code: def rowgroupmap(table, key, mapper, header=None, presorted=False,
                  buffersize=None, tempdir=None, cache=True):
      
  
      return RowGroupMapView(table, key, mapper, header=header,
                             presorted=presorted,
                             buffersize=buffersize, tempdir=tempdir, cache=cache)
docs: Group rows under the given key then apply `mapper` to yield zero or more
      output rows for each input group of rows.