format_string):
        self.format_string = format_string

    def render(self, context):
        return datetime.datetime.now().strftime(self.format_string)

Notes:

  • __init__() gets the format_string from do_current_time(). Always pass any options/parameters/arguments to a Node via its __init__().
  • The render() method is where the work actually happens.
  • render() should generally fail silently, particularly in a production environment. In some cases however, particularly if context.template.engine.debug is True, this method may raise an exception to make debugging easier. For example, several core tags raise django.template.TemplateSyntaxError if they receive the wrong number or type of arguments.

Ultimately, this decoupling of compilation and rendering results in an efficient template system, because a template can render multiple contexts without having to be parsed multiple times.

Auto-escaping considerations

The output from template tags is not automatically run through the auto-escaping filters (with the exception of simple_tag() as described above). However, there are still a couple of things you should keep in mind when writing a template tag.

If the render() function of your template stores the result in a context variable (rather than returning the result in a string), it should take care to call mark_safe() if appropriate. When the variable is ultimately rendered, it will be affected by the auto-escape setting in effect at the time, so content that should be safe from further escaping needs to be marked as such.

Also, if your template tag creates a new context for performing some sub-rendering, set the auto-escape attribute to the current context's value. The __init__ method for the Context class takes a parameter called autoescape that you can use for this purpose. For example:

from django.template import Context

def render(self, context):
    # ...
    new_context = Context({'var': obj}, autoescape=context.autoescape)
    # ... Do something with new_context ...

This is not a very common situation, but it's useful if you're rendering a template yourself. For example:

def render(self, context):
    t = context.template.engine.get_template('small_fragment.html')
    return t.render(Context({'var': obj}, autoescape=context.autoescape))

If we had neglected to pass in the current context.autoescape value to our new Context in this example, the results would have always been automatically escaped, which may not be the desired behavior if the template tag is used inside a {% autoescape off %} block.

Thread-safety considerations

Once a node is parsed, its render method may be called any number of times. Since Django is sometimes run in multi-threaded environments, a single node may be simultaneously rendering with different contexts in response to two separate requests. Therefore, it's important to make sure your template tags are thread safe.

To make sure your template tags are thread safe, you should never store state information on the node itself. For example, Django provides a builtin cycle template tag that cycles among a list of given strings each time it's rendered:

{% for o in some_list %}
    <tr class="{% cycle 'row1' 'row2' %}">
        ...
    </tr>
{% endfor %}

A naive implementation of CycleNode might look something like this:

import itertools
from django import template

class CycleNode(template.Node):
    def __init__(self, cyclevars):
        self.cycle_iter = itertools.cycle(cyclevars)

    def render(self, context):
        return next(self.cycle_iter)

But, suppose we have two templates rendering the template snippet from above at the same time:

  1. Thread 1 performs its first loop iteration, CycleNode.render() returns 'row1'
  2. Thread 2 performs its first loop iteration, CycleNode.render() returns 'row2'
  3. Thread 1 performs its second loop iteration, CycleNode.render() returns 'row1'
  4. Thread 2 performs its second loop iteration, CycleNode.render() returns 'row2'

The CycleNode is iterating, but it's iterating globally. As far as Thread 1 and Thread 2 are concerned, it's always returning the same value. This is obviously not what we want!

To address this problem, Django provides a render_context that's associated with the context of the template that is currently being rendered. The render_context behaves like a Python dictionary, and should be used to store Node state between invocations of the render method.

Let's refactor our CycleNode implementation to use the render_context:

class CycleNode(template.Node):
    def __init__(self, cyclevars):
        self.cyclevars = cyclevars

    def render(self, context):
        if self not in context.render_context:
            context.render_context[self] = itertools.cycle(self.cyclevars)
        cycle_iter = context.render_context[self]
        return next(cycle_iter)

Note that it's perfectly safe to store global information that will not change throughout the life of the Node as an attribute. In the case of CycleNode, the cyclevars argument doesn't change after the Node is instantiated, so we don't need to put it in the render_context. But state information that is specific to the template that is currently being rendered, like the current iteration of the CycleNode, should be stored in the render_context.

