I am new to python and OpenERP and I apologise if my question is inconvenient. But I am trying to add a control that check if a field is empty, but I am getting this error on OpenERP

if isinstance(res2[record['id']], str): res2[record['id']] = eval(res2[record['id']]) #TOCHECK : why got string instend of dict in python2.6
KeyError

Please could you help me? What the problem could be?  Thanks in advance