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Title: itertools.islice() doesn't release reference to the source iterator when the slice is exhausted
Type: resource usage Stage: resolved
Components: Extension Modules Versions: Python 3.4, Python 3.5, Python 2.7
Status: closed Resolution: fixed
Dependencies: Superseder:
Assigned To: rhettinger Nosy List: Anton.Afanasyev, pitrou, python-dev, rhettinger
Priority: normal Keywords: patch

Created on 2014-04-21 12:13 by Anton.Afanasyev, last changed 2022-04-11 14:58 by admin. This issue is now closed.

File name Uploaded Description Edit
issue21321_3.4_8c8315bac6a8.diff Anton.Afanasyev, 2014-04-21 12:13
issue21321_2.7_e3217efa6edd.diff Anton.Afanasyev, 2014-04-21 12:41
issue21321_3.4_8c8315bac6a8_2.diff Anton.Afanasyev, 2014-04-22 16:52
issue21321_2.7_e3217efa6edd_3.diff Anton.Afanasyev, 2014-04-28 19:47
issue21321_3.4_8c8315bac6a8_3.diff Anton.Afanasyev, 2014-04-28 19:48
issue21321_2.7_e3217efa6edd_4.diff Anton.Afanasyev, 2014-04-29 06:27
issue21321_3.4_8c8315bac6a8_4.diff Anton.Afanasyev, 2014-04-29 06:27
issue21321_3.4_8c8315bac6a8_5.diff Anton.Afanasyev, 2014-04-29 09:55
Messages (17)
msg216939 - (view) Author: Anton Afanasyev (Anton.Afanasyev) * Date: 2014-04-21 12:13
This issue results in redundant memory consumption for e.g. in this case:

from itertools import *

def test_islice():

    items, lookahead = tee(repeat(1, int(1e9)))
    lookahead = islice(lookahead, 10)

    for item in lookahead:

    for item in items:

if __name__ == "__main__":
This demo is taken from real case where one needs to look ahead input stream before processing it. For my PC this demo stops with 'Segmentation fault' message after exhausting all PC memory, while running it with patched python consumes only 0.1% of memory till the end.

When one uses pure pythonic implementation of itertools.islice() (taken from docs), the issue goes away as well, since this implementation doesn't store redundant reference to source iterator.

def islice(iterable, *args):
    s = slice(*args)
    it = iter(xrange(s.start or 0, s.stop or sys.maxint, s.step or 1))
    nexti = next(it)
    for i, element in enumerate(iterable):
        if i == nexti:
            yield element
            nexti = next(it)

Attaching patch for this issue. Have to change '__reduce__()' method since now unpickling of exhausted 'islice()' object cannot return old source iterator.
msg216940 - (view) Author: Anton Afanasyev (Anton.Afanasyev) * Date: 2014-04-21 12:41
Added patch for 2.7 version (no need to change '__reduce__()' method since it's not implemented).
msg216992 - (view) Author: Raymond Hettinger (rhettinger) * (Python committer) Date: 2014-04-22 06:58
The ref-counts in the islice_reduce code don't look to be correct at first glance.
msg217014 - (view) Author: Anton Afanasyev (Anton.Afanasyev) * Date: 2014-04-22 16:52
Hi Raymond,
do you mean allocation exceptions handling should be more accurate?
Attaching fixed version for 3.4 branch.
msg217180 - (view) Author: Antoine Pitrou (pitrou) * (Python committer) Date: 2014-04-26 01:55
Haven't reviewed the patch, but you should definitely add a unit test for the bugfix.
msg217407 - (view) Author: Anton Afanasyev (Anton.Afanasyev) * Date: 2014-04-28 19:47
Hi Antoine,
I have no found a way to check resource usage in test infrastructure and I don't think it could be done carefully. The only method I found to test issue is straightforward: just to check source iterator is not referenced from itertools.islice() after the latter has been exhausted:

a = [random.random() for i in range(10)]
before = sys.getrefcount(a)
b = islice(a, 5)
for i in b: pass
after = sys.getrefcount(a)
self.assertEqual(before, after)

Attaching "issue21321_2.7_e3217efa6edd_3.diff" and "issue21321_3.4_8c8315bac6a8_3.diff" patches with this test included in "Lib/test/".
msg217410 - (view) Author: Antoine Pitrou (pitrou) * (Python committer) Date: 2014-04-28 19:54
Anton, the test is wrong: it is taking a reference to the iterable object (the list), not the iterator.

To check the reference to the original iterator is released, something like this would work:

>>> import itertools, weakref
>>> it = (x for x in (1, 2))
>>> wr = weakref.ref(it)
>>> it = itertools.islice(it, 1)
>>> wr() is None
>>> list(it)
>>> wr() is None  # returns True with the patch, False without
msg217411 - (view) Author: Antoine Pitrou (pitrou) * (Python committer) Date: 2014-04-28 19:55
(note I haven't looked at the C part of the patch)
msg217474 - (view) Author: Anton Afanasyev (Anton.Afanasyev) * Date: 2014-04-29 06:27
Hi Antoine,
my test works for me. It can be either
>>> a = [1, 2, 3]
>>> a = iter([1, 2, 3])
, no matter: both objects will be +1 referenced after taking
>>> b = islice(a, 1)
My test failed without patch and passed with one.

