code: def map(self, map_fn, desc=None):
          
          if desc is None:
              desc = getattr(map_fn, '__name__', '')
          desc = u'map({})'.format(desc)
  
          return self.transform(lambda xs: (map_fn(x) for x in xs), desc=desc)
docs: Return a copy of this query, with the values mapped through `map_fn`.
  
          Args:
              map_fn (callable): A callable that takes a single argument and returns a new value.
  
          Keyword Args:
              desc (str): A description of the mapping transform, for use in log message.
                  Defaults to the name of the map function.
  
          Returns:
              Query