Python's sets (and dictionaries) will iterate and print out in some order, but exactly what that order will be is arbitrary, and not guaranteed to remain the same after additions and removals.
Here's an example of a set changing order after a lot of values are added and then removed:
>>> s = set([1,6,8])
>>> print(s)
{8, 1, 6}
>>> s.update(range(10,100000))
>>> for v in range(10, 100000):
s.remove(v)
>>> print(s)
{1, 6, 8}
This is implementation dependent though, and so you should not rely upon it.