[Python-Dev] Issue 14417: consequences of new dict runtime error

[Guido van Rossum]

I'm confused. Are you saying that that program always raised
RuntimeError, or that it started raising RuntimeError with the new
behavior (3.3 alpha 2)?

On Tue, Apr 3, 2012 at 2:47 PM, Maciej Fijalkowski <fijall at gmail.com> wrote:
> On Sat, Mar 31, 2012 at 7:45 PM, R. David Murray <rdmurray at bitdance.com>
> wrote:
>>
>> On Sun, 01 Apr 2012 03:03:13 +1000, Nick Coghlan <ncoghlan at gmail.com>
>> wrote:
>> > On Sun, Apr 1, 2012 at 2:09 AM, Guido van Rossum <guido at python.org>
>> > wrote:
>> > > Here's a different puzzle. Has anyone written a demo yet that provokes
>> > > this RuntimeError, without cheating? (Cheating would be to mutate the
>> > > dict from *inside* the __eq__ or __hash__ method.) If you're serious
>> > > about revisiting this, I'd like to see at least one example of a
>> > > program that is broken by the change. Otherwise I think the status quo
>> > > in the 3.3 repo should prevail -- I don't want to be stymied by
>> > > superstition.
>> >
>> > I attached an attempt to *deliberately* break the new behaviour to the
>> > tracker issue. It isn't actually breaking for me, so I'd like other
>> > folks to look at it to see if I missed something in my implementation,
>> > of if it's just genuinely that hard to induce the necessary bad timing
>> > of a preemptive thread switch.
>>
>> Thanks, Nick.  It looks reasonable to me, but I've only given it a quick
>> look so far (I'll try to think about it more deeply later today).
>>
>> If it is indeed hard to provoke, then I'm fine with leaving the
>> RuntimeError as a signal that the application needs to add some locking.
>> My concern was that we'd have working production code that would start
>> breaking.  If it takes a *lot* of threads or a *lot* of mutation to
>> trigger it, then it is going to be a lot less likely to happen anyway,
>> since such programs are going to be much more careful about locking
>> anyway.
>>
>> --David
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>
>
> Hm
>
> I might be missing something, but if you have multiple threads accessing a
> dict, already this program: http://paste.pocoo.org/show/575776/ raises
> RuntimeError. You'll get slightly more obscure cases than changing a size
> raise RuntimeError during iteration under PyPy. As far as I understood, if
> you're mutating while iterating, you *can* get a runtime error.
>
> This does not even have a custom __eq__ or __hash__. Are you never iterating
> over dicts?
>
> Cheers,
> fijal



-- 
--Guido van Rossum (python.org/~guido)