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

Maciej Fijalkowski fijall at gmail.com
Tue Aug 30 09:11:25 EDT 2016

On Tue, Aug 30, 2016 at 2:31 PM, Dima Tisnek <dimaqq at gmail.com> wrote:

On 30 August 2016 at 14:13, Serhiy Storchaka <storchaka at gmail.com> wrote:

1. Detect length change and raise.

It would be simpler solution. But I afraid that this can break third-party code that "just works" now. For example slicing a list "just works" if step is 1. It can return not what the author expected if a list grows, but it never crashes, and existing code can depends on current behavior. This solution is not applicable in maintained versions. Serhiy, If dictionary is iterated in thread1 while thread2 changes the dictionary, thread1 currently raises RuntimeError. Would cloning current dict behaviour to slice with overridden index make sense? I'd argue 3rd party code depends on slicing not to raise an exception, is same as 3rd party code depending on dict iteration not to raise and exception; If same container may be concurrently used in another thread, then 3rd party code is actually buggy. It's OK to break such code. Just my 2c.

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I'm with Dima here.

It's more complicated - if the third party rely on the code working when one thread slices while the other thread modifies that gives implicit atomicity requirements. Those specific requirements are very hard to maintain across the python versions and python implementations. Replicating the exact CPython behavior (for each CPython version too!) is a major nightmare for such specific scenarios.

I propose the following:

-or-

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