注釈

Notice how we used self to scope the CycleNode specific information within the render_context. There may be multiple CycleNodes in a given template, so we need to be careful not to clobber another node's state information. The easiest way to do this is to always use self as the key into render_context. If you're keeping track of several state variables, make render_context[self] a dictionary.

Registering the tag

Finally, register the tag with your module's Library instance, as explained in writing custom template filters above. Example:

register.tag('current_time', do_current_time)

The tag() method takes two arguments:

  1. The name of the template tag -- a string. If this is left out, the name of the compilation function will be used.
  2. 編集用の関数 -- Python の関数です (文字列としての関数名ではありません)。

As with filter registration, it is also possible to use this as a decorator:

@register.tag(name="current_time")
def do_current_time(parser, token):
    ...

@register.tag
def shout(parser, token):
    ...

If you leave off the name argument, as in the second example above, Django will use the function's name as the tag name.

Passing template variables to the tag

Although you can pass any number of arguments to a template tag using token.split_contents(), the arguments are all unpacked as string literals. A little more work is required in order to pass dynamic content (a template variable) to a template tag as an argument.

While the previous examples have formatted the current time into a string and returned the string, suppose you wanted to pass in a DateTimeField from an object and have the template tag format that date-time:

<p>This post was last updated at {% format_time blog_entry.date_updated "%Y-%m-%d %I:%M %p" %}.</p>

Initially, token.split_contents() will return three values:

  1. The tag name format_time.
  2. The string 'blog_entry.date_updated' (without the surrounding quotes).
  3. The formatting string '"%Y-%m-%d %I:%M %p"'. The return value from split_contents() will include the leading and trailing quotes for string literals like this.

Now your tag should begin to look like this:

from django import template

def do_format_time(parser, token):
    try:
        # split_contents() knows not to split quoted strings.
        tag_name, date_to_be_formatted, format_string = token.split_contents()
    except ValueError:
        raise template.TemplateSyntaxError(
            "%r tag requires exactly two arguments" % token.contents.split()[0]
        )
    if not (format_string[0] == format_string[-1] and format_string[0] in ('"', "'")):
        raise template.TemplateSyntaxError(
            "%r tag's argument should be in quotes" % tag_name
        )
    return FormatTimeNode(date_to_be_formatted, format_string[1:-1])

You also have to change the renderer to retrieve the actual contents of the date_updated property of the blog_entry object. This can be accomplished by using the Variable() class in django.template.

To use the Variable class, simply instantiate it with the name of the variable to be resolved, and then call variable.resolve(context). So, for example:

class FormatTimeNode(template.Node):
    def __init__(self, date_to_be_formatted, format_string):
        self.date_to_be_formatted = template.Variable(date_to_be_formatted)
        self.format_string = format_string

    def render(self, context):
        try:
            actual_date = self.date_to_be_formatted.resolve(context)
            return actual_date.strftime(self.format_string)
        except template.VariableDoesNotExist:
            return ''

Variable resolution will throw a VariableDoesNotExist exception if it cannot resolve the string passed to it in the current context of the page.

Setting a variable in the context

The above examples simply output a value. Generally, it's more flexible if your template tags set template variables instead of outputting values. That way, template authors can reuse the values that your template tags create.

To set a variable in the context, just use dictionary assignment on the context object in the render() method. Here's an updated version of CurrentTimeNode that sets a template variable current_time instead of outputting it:

import datetime
from django import template

class CurrentTimeNode2(template.Node):
    def __init__(self, format_string):
        self.format_string = format_string
    def render(self, context):
        context['current_time'] = datetime.datetime.now().strftime(self.format_string)
        return ''

Note that render() returns the empty string. render() should always return string output. If all the template tag does is set a variable, render() should return the empty string.

Here's how you'd use this new version of the tag:

{% current_time "%Y-%M-%d %I:%M %p" %}<p>The time is {{ current_time }}.</p>

Variable scope in context

Any variable set in the context will only be available in the same block of the template in which it was assigned. This behavior is intentional; it provides a scope for variables so that they don't conflict with context in other blocks.

