code: def widget(self, f):
          
          return self.cls(f, self.opts, **self.kwargs)
docs: Return an interactive function widget for the given function.
  
          The widget is only constructed, not displayed nor attached to
          the function.
  
          Returns
          -------
          An instance of ``self.cls`` (typically :class:`interactive`).
  
          Parameters
          ----------
          f : function
              The function to which the interactive widgets are tied.