weird dict problem, how can this even happen?

[Arnaud Delobelle]

Joel Hedlund <yohell at ifm.liu.se> writes:

> I'm having a very hard time explaining why this snippet *sometimes*
> raises KeyError:
>
> snippet:
>> print type(self.pool)
>> for frag in self.pool.keys():
>>     if frag is fragment_info:
>>         print "the fragment_info *is* in the pool", hash(frag), hash(fragment_info), hash(frag) == hash(fragment_info), frag == fragment_info, frag in self.pool, frag in self.pool.keys()
>> try:
>>     renderer_index = self.pool.pop(fragment_info)
>> except KeyError:
>>     print "Glorious KeyError!"
>>     for frag in self.pool.keys():
>>         if frag is fragment_info:
>>             print "the fragment_info *is* in the pool", hash(frag), hash(fragment_info), hash(frag) == hash(fragment_info), frag == fragment_info, frag in self.pool, frag in self.pool.keys()
>>     raise
>
>
> output:
>> <type 'dict'>
>> the fragment_info *is* in the pool 987212075 987212075 True True False True
>> Glorious KeyError!
>> the fragment_info *is* in the pool 987212075 987212075 True True False True
>> Traceback (most recent call last):
>>   File "/home/yohell/workspace/missy/core/gui.py", line 92, in process_job
>>     renderer_index = self.pool.pop(fragment_info)
>> KeyError: <core.gui.FragmentInfo object at 0x8fc906c>
>
> This snippet is part of a much larger gtk program, and the problem
> only from time to time, predominantly when the cpu is under heavy load
> and this method gets called a lot. If I didn't know better I'd say
> it's a bug in python's dict implementation, but I do know better, so I
> know it's far more likely that I've made a mistake somewhere. I'll be
> damned if I can figure out what and where though. I've reproduced this
> bug (?) with python-2.5.2 on Ubuntu 8.10 and python-2.5.1 on WinXP.
>
> I would very much like an explanation to this that does not involve
> threads, because I haven't made any that I'm aware of. I can't even
> understand how this could happen. How do I even debug this?
>
> Please help, I feel like I've taken crazy pills here!
> /Joel Hedlund

It could happen if two object x and y are such that hash(x) != hash(y)
but x == y, breaking the requirement on hash values.

HTH

-- 
Arnaud