In gdb we supply a class whose nb_int method can throw
an exception.

A user wrote code like this:

    return '%x' % value

... where "value" was an instance of this class.
This caused funny exception behavior later on.
You can see the original report here:

http://sourceware.org/bugzilla/show_bug.cgi?id=14320

I tracked down the odd behavior to this code in
stringobject.c:PyString_Format:

                        iobj = PyNumber_Int(v);
                        if (iobj==NULL) iobj = PyNumber_Long(v);

Here, PyNumber_Int fails and sets the exception.
I think this ought to be cleared before calling
PyNumber_Long.  In our case, PyNumber_Long succeeds,
and the program keeps executing until something happens
to call PyErr_Occurred.

I think this patch ought to fix the problem:

diff -r e0eb7dea245f Objects/stringobject.c
--- a/Objects/stringobject.c	Mon Jul 30 04:07:49 2012 -0700
+++ b/Objects/stringobject.c	Tue Jul 31 13:58:07 2012 -0600
@@ -4489,7 +4489,10 @@
                     }
                     else {
                         iobj = PyNumber_Int(v);
-                        if (iobj==NULL) iobj = PyNumber_Long(v);
+                        if (iobj==NULL) {
+			    PyErr_Clear();
+			    iobj = PyNumber_Long(v);
+			}
                     }
                     if (iobj!=NULL) {
                         if (PyInt_Check(iobj)) {

I haven't written a test case yet; David Malcolm suggested I file
a bug here first.

Hi, and thank you for reporting. The diagnosis looks good, please proceed and add a test case if possible.

Here is a patch that includes a test case.
The test fails before the stringobject.c patch is applied,
and passes after.

Wouldn't it be easier to write a Python class that raises an error from __int__() but succeeds in __long__()?

It sure would.
Here's a new patch.

Hmm, well I dont't think that test will fail without the patch. You probably need to add a special function to _testcapi to check if an exception is set (PyErr_Occurred()).

It does fail without the patch:

test_format
XXX undetected error
test test_format failed -- Traceback (most recent call last):
  File "/home/tromey/Space/Python/cpython/Lib/test/test_format.py", line 251, in test_format
    def test_exc(formatstr, args, exception, excmsg):
  File "/home/tromey/Space/Python/cpython/Lib/test/test_format.py", line 240, in __int__
    raise TestFailed
TestFailed

New changeset 80cfd4caa726 by Benjamin Peterson in branch '2.7':
call PyErr_Clear() when ignoring error from PyNumber_Int (closes #15516)
http://hg.python.org/cpython/rev/80cfd4caa726

Thanks for the patch. For future reference, we use spaces instead of tabs.