code: def set_if_unset(self, key, value):
          
          if key not in self._conf_dict:
              self.set(key, value)
          return self
docs: Set a particular spark property by the string key name if it hasn't already been set.
  
          This method allows chaining so that i can provide a similar feel to the standard Scala way of setting
          multiple configurations
  
          Parameters
          ----------
          key : string
          value : string
  
          Returns
          -------
          self