The empty tuple is usually a singleton with a much higher refcnt than 1
diff --git a/Modules/itertoolsmodule.c b/Modules/itertoolsmodule.c
index 4efde75..2ee947d 100644
--- a/Modules/itertoolsmodule.c
+++ b/Modules/itertoolsmodule.c
@@ -2111,8 +2111,11 @@
}
Py_DECREF(old_result);
}
- /* Now, we've got the only copy so we can update it in-place */
- assert (Py_REFCNT(result) == 1);
+ /* Now, we've got the only copy so we can update it in-place
+ * CPython's empty tuple is a singleton and cached in
+ * PyTuple's freelist.
+ */
+ assert(r == 0 || Py_REFCNT(result) == 1);
/* Scan indices right-to-left until finding one that is not
at its maximum (i + n - r). */