If you do not want to use the MARS syntax with the mars script we provide, you can directly use Python. The execute function accepts both a Python dictionary or a string. To use the 'list' function, the argument must be a string starting with 'list,'. In this example we convert the request dictionary to a string with 'list':

server = ECMWFService("mars")
req = {
    "class": "od",
    "date": "20150101",
    "expver": "1",
    "levtype": "sfc",
    "param": "167.128",
    "step": "0/to/240/by/12",
    "stream": "oper",
    "time": "00",
    "type": "fc"

kv=['{}={}'.format(k, v) for k, v in req.items()]