code: def set_default(self, name, value):
          
          if self.vars is None:
              self.vars = {}
          if name not in self.vars:
              self.vars[name] = value
docs: Like set(), but only sets the value if the variable is not already
          defined.
  
          :type  name: string
          :param name: The name of the variable.
          :type  value: object
          :param value: The value of the variable.