add_class filter correction

smash/web/templatetags/filters.py

@@ -5,9 +5,14 @@ register = template.Library()
 
 @register.filter(name='add_class')
 def add_class(value, arg):
+    # Get all the field's initial css-classes
     css_classes = value.field.widget.attrs.get('class', ' ').split(' ')
-    if css_classes and arg not in css_classes:
-        css_classes = '%s %s' % (css_classes, arg)
+    # Filter out zero-length class names ('')
+    css_classes = filter(lambda x: len(x) > 0, css_classes)
+    # Convert list to string
+    css_classes = reduce(lambda a, x: "%s %s" % (a, x), css_classes, "")
+    css_classes = '%s %s' % (css_classes, arg)
+    # Return the widget with freshly crafted classes
     return value.as_widget(attrs={'class': css_classes})
 
 @register.filter(name='disable')