Django snippets

from django import template
from django.template import Node
from django.template import TemplateSyntaxError, VariableDoesNotExist

register = template.Library()

class SmartWidthRatioNode(Node):
    def __init__(self, val_expr, max_expr, max_width):
        self.val_expr = val_expr
        self.max_expr = max_expr
        self.max_width = max_width

    def render(self, context):
        try:
            value = self.val_expr.resolve(context)
            maxvalue = self.max_expr.resolve(context)
            max_width = int(self.max_width.resolve(context))
        except VariableDoesNotExist:
            return ''
        except ValueError:
            raise TemplateSyntaxError("widthratio final argument must be an number")
        try:
            value = float(value)
            maxvalue = float(maxvalue)
            if value > maxvalue:
                value = maxvalue
            ratio = (value / maxvalue) * max_width
        except (ValueError, ZeroDivisionError):
            return str(max_width)
        return str(int(round(ratio)))

def smart_widthratio(parser, token):
    bits = token.contents.split()
    if len(bits) != 4:
        raise TemplateSyntaxError("widthratio takes three arguments")
    tag, this_value_expr, max_value_expr, max_width = bits
        
    return SmartWidthRatioNode(parser.compile_filter(this_value_expr),
                               parser.compile_filter(max_value_expr),
                               parser.compile_filter(max_width))

smart_widthratio = register.tag(smart_widthratio)