[Python-Dev] Bug or not? Different behaviour iterating list and collections.deque (original) (raw)

Christos Georgiou tzot at mediconsa.com
Sun Jan 7 11:38:04 CET 2007

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 [item20+1, item30+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:

processlist(numbers, list, False) got 1 got 2 got 61 got 91

List works as expected.

processlist(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:

processlist(numbers, collections.deque, True) got 1 got 2 Traceback (most recent call last): File "", line 1, in 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.

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