code: def setdefaultlist(self, key, defaultlist=[None]):
          
          if key in self:
              return self.getlist(key)
          self.addlist(key, defaultlist)
          return defaultlist
docs: Similar to setdefault() except <defaultlist> is a list of values to set
          for <key>. If <key> already exists, its existing list of values is
          returned.
  
          If <key> isn't a key and <defaultlist> is an empty list, [], no values
          are added for <key> and <key> will not be added as a key.
  
          Returns: List of <key>'s values if <key> exists in the dictionary,
          otherwise <default>.