See Brief request syntax to understand each keyword.

#!/usr/bin/env python
from ecmwfapi import ECMWFDataServer
   
server = ECMWFDataServer()
   
server.retrieve({
    'stream'    : "oper",
    'levtype'   : "sfc",
    'expver'    : "rean",
    'dataset'   : "macc",
    'step'      : "0",
    'grid'      : "0.75/0.75",
    'param'     : "31.128",
    'time'      : "00/06/12/18",
    'date'      : "2012-08-01/to/2012-08-31",
    'type'      : "an",
    'class'     : "mc",
    'target'    : "macc_2012-08-01to2012-08-31_00061218.grib"
})