code: def process_keys(func):
      
  
      @wraps(func)
      def decorated(self, k, *args):
          if not isinstance(k, str):
              msg = "%s: key must be a string" % self.__class__.__name__
              raise ValueError(msg)
  
          if not k.startswith(self.prefix):
              k = self.prefix + k
  
          return func(self, k, *args)
  
      return decorated
docs: Raise error for keys that are not strings
      and add the prefix if it is missing