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

Python set an unordered collection which do not record element position or order of insertion. There is no index attached to any element in a python set. So they do not support any indexing or slicing operation.

So don't expect your for loop will work in a defined order.

Why does it do 16 iterations?

user2357112 supports Monica already explains the main cause. Here, is another way of thinking.

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

When you run this code it gives you output this :

{0, 1}                                                                                                                               
{1, 2}                                                                                                                               
{2, 3}                                                                                                                               
{3, 4}                                                                                                                               
{4, 5}                                                                                                                               
{5, 6}                                                                                                                               
{6, 7}                                                                                                                               
{7, 8}
{8, 9}                                                                                                                               
{9, 10}                                                                                                                              
{10, 11}                                                                                                                             
{11, 12}                                                                                                                             
{12, 13}                                                                                                                             
{13, 14}                                                                                                                             
{14, 15}                                                                                                                             
{16, 15}                                                                                                                             
{16}       

When we access all the elements together like loop or printing the set, there must be a predefined order for it to traverse the whole set. So, in last iteration you will see order is changed like from {i,i+1} to {i+1,i}.

After the last iteration it happened that i+1 is already traversed so loop exit.

Interesting Fact: Use any value less than 16 except 6 and 7 will always gives you result 16.