docs: def form_group_classes(self):
          """
          Full list of classes for the class attribute of the form group. Returned as a string
          with spaces separating each class, ready for insertion into the class attribute.
  
          This will generally look like the following:
  
          'form-group has-error custom-class'
          """
          classes = ['form-group']
          if self.style == styles.BOOTSTRAP_4 and self.form_type == formtype.HORIZONTAL:
              classes.append('row')
          if self.error and self.style == styles.BOOTSTRAP_3:
              classes.append('has-error')
          if self.form_group_css_class:
              classes.append(self.form_group_css_class)
  
          return ' '.join(classes)