[Python-bugs-list] [ python-Bugs-575229 ] multiple inheritance w/ slots dumps core

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Bugs item #575229, was opened at 2002-06-29 01:33
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Category: Python Interpreter Core
Group: Python 2.2.1
Status: Open
Resolution: None
Priority: 5
Submitted By: Cesar Douady (douady)
Assigned to: Nobody/Anonymous (nobody)
Summary: multiple inheritance w/ slots dumps core

Initial Comment:
The following script dumps a core (revision 2.2.1) :

class A(object): __slots__=()
class B(object): pass
class C(A,B) : __slots__=()
C().x=2

basic sizes for those classes are:
A => 8 (correct : no dict, no weakref)
B => 16 (correct : dict and weakref slots added)
C => 8 (incorrect : dict and weakref not inherited from
B)

in the last lines of type_new, variables add_dict and
add_weak
are exploited to add those slots, but only when
slots==NULL.
The best base selected for C is A, which is correct in
my opinion
since slots come from it, but dict and weak refs must
be treated
specially and can come from another base.

I have seen nothing in the cvs repository which may fix
this bug.

I did not submit a patch since this is a touchy part of
the interpreter,
but I may do it upon request.

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>Comment By: Cesar Douady (douady)
Date: 2002-07-03 18:51

Message:
Logged In: YES 
user_id=428521

oops!
I meant C.__basicsize__ is now 12 (it has slot 'a'), but
this is still less than 16.

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Comment By: Cesar Douady (douady)
Date: 2002-07-03 18:47

Message:
Logged In: YES 
user_id=428521

I could not replicate the core dump with the latest 2.2 from
CVS,  but the problem still exist : C.__basicsize__ still is
8, which should not possible when
deriving from a class whose basic size is 16.
The following script show an abnormal behavior (actually the
one I had initially in my application):

class A(object): __slots__=('a')
class B(object): pass
class C(A,B) : __slots__=()
c=C()
c.x=2
c.a  ==> {'x':2}

In the original example, the B.__dictoffset__ would point
out of C objects, which may or may not create a core dump,
depending on a lot of factors.
In this script, B.__dictoffset__ points to slot 'a' of C,
hence 'c.x=2' creates the dictionary which is then seen as
the 'a' slot of c.

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Comment By: Neal Norwitz (nnorwitz)
Date: 2002-07-02 21:13

Message:
Logged In: YES 
user_id=33168

I could not replicate this problem from current CVS
nor from 2.2.1+ (cvs).  Can you test with the CVS
versions to see if you still have a problem?

for 2.2.1+ the tag is release22-maint

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