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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.

This fixed an issue for str, but not for unicode. See issue13410.

New changeset a06654ca0134 by Serhiy Storchaka in branch '2.7':
Issue #13410: Fixed a bug in PyUnicode_Format where it failed to properly
https://hg.python.org/cpython/rev/a06654ca0134