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HomeBackend DevelopmentPython Tutorial在Python的Django框架中编写编译函数

当遇到一个模板标签(template tag)时,模板解析器就会把标签包含的内容,以及模板解析器自己作为参数调用一个python函数。 这个函数负责返回一个和当前模板标签内容相对应的节点(Node)的实例。

例如,写一个显示当前日期的模板标签:{% current_time %}。该标签会根据参数指定的 strftime 格式(参见:http://www.djangoproject.com/r/python/strftime/)显示当前时间。首先确定标签的语法是个好主意。 在这个例子里,标签应该这样使用:

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

注意

没错, 这个模板标签是多余的,Django默认的 {% now %} 用更简单的语法完成了同样的工作。 这个模板标签在这里只是作为一个例子。

这个函数的分析器会获取参数并创建一个 Node 对象:

from django import template

register = template.Library()

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

这里需要说明的地方很多:

    每个标签编译函数有两个参数,parser和token。parser是模板解析器对象。 我们在这个例子中并不使用它。 token是正在被解析的语句。

    token.contents 是包含有标签原始内容的字符串。 在我们的例子中,它是 'current_time "%Y-%m-%d %I:%M %p"' 。

    token.split_contents() 方法按空格拆分参数同时保证引号中的字符串不拆分。 应该避免使用 token.contents.split() (仅使用Python的标准字符串拆分)。 它不够健壮,因为它只是简单的按照所有空格进行拆分,包括那些引号引起来的字符串中的空格。

    这个函数可以抛出 django.template.TemplateSyntaxError ,这个异常提供所有语法错误的有用信息。

    不要把标签名称硬编码在你的错误信息中,因为这样会把标签名称和你的函数耦合在一起。 token.split_contents()[0]总是记录标签的名字,就算标签没有任何参数。

    这个函数返回一个 CurrentTimeNode (稍后我们将创建它),它包含了节点需要知道的关于这个标签的全部信息。 在这个例子中,它只是传递了参数 "%Y-%m-%d %I:%M %p" 。模板标签开头和结尾的引号使用 format_string[1:-1] 除去。

    模板标签编译函数 必须 返回一个 Node 子类,返回其它值都是错的。

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