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

Maciej Fijalkowski fijall at gmail.com
Tue Apr 3 23:47:01 CEST 2012

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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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?

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