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register.tag as a class decorator

Author:
gsakkis
Posted:
January 27, 2010
Language:
Python
Version:
1.1
Score:
0 (after 0 ratings)

Defining a custom template tag consists of three parts: a compiling function, a rendering Node subclass and a tag registration with register.tag(). The latter can be used as a (function) decorator on the compiling function, simplifying things into two parts.

A neat fact is that register.tag() can actually be used as a class decorator in Python 2.6+ to condense all steps into the Node subclass. The compiling function simply becomes the __init__(). Below are two variants of the 'current_time' tag example from the Django docs: the first passing an explicit tag name and the second using the class name instead.

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import datetime
from django import template
register = template.Library()

if 1: # explicit tag name

    @register.tag('current_time')
    class CurrentTimeNode(template.Node):
        def __init__(self, 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
            self.format_string = str(format_string[1:-1])
            
        def render(self, context):
            return datetime.datetime.now().strftime(self.format_string)

else: # implicit tag name
    
    @register.tag
    class current_time(template.Node):
        def __init__(self, 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
            self.format_string = str(format_string[1:-1])
            
        def render(self, context):
            return datetime.datetime.now().strftime(self.format_string)

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Comments

culebron (on June 21, 2010):

I see one disadvantage: can't inherit from a decorated class. This makes it impossible to reuse them. So, unless Library.tag is modified to handle classes as well, its use is limited. Except for that works perfect.

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