[Python-Dev] Strange bug only happens with Python 2.2

[Guido van Rossum]

> Ok, I've now further narrowed down this isinstance issue:
> 
> python22-maint                                   ==> bug does not appear
> python22-maint with abstract.c from Python 2.2.1 ==> bug appears
> 
> So for what it's worth (i. e. not much), I'd say please /do/ include the
> abstract.c changes into the upcoming Python 2.2.2 :-)

I'm sure it's this change, which was backported to the 2.2 maintenance
branch (and hence will be in 2.2.2).  It fixes several several
occurrences where an error is not cleared.

----------------------------
revision 2.101
date: 2002/04/23 22:45:44;  author: bwarsaw;  state: Exp;  lines: +46 -9
abstract_get_bases(): Clarify exactly what the return values and
states can be for this function, and ensure that only AttributeErrors
are masked.  Any other exception raised via the equivalent of
getattr(cls, '__bases__') should be propagated up.

abstract_issubclass(): If abstract_get_bases() returns NULL, we must
call PyErr_Occurred() to see if an exception is being propagated, and
return -1 or 0 as appropriate.  This is the specific fix for a problem
whereby if getattr(derived, '__bases__') raised an exception, an
"undetected error" would occur (under a debug build).  This nasty
situation was uncovered when writing a security proxy extension type
for the Zope3 project, where the security proxy raised a Forbidden
exception on getattr of __bases__.

PyObject_IsInstance(), PyObject_IsSubclass(): After both calls to
abstract_get_bases(), where we're setting the TypeError if the return
value is NULL, we must first check to see if an exception occurred,
and /not/ mask an existing exception.

Neil Schemenauer should double check that these changes don't break
his ExtensionClass examples (there aren't any test cases for those
examples and abstract_get_bases() was added by him in response to
problems with ExtensionClass).  Neil, please add test cases if
possible!

I belive this is a bug fix candidate for Python 2.2.2.
----------------------------

--Guido van Rossum (home page: http://www.python.org/~guido/)