# Django 4.1.8.dev20230308103347 ドキュメント

# How to create custom template tags and filters

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。それでもなお、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が必要になることもあるでしょう。そのときは Python を使用し、独自の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できます。その上で、`{% load %}` タグを使用すると、テンプレートでそれらの機能を利用することができるようになります。

## 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、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。

`templatetags` パッケージに入れられるモジュールの数に制限はありません。ただ、`{% load %}` ステートメントは、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

For a ton of examples, read the source code for Django's default filters and tags. They're in django/template/defaultfilters.py and django/template/defaulttags.py, respectively.

`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Pythonの関数です:<br>

- 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、次のように関数の第2引数を省略してください:

    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_safe`、`needs_autoescape`、 `expects_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 と 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()

これにより、このフィルタに整数を渡したとしても `AttributeError` は発生しません。(整数は `lower()` メソッドを持ちませんが。)

### フィルタと自動エスケープ

独自のフィルタを作成する場合、フィルタがDjangoの自動エスケープの挙動とどのように関連するか考慮してください。2種類の文字列がテンプレートコードに渡されることに留意してください:

- Raw strings はPythonのネイティブの文字列です。出力時に、自動エスケープが有効な場合はエスケープされ、それ以外の場合は変更されません。

- Safe strings は出力時にさらなるエスケープがされないように安全とマークされた文字列です。必要なエスケープはすでに行われています。これらはクライアント側でそのまま解釈されることを目的とした生のHTMLを含む出力によく利用されます。

  内部では、これらの文字列は `SafeString` 型になります。次のようなコードでこれらの値を検証することができます:

      from django.utils.safestring import SafeString

      if isinstance(value, SafeString):
          # 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 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 receive a plain string.
- もし引数がテンプレート変数だった場合、テンプレート中でタグが呼ばれた時点の値が渡されます。テンプレート変数そのものが渡されるわけではありません。

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.

テンプレートタグの中からコンテキストにアクセスしたい場合、タグを登録する際に``takes_context``引数を使うことでできるようになります。

    @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`.

`takes_context` オプションがどのように動くかについて、詳しくは inclusion tags を参照してください。

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 ...

このようにすることで、テンプレートからはスペースで区切られた変数をいくつでもテンプレートタグに渡すことができます。Pythonの文法と同じように、キーワード引数の値は等号（"`=`"）を用いて位置引数の後に記述します。たとえば：

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

タグの処理結果を直接出力することのほかに、テンプレート変数に格納することも可能です。これは``as``に続けて変数名を書くことで実現可能です。これによって、タグの処理結果を好きなように出力することができるようになります。

    {% 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 short:

    <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 ...

このようにすることで、テンプレートからはスペースで区切られた変数をいくつでもテンプレートタグに渡すことができます。Pythonの文法と同じように、キーワード引数の値は等号（"`=`"）を用いて位置引数の後に記述します。たとえば：

    {% 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 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 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])

メモ:

- `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 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)

メモ:

- `__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()` method of your template tag 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 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 tags 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, 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 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, 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()` 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`.