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

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 [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. 




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