Why do I get this many iterations when adding to and removing from a set while iterating over it?

Question

Trying to understand the Python for-loop, I thought this would give the result {1} for one iteration, or just get stuck in an infinite loop, depending on if it does

Answer 1

From the python 3 documentation:

Code that modifies a collection while iterating over that same collection can be tricky to get right. Instead, it is usually more straight-forward to loop over a copy of the collection or to create a new collection:

Iterate over a copy

s = {0}
s2 = s.copy()
for i in s2:
     s.add(i + 1)
     s.remove(i)

which should iterate only 1 time

>>> print(s)
{1}
>>> print(s2)
{0}

Edit: A Possible reason for this iteration is because a set is unordered, causing some kind of stack trace sort of thing. If you do it with a list and not a set, then it will just end, with s = [1] because lists are ordered so the for loop will start with index 0 and then move on to the next index, finding that there isn't one, and exiting the loop.