old_contents: """ContainerWidget class.  
  
  Represents a container that can be used to group other widgets.
  """
  
  # Copyright (c) IPython Development Team.
  # Distributed under the terms of the Modified BSD License.
  
  from .widget import DOMWidget
  from IPython.utils.traitlets import Unicode, Tuple, TraitError
  
  class ContainerWidget(DOMWidget):
      _view_name = Unicode('ContainerView', sync=True)
  
      # Child widgets in the container.
      # Using a tuple here to force reassignment to update the list.
      # When a proper notifying-list trait exists, that is what should be used here.
      children = Tuple(sync=True)
  
      def __init__(self, **kwargs):
          super(ContainerWidget, self).__init__(**kwargs)
          self.on_displayed(ContainerWidget._fire_children_displayed)
  
      def _fire_children_displayed(self):
          for child in self.children:
              child._handle_displayed()
  
  
  class PopupWidget(ContainerWidget):
      _view_name = Unicode('PopupView', sync=True)
      
      description = Unicode(sync=True)
      button_text = Unicode(sync=True)
  
new_contents: """ContainerWidget class.  
  
  Represents a container that can be used to group other widgets.
  """
  
  # Copyright (c) IPython Development Team.
  # Distributed under the terms of the Modified BSD License.
  
  from .widget import DOMWidget
  from IPython.utils.traitlets import Unicode, Tuple, TraitError
  
  class ContainerWidget(DOMWidget):
      _view_name = Unicode('ContainerView', sync=True)
  
      # Child widgets in the container.
      # Using a tuple here to force reassignment to update the list.
      # When a proper notifying-list trait exists, that is what should be used here.
      children = Tuple(sync=True)
  
      def __init__(self, children = None, **kwargs):
          kwargs['children'] = children
          super(ContainerWidget, self).__init__(**kwargs)
          self.on_displayed(ContainerWidget._fire_children_displayed)
  
      def _fire_children_displayed(self):
          for child in self.children:
              child._handle_displayed()
  
  
  class PopupWidget(ContainerWidget):
      _view_name = Unicode('PopupView', sync=True)
      
      description = Unicode(sync=True)
      button_text = Unicode(sync=True)
  
subject: Make Container widgets take children as the first positional argument