[Python-Dev] Negative times behaviour in itertools.repeat for Python maintenance releases (2.7, 3.3 and maybe 3.4) (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Mon Jan 27 13:43:39 CET 2014

On 27 January 2014 22:29, Antoine Pitrou <solipsis at pitrou.net> wrote:

On Mon, 27 Jan 2014 20:22:53 +0800 Vajrasky Kok <sky.kok at speaklikeaking.com> wrote:

>>> from itertools import repeat >>> list(repeat('a', 2**31)) Traceback (most recent call last): File "", line 1, in MemoryError Sure, just adjust the number to fit the available memory (here, 2**29 does the trick).

And for anyone interested in why a sufficiently large positive value that won't fit in available RAM fails gracefully with MemoryError:

repeat('a', 2**31).lengthhint() 2147483648 repeat('a', -1).lengthhint() 0

list() uses length_hint() for preallocation, so a sufficiently large length hint means the preallocation attempt fails with MemoryError. As Antoine showed though, you still can't feed it untrusted data, because a large enough value that just fits into RAM can still cause you a lot of grief.

Everything points to "times=-1" behaving as it does being a bug, but not a sufficiently critical one to risk breaking working code in a maintenance release. That makes deprecating the current behaviour of "times=-1" and accepting "times=None" in 3.5 the least controversial course of action.

Cheers, Nick.

-- Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia

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