[issue12029] Catching virtual subclasses in except clauses

[Georg Brandl]

Georg Brandl added the comment:

I'm attaching a patch that works without changing the recursion limit, and adds some tests for pathological cases.

Instead, PyErr_GivenExceptionMatches is changed so that if an exception was previously set, it is replaced by an exception that PyObject_IsSubclass raises.  In that way recursion errors should be propagated properly.

In exception matching, this means that exceptions (including recursion errors) from PyObject_IsSubclass are ignored.  There is already an explicit test for this behavior in test_exceptions.

This behavior *could* be changed if intended by introducing a variant of PyErr_GivenExceptionMatches that can set an exception even if none was set before, and calling that in cmp_outcome in ceval.

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nosy: +georg.brandl
Added file: http://bugs.python.org/file36776/exception_proper_subclass_matching.patch

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