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__() of the class. Here's the 'current_time' tag example from the docs:

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

@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)

or even simpler, leave off the name argument and use the class name as the tag name:

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

@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)