Adrian McCarthy Adrian McCarthy - 8 days ago 4
Python Question

Python set iteration order varies from run to run

Why does the iteration order of a Python set (with the same contents) vary from run to run, and what are my options for making it consistent from run to run?

I understand that the iteration order for a Python set is arbitrary. If I put 'a', 'b', and 'c' into a set and then iterate them, they may come back out in any order.

What I've observed is that the order remains the same within a run of the program. That is, if my program iterates the same set twice in a row, I get the same order both times. However, if I run the program twice in a row, the order changes from run to run.

Unfortunately, this breaks one of my automated tests, which simply compares the output from two runs of my program. I don't care about the actual order, but I would like it to be consistent from run to run.

The best solution I've come up with is:


  1. Copy the set to a list.

  2. Apply an arbitrary sort to the list.

  3. Iterate the list instead of the set.



Is there a simpler solution?

Note: I've found similar questions on StackOverlow, but none that address this specific issue of getting the same results from run to run.

Answer

What you want isn't possible. Arbitrary means arbitrary.

My solution would be the same as yours, you have to sort the set if you want to be able to compare it to another one.