Repair widespread misuse of _PyString_Resize. Since it's clear people don't understand how this function works, also beefed up the docs. The most common usage error is of this form (often spread out across gotos): if (_PyString_Resize(&s, n) < 0) { Py_DECREF(s); s = NULL; goto outtahere; } The error is that if _PyString_Resize runs out of memory, it automatically decrefs the input string object s (which also deallocates it, since its refcount must be 1 upon entry), and sets s to NULL. So if the "if" branch ever triggers, it's an error to call Py_DECREF(s): s is already NULL! A correct way to write the above is the simpler (and intended) if (_PyString_Resize(&s, n) < 0) goto outtahere; Bugfix candidate.

--- a/Python/bltinmodule.c	Sat Apr 27 18:03:26 2002 +0000
+++ b/Python/bltinmodule.c	Sat Apr 27 18:44:32 2002 +0000
@@ -1993,8 +1993,8 @@
 		Py_DECREF(item);
 	}
 
-	if (j < len && _PyString_Resize(&result, j) < 0)
-		return NULL;
+	if (j < len)
+		_PyString_Resize(&result, j);
 
 	return result;