[Python-bugs-list] [ python-Bugs-660098 ] New style classes and __hash__

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Bugs item #660098, was opened at 2002-12-30 18:39
Message generated for change (Comment added) made by jhylton
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Category: None
Group: None
Status: Open
Resolution: None
Priority: 3
Submitted By: Thomas Heller (theller)
Assigned to: Guido van Rossum (gvanrossum)
Summary: New style classes and __hash__

Initial Comment:
New style classes obviously inherit a __hash__ 
implementation from object which returns the id. Per 
default this allows using instances as dictionary keys, 
but usually with the wrong behaviour, because most 
often user classes are mutable, and their contained data 
should be used to calculate the hash value.

IMO one possible solution would be to change 
typeobject.c:object_hash() to raise TypeError, and 
change all the immutable (core) Python objects to use 
_Py_HashPointer in their tp_hash slot.

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>Comment By: Jeremy Hylton (jhylton)
Date: 2003-05-09 17:32

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Currently, a new-style class that defines __cmp__ but not
__hash__ is usable as a dictionary key.  That seems related
to this bug.  Should I paste the example here and bump the
priority?  Or should I open a separate bug report?


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Comment By: Guido van Rossum (gvanrossum)
Date: 2003-02-11 23:01

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I spent an afternoon looking into this, and I can't see an
easy solution. The idea of only inheriting __hash__ together
with certain other slots is really flawed; it may be better
if object DIDN'T define a default implementation for
__hash__, comparisons (both flavors), and other things, or
maybe the default __hash__ should raise an exception when
the comparisons are not those inherited from object, or
maybe PyType_Ready should insert a dummy __hash__ when it
sees that you redefine __eq__, or...  I really don't know.
I'm going to sleep on this some more, and lower the
priority. You can always get the right behavior by
explicitly defining __hash__.

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Comment By: Thomas Heller (theller)
Date: 2002-12-30 18:50

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You mean at the end of the inherit_slots() function?
For my extension which I'm currently debugging, tp_compare, 
tp_richcompare, and tp_hash are inherited from base, but 
only tp_hash is != NULL there.

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Comment By: Guido van Rossum (gvanrossum)
Date: 2002-12-30 18:44

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There seems to be code that tries to inherit tp_hash only
when tp_compare and tp_richcompare are also inherited, but
it seems to be failing.

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