class_docstr: Widget that can be inserted into the DOM
docstr: Adds a class to the top level element of the widget.
  
  Doesn't add the class if it already exists.
code: def add_class(self, className):
      """
      Adds a class to the top level element of the widget.
  
      Doesn't add the class if it already exists.
      """
      if className not in self._dom_classes:
          self._dom_classes = list(self._dom_classes) + [className]
      return self
  
file_path: catboost/contrib/python/ipywidgets/py2/ipywidgets/widgets/domwidget.py
imports: from traitlets import Unicode
  from .widget import Widget, widget_serialization
  from .trait_types import InstanceDict, TypedTuple
  from .widget_layout import Layout
  from .widget_style import Style
label_desc: Use image node_id 1 for calling the DOMWidget obj's underlying member method code with example usage: obj.add_class(className) and returns: self