Guido van Rossum wrote:
On Wed, Jul 2, 2008 at 5:31 AM, Nick Coghlan <[EMAIL PROTECTED]> 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 | [EMAIL PROTECTED] | Brisbane, Australia
---------------------------------------------------------------
http://www.boredomandlaziness.org
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