[Python-Dev] What do we do about bad slicing and possible crashes (issue 27867)

Nick Coghlan ncoghlan at gmail.com
Sun Aug 28 12:26:59 EDT 2016

On 28 August 2016 at 08:25, Terry Reedy <tjreedy at udel.edu> wrote:
> Slicing can be made to malfunction and even crash with an 'evil' __index__
> method. https://bugs.python.org/issue27867
> The crux of the problem is this: PySlice_GetIndicesEx
> receives a slice object and a sequence length.  Calling __index__ on the
> start, stop, and step components can mutate the sequence and invalidate the
> length.  Adjusting the int values of start and stop according to an invalid
> length (in particular, one that is too long) will result in invalid results
> or a crash.
> Possible actions -- very briefly.  For more see end of
> https://bugs.python.org/issue27867?@ok_message=msg 273801
> 0. Do nothing.
> 1. Detect length change and raise.

I suggest taking this path - it's the lowest impact, and akin to the
"dictionary changed size during iteration" runtime error.

__index__ having side effects is pathological code behaviour, so we
really just need to prevent the interpreter crash, rather than trying
to make it sense of it.


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

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