Django snippets

from django.template import Library
from django.template import FilterExpression, Node, NodeList
from django.template import TemplateSyntaxError, VariableDoesNotExist

register = Library()

def do_ifvalue(parser, token, name, negate=False):
    bits = list(token.split_contents())
    if len(bits) != 2 and len(bits) != 4:
        raise TemplateSyntaxError, "%r takes one or three arguments" % bits[0]
    tagname, variable = tuple(bits)[:2]
    if len(bits) == 4:
        if bits[2] != "as":
            raise TemplateSyntaxError, "%r with three arguments must be 'value as name'" % tagname
        name = bits[3]
    
    end_tag = 'end' + tagname
    nodelist_true = parser.parse(('else', end_tag))
    token = parser.next_token()
    if token.contents == 'else':
        nodelist_false = parser.parse((end_tag,))
        parser.delete_first_token()
    else:
        nodelist_false = NodeList()
    expr = FilterExpression(variable, parser)
    return IfValueNode(expr, name, nodelist_true, nodelist_false, negate)

@register.tag
def ifvalue(parser, token, name="value"):
    """
    Output the contents of the block if the argument is true, assigning
    the value of the argument to a context variable ("value" by default).

    Examples::

        {% ifvalue user.name %}
            {{ value }}
            ...
        {% else %}
            ...
        {% endifvalue %}

        {% ifvalue user.name as username %}
            {{ username }}
            ...
        {% else %}
            ...
        {% endifvalue %}
    """
    return do_ifvalue(parser, token, name, False)

@register.tag
def ifnotvalue(parser, token, name="value"):
    """Output the contents of the block if the argument is false. See ifvalue."""
    return do_ifvalue(parser, token, name, True)

class IfValueNode(Node):
    def __init__(self, var, name, nodelist_true, nodelist_false, negate):
        self.var = var
        self.name = name
        self.nodelist_true, self.nodelist_false = nodelist_true, nodelist_false
        self.negate = negate

    def __repr__(self):
        return "<IfValueNode>"

    def render(self, context):
        val = self.var.resolve(context, ignore_failures=True)
        context[self.name] = val

        if (self.negate and not val) or (not self.negate and val):
            return self.nodelist_true.render(context)
        return self.nodelist_false.render(context)

    def get_nodes_by_type(self, nodetype):
        nodes = []
        if isinstance(self, nodetype):
            nodes.append(self)
        nodes.extend(self.nodelist_true.get_nodes_by_type(nodetype))
        nodes.extend(self.nodelist_false.get_nodes_by_type(nodetype))
        return nodes