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。
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。
独自のフィルタは、1つか2つの引数を取るPythonの関数です:<br>
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()
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:
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。
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。)
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:
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 & into &, 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').
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 }}
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.
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。
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:
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>
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 %}
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.
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.
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"'.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().django.template.TemplateSyntaxError, with helpful messages, for
any syntax error.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.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 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__().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.
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.
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:
CycleNode.render()
returns 'row1'CycleNode.render()
returns 'row2'CycleNode.render()
returns 'row1'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.
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:
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.
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:
format_time.'blog_entry.date_updated' (without the surrounding
quotes).'"%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.
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.
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.
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.
2022年6月01日