from django import template
register = template.Library()

@register.filter(name='addcss')
def addcss(field, css):
    """
    Adds the given class to the class attribute of the given field.
    
    E.g. adds the class 'bar' to the foo widget:
     
        {{ form.foo|addcss:"bar" }}
    """
    return field.as_widget(attrs={"class": css})