But your test is more straightforward, thanks.
Attaching patches with your test.
msg217495 - (view) Author: Antoine Pitrou (pitrou) * (Python committer) Date: 2014-04-29 08:39
Thanks. Could you also add a test for the islice_reduce additions? Or is it already tested?
I suspect there's a reference leak there: after calling PyObject_GetIter, you should always Py_DECREF(empty_list).
Also, with the "O" code, Py_BuildValue will take a new reference to empty_it, so you should use the "N" code instead.
msg217503 - (view) Author: Anton Afanasyev (Anton.Afanasyev) * Date: 2014-04-29 09:55
Hi Antoine,
oops you are right about leaks: fixed them in new attached patch.
As for testing changes in "reduce()": they are already covered by "self.pickletest(islice(range(100), *args))". Function "pickletest()" covers case for pickle dumping/loading of exhausted iterator.
msg217504 - (view) Author: Antoine Pitrou (pitrou) * (Python committer) Date: 2014-04-29 10:12
For the record, checks such as:

        self.assertEqual(wr() is None, False)

are better written:


No need to upload a new patch, I'm gonna make the change while committing :-)
msg217505 - (view) Author: Roundup Robot (python-dev) (Python triager) Date: 2014-04-29 10:14
New changeset b795105db23a by Antoine Pitrou in branch '3.4':
Issue #21321: itertools.islice() now releases the reference to the source iterator when the slice is exhausted.

New changeset a627b3e3c9c8 by Antoine Pitrou in branch 'default':
Issue #21321: itertools.islice() now releases the reference to the source iterator when the slice is exhausted.
msg217506 - (view) Author: Antoine Pitrou (pitrou) * (Python committer) Date: 2014-04-29 10:15
Patch committed, thank you!
If you want to provide a patch for 2.7, please say so, otherwise I'll close the issue.
msg217508 - (view) Author: Anton Afanasyev (Anton.Afanasyev) * Date: 2014-04-29 10:21
Antoine, not sure about 2.7. The issue first arose for me at Python 2.7, so I would prefer "issue21321_2.7_e3217efa6edd_4.diff" patch be applied.
msg217509 - (view) Author: Roundup Robot (python-dev) (Python triager) Date: 2014-04-29 10:26
New changeset 8ee76e1b5aa6 by Antoine Pitrou in branch '2.7':
Issue #21321: itertools.islice() now releases the reference to the source iterator when the slice is exhausted.
msg217510 - (view) Author: Antoine Pitrou (pitrou) * (Python committer) Date: 2014-04-29 10:27
Ok, then I've committed to 2.7 too. Thank you very much for contributing!
Date User Action Args
2022-04-11 14:58:02adminsetgithub: 65520
2014-04-29 10:27:27pitrousetstatus: open -> closed

messages: + msg217510
2014-04-29 10:26:59python-devsetmessages: + msg217509
2014-04-29 10:21:38Anton.Afanasyevsetmessages: + msg217508
2014-04-29 10:15:48pitrousetresolution: fixed
messages: + msg217506
stage: resolved
2014-04-29 10:14:59python-devsetnosy: + python-dev
messages: + msg217505
2014-04-29 10:12:04pitrousetmessages: + msg217504
2014-04-29 09:55:51Anton.Afanasyevsetfiles: + issue21321_3.4_8c8315bac6a8_5.diff

messages: + msg217503
2014-04-29 08:39:22pitrousetmessages: + msg217495
2014-04-29 06:27:51Anton.Afanasyevsetfiles: + issue21321_3.4_8c8315bac6a8_4.diff
2014-04-29 06:27:22Anton.Afanasyevsetfiles: + issue21321_2.7_e3217efa6edd_4.diff
2014-04-29 06:27:03Anton.Afanasyevsetmessages: + msg217474
2014-04-28 19:55:20pitrousetmessages: + msg217411
2014-04-28 19:54:56pitrousetmessages: + msg217410
2014-04-28 19:48:34Anton.Afanasyevsetfiles: + issue21321_3.4_8c8315bac6a8_3.diff
2014-04-28 19:47:57Anton.Afanasyevsetfiles: + issue21321_2.7_e3217efa6edd_3.diff

messages: + msg217407
2014-04-26 01:55:17pitrousetmessages: + msg217180
2014-04-25 22:25:51terry.reedysetversions: + Python 3.5, - Python 3.1, Python 3.2, Python 3.3
2014-04-23 00:04:36pitrousetnosy: + pitrou
2014-04-22 16:52:50Anton.Afanasyevsetfiles: + issue21321_3.4_8c8315bac6a8_2.diff

messages: + msg217014
2014-04-22 06:58:12rhettingersetassignee: rhettinger
messages: + msg216992
2014-04-21 12:41:36Anton.Afanasyevsetfiles: + issue21321_2.7_e3217efa6edd.diff

messages: + msg216940
2014-04-21 12:13:50Anton.Afanasyevcreate