On 08/28/2016 09:26 AM, Nick Coghlan wrote:
On 28 August 2016 at 08:25, Terry Reedy <tjre...@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.
+1. Being able to do such strange things with list but not dict would be
irritating and a nuisance (although maybe not attractive ;) .
__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.
Agreed.
--
~Ethan~
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