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If you look at Object/abstract.c (line 2119 or so) for 2.4 CVS you 
will notice that PyObject_IsSubclass goes into a 'for' loop for each 
item in the tuple passed in and calls PyObject_IsSubclass .  
Unfortunately it makes no check for whether the argument it is 
passing is a class itself or not.  This allows it to keep making calls 
as long as the second argument is either a class or a tuple.  This 
is what is leads to the stack being blown and the subsequent 
segfault.

Obvious solution is to put in a check that the argument about to be 
passed is a class itself so as to not have such a deep call chain.  
But since ``help(issubclass)`` actually makes the above use legit 
(it says using a tuple as a second argument is equivalent as 
passing each item to issubclass which is what it is doing, albeit in 
a rather uncommon and pointless way), is it worth putting the 
check in?  Since this is such an obvious mis-use, I say no.  But if 
someone else on python-dev steps in and says otherwise I will 
patch it.