[Python-Dev] Review needed: proposed fix for 2.6 __hash__ incompatibility (issue 2235)

[Nick Coghlan]

Guido van Rossum wrote:
> On Wed, Jul 2, 2008 at 5:31 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:
>> I've posted a possible fix for the __hash__ backwards incompatibilities
>> described in issue 2235 [1].
>>
>> The patch uses a model similar to that used in Py3k (using None is indicate
>> "don't inherit __hash__"), but extends it to allowing Py_None to be
>> explicitly stored in the tp_hash slot. The major downside is that we suffer
>> the cost of an extra pointer comparison on every call to PyObject_Hash, but
>> I wasn't able to come up with another solution that preserved backwards
>> compatibility while still allowing collections.Hashable to function
>> correctly.
> 
>>From your description it seems storing Py_None in the slot acts as a
> magic value meaning "this is defined but not usable". However it used
> to be pretty common for various code around to call various slots
> directly (after a NULL) check. That would have disastrous results if
> the slot value was Py_None. Would it be terribly inconvenient if the
> magic value was in fact another function, with a public name), whose
> sole purpose was to raise an exception?

Not only not inconvenient, but a significant improvement - as well as 
addressing your concern that I missed some code that calls tp_hash 
directly (a concern that I share, particularly since it could be an 
extension module we don't control that ends up doing it), it also gets 
rid of that extra pointer comparison in PyObject_Hash that was bothering me.

>> The patch involves a few changes to fairly deep components in typeobject.c
>> though, so I'd like at least some kind of sanity check before I commit it.
>>
>> Cheers,
>> Nick.
>>
>> [1] http://bugs.python.org/issue2235
> 
> I can't promise I'll have time to look at this before my EuroPython
> keynote, but it's important for me to get it right, so if nobody else
> jumps in, remind me Tuesday.

I'd now advise waiting until I have a chance to implement your idea 
anyway :)

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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