[Python-Dev] Issue 19332: Guard against changing dict during iteration (original) (raw)

Antoine Pitrou solipsis at pitrou.net
Wed Nov 6 18:16:59 CET 2013

Le 06/11/2013 06:41, Nick Coghlan a écrit :

The behaviour of mutating builtin containers while iterating over them is formally undefined beyond "it won't segfault" (one of the few such undefined behaviours in Python). The associated exceptions are thus strictly "best effort given other constraints".

Not sure what you mean with "formally undefined". For example, you can perfectly well change a list's contents while iterating over it, and I bet there's a lot of code that relies on that, as in::

for i, value in enumerate(mylist): if some_condition(value): mylist[i] = some_function(value)

If you change "builtin containers" to "builtin unordered containers", then you probably are closer to the truth :)

>or perhaps some performance measurements that would show there is no performance based reason to not have the patch added. (I can try to do the performance part myself if necessary, I'm just not sure what all the steps are yet.)

If the benchmark suite indicates there's no measurable speed penalty then such a patch may be worth reconsidering. I'd be astonished if that was actually the case, though - the lowest impact approach I can think of is to check for live iterators when setting a dict entry, and that still has non-trivial size and speed implications.

I think Serhiy's patch is much simpler than that (is it Serhiy's? I think so).

Regards

Antoine.

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