EntityTemplate has no property of parent_type

[xiaodong shang]

EntityTemplate is the parent of NodeTemplate，but there is no parent_type definition, so the method of is_derived_from may be error when judge one nodetemplate whether is a instance of grandparent node type, the current code logic in EntityTemplate class is below:

     def is_derived_from(self, type_str):
        '''Check if object inherits from the given type.

        Returns true if this object is derived from 'type_str'.
        False otherwise.
        '''
        if not self.type:
            return False
        elif self.type == type_str:
            return True
        elif self.parent_type: # Here may be error
            return self.parent_type.is_derived_from(type_str)
        else:
            return False