To get a set of elements that are present in data1, but not in data2, we use the difference() instruction.
Using difference() gives us the elements that the left set, here data1 has, but the right set, here data2, doesn't.
>>> data1 = {1,2,3}
>>> data2 = {3,4,5}
>>> data1.difference(data2)
{1, 2}