On Wed, Oct 14, 2015 at 3:11 PM, candide via Python-list <python-list@python.org> wrote: > If set size changes during a for loop, a runtime exception is raised: > > [SNIP] > > Surprisingly, if a for loop changes the size of a list it is iterating on, no > exception is raised : > > [SNIP] > > So why lists and sets don't react the same way?
Because the behavior of iteration over a set (or dict) that changes while it's being iterated over is much more difficult to predict. Though you shouldn't insert or delete items in a list before the current iteration point, it's generally safe to append items while iterating. With a set or dict, *any* items that are added or deleted [1] during iteration could cause the entire data structure to be rebuilt, changing the buckets that everything is in and fundamentally altering the order of iteration. If that happens, all bets are off as to which items will be iterated over multiple times, which will not be iterated at all, or how the number of items iterated over will compare to the actual size of the set or dict. [1] Merely changing the value of a particular key in a dict is perfectly safe, however. -- https://mail.python.org/mailman/listinfo/python-list
