Hello, people. I am not sure whether this is a bug or intentional, so I thought checking it with you before opening a bug. I will explain this issue, but please understand this is not a question for help to change the algorithm (this has been done already), so it's not a question of c.l.py. It's a matter of discrepancy.
A list that changes while iterating produces no errors, while a deque fails. Given the following example code: #code start import itertools, collections def item_is_special(item): "Just a dummy check in this example" return item % 3 == 0 def item_products(item): "Also a dummy function for the example" return [item*20+1, item*30+1] def process_list(items, type_of_que, special_case): # we want to process some items, but some of them # produce other items to be processed by the # same algorithm products= type_of_que() if special_case: products.append(-1) for item in itertools.chain(items, products): if item_is_special(item): for product in item_products(item): products.append(product) else: print "got", item #code end we have the following cases: >>> process_list(numbers, list, False) got 1 got 2 got 61 got 91 List works as expected. >>> process_list(numbers, collections.deque, False) got 1 got 2 deque does not work, most probably because deque.__iter__ of an empty deque ignores later changes. For this reason the `special_case' parameter was inserted in the code above, so that there is at least an item when itertools.chain calls iter on the deque: >>> process_list(numbers, collections.deque, True) got 1 got 2 Traceback (most recent call last): File "<stdin>", line 1, in <module> File "testdequeiter.py", line 17, in process_list for item in itertools.chain(items, products): RuntimeError: deque mutated during iteration Is that intentional? If not, let me know so that I open a bug. _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com