old_contents: import six
  import json
  
  from django.utils.html import format_html, mark_safe
  from django.core.urlresolvers import reverse
  from django.utils.encoding import python_2_unicode_compatible
  
  CHARTS = {}
  
  class ChartMeta(type):
      def __new__(cls, name, bases, attrs):
          klass = super(ChartMeta, cls).__new__(cls, name, bases, attrs)
  
          if klass.chart_slug:
              CHARTS[klass.chart_slug] = klass
  
          return klass
  
  @six.add_metaclass(ChartMeta)
  @python_2_unicode_compatible
  class Chart(object):
      options = {}
      chart_slug = None
      columns = None
  
      def get_data(self):
          raise NotImplementedError
  
      def __str__(self):
          return format_html(
              "<div "
                "data-chart-options='{0}'"
                "data-chart-url='{1}'"
              "></div>",
              json.dumps(self.options),
              reverse(
                  'djgc-chart-data',
                  args=(self.chart_slug,),
              ),
          )
new_contents: import json
  
  from django.utils import six
  from django.utils.html import format_html, mark_safe
  from django.core.urlresolvers import reverse
  from django.utils.encoding import python_2_unicode_compatible
  
  CHARTS = {}
  
  class ChartMeta(type):
      def __new__(cls, name, bases, attrs):
          klass = super(ChartMeta, cls).__new__(cls, name, bases, attrs)
  
          if klass.chart_slug:
              CHARTS[klass.chart_slug] = klass
  
          return klass
  
  @six.add_metaclass(ChartMeta)
  @python_2_unicode_compatible
  class Chart(object):
      options = {}
      chart_slug = None
      columns = None
  
      def get_data(self):
          raise NotImplementedError
  
      def __str__(self):
          return format_html(
              "<div "
                "data-chart-options='{0}'"
                "data-chart-url='{1}'"
              "></div>",
              json.dumps(self.options),
              reverse(
                  'djgc-chart-data',
                  args=(self.chart_slug,),
              ),
          )
subject: Use django's bundled and customised version of six