Django template tag for replacing substrings

replace.py

from django.template import Library, Node, Variable, \
    VariableDoesNotExist, TemplateSyntaxError

register = Library()

def get_var(v, context):
    try:
        return v.resolve(context)
    except VariableDoesNotExist:
        return v.var

class ReplaceNode(Node):

    def __init__(self, s, old, new):
        self.s = Variable(s)
        self.old = Variable(old)
        self.new = Variable(new)

    def render(self, context):
        s = unicode(get_var(self.s, context))
        old = unicode(get_var(self.old, context))
        new = unicode(get_var(self.new, context))
        return s.replace(old, new)

@register.tag
def replace(parser, token):
    args = token.split_contents()[1:]
    if len(args) != 3:
        raise TemplateSyntaxError, '%r tag requires a string, an old value, and a new value.' % token.contents.split()[0]
    return ReplaceNode(*args)