I'm doing a set difference operation in Python: from sets import Set from mongokit import ObjectId x = [ObjectId("4f7aba8a43f1e51544000006"), ObjectId("4f7abaa043f1e51544000007"), ObjectId("4f7ac02543f1e51a44000001")] y = [ObjectId("4f7acde943f1e51fb6000003")] print list(Set(x).difference(Set(y))) I'm getting: [ObjectId('4f7abaa043f1e51544000007'), ObjectId('4f7ac02543f1e51a44000001'), ObjectId('4f7aba8a43f1e51544000006')] I need to get the first element for next operation which is important. How can I retain the list x in original format?