On Tue, Dec 6, 2016, at 05:27, Patrick Westerhoff wrote: > Hey all, > > I just stumbled on the following comment in the C source of the repr > implementation for the list object: > > /* Do repr() on each element. Note that this may mutate the list, > so must refetch the list size on each iteration. */ > > (as seen in list_repr implementation [1]) > > I’m honestly very surprised about this remark since I can neither > understand why this would be the case (repr shouldn’t mutate the > list)
It *shouldn't*, but it can't be enforced. It's one of those things where if Python assumes all user code is sane (in this case, overridden __repr__ not messing with the list) it can bite in a way that could cause the interpreter to crash. >>> class EvilClass: ... def __repr__(x): ... l.pop() ... return 'x' ... >>> l = [EvilClass()]*10 >>> l [x, x, x, x, x] >, and I also don’t see any clue in the source as to when this > would actually happen. Since inside the loop, the list object `v` is > never accessed other than passing `v->ob_item[i]` to the recursive > repr call, there shouldn’t be any mutation on the list object itself. The item may have or be able to a reference to the list object otherwise, as I demonstrated. _______________________________________________ Python-Dev mailing list Python-Dev@python.org https://mail.python.org/mailman/listinfo/python-dev Unsubscribe: https://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
