The array of IDs may have duplicates but don't need to find or remove them.

so, to make the IDs unique without removing or finding them, I passed the arrays to the set

They iterate lst, not x, it's called list comprehension, you can read more here: https://www.programiz.com/python-programming/list-comprehension

Edit: didn't mean to post this

Read the error message you get:

TypeError: 'int' object is not iterable

x is an int, you can't make a list of it. Even if you could, it would return a list of the first number.