But, there's a problem with CurrentTimeNode2: The variable name current_time is hard-coded. This means you'll need to make sure your template doesn't use {{ current_time }} anywhere else, because the {% current_time %} will blindly overwrite that variable's value. A cleaner solution is to make the template tag specify the name of the output variable, like so:

{% current_time "%Y-%M-%d %I:%M %p" as my_current_time %}
<p>The current time is {{ my_current_time }}.</p>

To do that, you'll need to refactor both the compilation function and Node class, like so:

import re

class CurrentTimeNode3(template.Node):
    def __init__(self, format_string, var_name):
        self.format_string = format_string
        self.var_name = var_name
    def render(self, context):
        context[self.var_name] = datetime.datetime.now().strftime(self.format_string)
        return ''

def do_current_time(parser, token):
    # This version uses a regular expression to parse tag contents.
    try:
        # Splitting by None == splitting by spaces.
        tag_name, arg = token.contents.split(None, 1)
    except ValueError:
        raise template.TemplateSyntaxError(
            "%r tag requires arguments" % token.contents.split()[0]
        )
    m = re.search(r'(.*?) as (\w+)', arg)
    if not m:
        raise template.TemplateSyntaxError("%r tag had invalid arguments" % tag_name)
    format_string, var_name = m.groups()
    if not (format_string[0] == format_string[-1] and format_string[0] in ('"', "'")):
        raise template.TemplateSyntaxError(
            "%r tag's argument should be in quotes" % tag_name
        )
    return CurrentTimeNode3(format_string[1:-1], var_name)

The difference here is that do_current_time() grabs the format string and the variable name, passing both to CurrentTimeNode3.

Finally, if you only need to have a simple syntax for your custom context-updating template tag, consider using the simple_tag() shortcut, which supports assigning the tag results to a template variable.

Parsing until another block tag

Template tags can work in tandem. For instance, the standard {% comment %} tag hides everything until {% endcomment %}. To create a template tag such as this, use parser.parse() in your compilation function.

Here's how a simplified {% comment %} tag might be implemented:

def do_comment(parser, token):
    nodelist = parser.parse(('endcomment',))
    parser.delete_first_token()
    return CommentNode()

class CommentNode(template.Node):
    def render(self, context):
        return ''

注釈

The actual implementation of {% comment %} is slightly different in that it allows broken template tags to appear between {% comment %} and {% endcomment %}. It does so by calling parser.skip_past('endcomment') instead of parser.parse(('endcomment',)) followed by parser.delete_first_token(), thus avoiding the generation of a node list.

parser.parse() takes a tuple of names of block tags ''to parse until''. It returns an instance of django.template.NodeList, which is a list of all Node objects that the parser encountered ''before'' it encountered any of the tags named in the tuple.

In "nodelist = parser.parse(('endcomment',))" in the above example, nodelist is a list of all nodes between the {% comment %} and {% endcomment %}, not counting {% comment %} and {% endcomment %} themselves.

After parser.parse() is called, the parser hasn't yet "consumed" the {% endcomment %} tag, so the code needs to explicitly call parser.delete_first_token().

CommentNode.render() simply returns an empty string. Anything between {% comment %} and {% endcomment %} is ignored.

Parsing until another block tag, and saving contents

In the previous example, do_comment() discarded everything between {% comment %} and {% endcomment %}. Instead of doing that, it's possible to do something with the code between block tags.

For example, here's a custom template tag, {% upper %}, that capitalizes everything between itself and {% endupper %}.

Usage:

{% upper %}This will appear in uppercase, {{ your_name }}.{% endupper %}

As in the previous example, we'll use parser.parse(). But this time, we pass the resulting nodelist to the Node:

def do_upper(parser, token):
    nodelist = parser.parse(('endupper',))
    parser.delete_first_token()
    return UpperNode(nodelist)

class UpperNode(template.Node):
    def __init__(self, nodelist):
        self.nodelist = nodelist
    def render(self, context):
        output = self.nodelist.render(context)
        return output.upper()

The only new concept here is the self.nodelist.render(context) in UpperNode.render().

For more examples of complex rendering, see the source code of {% for %} in django/template/defaulttags.py and {% if %} in django/template/smartif.py.

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