Idea for a nicer way to write template tags in Django

doctags.py

@doctag('''Sums two numbers
Usage:
    {% sum <a> <b> [as <c>] %}
''')
def sum(context, a, b, c=None):
    result = int(a) + int(b)
    if c:
        context[c] = result
    else:
        return result

@doctag('''Adds a new thing to the context
Usage:
    {% with (<a>=<b>)... %}...{% endwith %}
''')
def with_tag(context, a, b, nodelist):
    context.push()
    for key, value in zip(a, b):
        context[key] = value
    output = nodelist.render(context)
    context.pop()
    return output