code: def add_widget(self, widget):
          
          self._widgets.append(widget)
          widget.parent = self
          self._update_child_widgets()
          return widget
docs: Add a Widget as a managed child of this Widget.
  
          The child will be
          automatically positioned and sized to fill the entire space inside
          this Widget (unless _update_child_widgets is redefined).
  
          Parameters
          ----------
          widget : instance of Widget
              The widget to add.
  
          Returns
          -------
          widget : instance of Widget
              The widget.