A django template tag for summing sequences

sumtag.py

from django import template

register = template.Library()


class SumNode(template.Node):
    def __init__(self, sequence, loopvar, loopitem):
        self.sequence = template.Variable(sequence)
        self.loopvar = loopvar
        self.loopitem = template.Variable(loopitem)
    
    def render(self, context):
        sequence = self.sequence.resolve(context)
        return sum(self.loopitem.resolve({self.loopvar: x}) for x in sequence)


@register.tag('sum')
def do_sum(parser, token):
    """{% sum <foo.bar> for <foo> in <sequence> %}"""
    bits = token.split_contents()[1:]
    try:
        loopitem, _for, loopvar, _in, sequence = bits
    except ValueError:
        raise template.TemplateSyntaxError("Invalid tag syntax expected"
                                           " '{% sum <foo.bar> for <foo> in <sequence> %}'")
    
    return SumNode(sequence, loopvar, loopitem)