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The right format if you want to specify params for each field is :
fields = ['CONTR_MNTH(CH=Fd RH=IN)',
'CF_NETCHNG(CH=Fd RH=IN)',
'CF_ASK(CH=Fd RH=IN)',
'CF_BID(CH=Fd RH=IN)',
'SETTLE(CH=Fd RH=IN)',
'HST_VOL(CH=Fd RH=IN)',
'CF_VOLUME(CH=Fd RH=IN)',
'OPINT_1(CH=Fd RH=IN)',
'EXPIR_DATE(CH=Fd RH=IN)',
'ACVOL(CH=Fd RH=IN)',
'GEN_VAL1(CH=Fd RH=IN)',
'GEN_VAL2(CH=Fd RH=IN)',
'GEN_VAL3(CH=Fd RH=IN)',
'GEN_VAL4(CH=Fd RH=IN)',
'LOT_SIZE_A(CH=Fd RH=IN)']