old_contents: from django.utils.safestring import mark_safe
  from pygments import highlight
  from pygments.formatters import get_formatter_by_name
  from pygments.lexers import get_lexer_by_name
  from wagtail.wagtailcore import blocks
  
  
  class CodeBlock(blocks.StructBlock):
      """
      Code Highlighting Block
      """
  
      LANGUAGE_CHOICES = (
          ('python', 'Python'),
          ('javascript', 'Javascript'),
          ('json', 'JSON'),
          ('bash', 'Bash/Shell'),
          ('html', 'HTML'),
          ('css', 'CSS'),
          ('scss', 'SCSS'),
          ('yaml', 'YAML'),
      )
  
      language = blocks.ChoiceBlock(choices=LANGUAGE_CHOICES)
      code = blocks.TextBlock()
  
      class Meta:
          icon = 'code'
  
      def render(self, value):
          src = value['code'].strip('\n')
          lang = value['language']
  
          lexer = get_lexer_by_name(lang)
          formatter = get_formatter_by_name(
              'html',
              linenos=None,
              cssclass='codehilite',
              style='default',
              noclasses=False,
          )
          return mark_safe(highlight(src, lexer, formatter))
  
new_contents: from django.utils.safestring import mark_safe
  from pygments import highlight
  from pygments.formatters import get_formatter_by_name
  from pygments.lexers import get_lexer_by_name
  from wagtail.wagtailcore import blocks
  
  
  class CodeBlock(blocks.StructBlock):
      """
      Code Highlighting Block
      """
  
      LANGUAGE_CHOICES = (
          ('python', 'Python'),
          ('javascript', 'Javascript'),
          ('json', 'JSON'),
          ('bash', 'Bash/Shell'),
          ('html', 'HTML'),
          ('css', 'CSS'),
          ('scss', 'SCSS'),
          ('yaml', 'YAML'),
      )
  
      language = blocks.ChoiceBlock(choices=LANGUAGE_CHOICES)
      code = blocks.TextBlock()
  
      class Meta:
          icon = 'code'
  
      def render(self, value, context=None):
          src = value['code'].strip('\n')
          lang = value['language']
  
          lexer = get_lexer_by_name(lang)
          formatter = get_formatter_by_name(
              'html',
              linenos=None,
              cssclass='codehilite',
              style='default',
              noclasses=False,
          )
          return mark_safe(highlight(src, lexer, formatter))
  
subject: Add context=None to render as well.