It is because you set y_2 to be the same list as y. That list is an object, and you created two references to that object. If you want to create an independent copy of the list you could use y_2 = list(y), or use the copy module in more complicated cases.
Btw please just change that first line in to three lines; it would be a tremendous increase of readability.
solved Changing one list happen to changed back up list [duplicate]