# -*- coding: utf-8 -*-
from __future__ import absolute_import, unicode_literals


def add_css_class(widget, css_class):
    css_classes = widget.attrs.get('class', '').split()
    if css_class not in css_classes:
        css_classes.append(css_class)
    widget.attrs.update({'class': ' '.join(css_classes)})
    return widget

# -*- coding: utf-8 -*-
from __future__ import absolute_import, unicode_literals


def add_css_class(widget, css_class):
    css_classes = widget.attrs.get('class', '').split()
    if not css_class in css_classes:
        css_classes.append(css_class)
    widget.attrs.update({'class': ' '.join(css_classes)})
    return widget

--- 
+++ 
@@ -4,7 +4,7 @@
 
 def add_css_class(widget, css_class):
     css_classes = widget.attrs.get('class', '').split()
-    if not css_class in css_classes:
+    if css_class not in css_classes:
         css_classes.append(css_class)
     widget.attrs.update({'class': ' '.join(css_classes)})
     return widget
Make if statement flake8 compliant
