class_docstr: A proxy to access variables defined in a layer. 
code: def __setattr__(self, name, var):
      if not name.startswith("_"):
          self._add_variable(name, var)
      else:
          # private attributes
          super(VariableHolder, self).__setattr__(name, var)
  
imports: import six
label_desc: Use image node_id 3 for calling the VariableHolder obj's underlying member method code with example usage: obj.__setattr__(name, var) without return types