Crashers of the day: Py_CLEAR must be used when there is a chance that the function can be called recursively. This was discussed in issue1020188. In python codebase, all occurrences of Py_[X]DECREF(xxx->yyy) are suspect, except when they appear in tp_new or tp_dealloc functions, or when the member cannot be of a user-defined class. Note that tp_init is not safe. I do have a (crashing) example for every changed line. Is it worth adding them to the test suite? Example: class SpecialStr(str): def __del__(self): s.close() import cStringIO s = cStringIO.StringIO(SpecialStr("text")) s.close() # Segfault

--- a/Modules/_struct.c	Sat Feb 16 09:37:32 2008 +0000
+++ b/Modules/_struct.c	Sat Feb 16 14:34:57 2008 +0000
@@ -1471,7 +1471,7 @@
 		return -1;
 
 	Py_INCREF(o_format);
-	Py_XDECREF(soself->s_format);
+	Py_CLEAR(soself->s_format);
 	soself->s_format = o_format;
 
 	ret = prepare_s(soself);