bpo-31478: Fix an assertion failure in random.seed() in case a seed has a bad…

bpo-31478: Fix an assertion failure in random.seed() in case a seed has a bad __abs__() method. (#3596)

Lib/test/test_random.py

@@ -430,6 +430,17 @@ class MersenneTwister_TestBasicOps(TestBasicOps, unittest.TestCase):
             ['0x1.b0580f98a7dbep-1', '0x1.84129978f9c1ap-1',
              '0x1.aeaa51052e978p-2', '0x1.092178fb945a6p-2'])
 
+    def test_bug_31478(self):
+        # There shouldn't be an assertion failure in _random.Random.seed() in
+        # case the argument has a bad __abs__() method.
+        class BadInt(int):
+            def __abs__(self):
+                return None
+        try:
+            self.gen.seed(BadInt())
+        except TypeError:
+            pass
+
     def test_bug_31482(self):
         # Verify that version 1 seeds are unaffected by hash randomization
         # when the seeds are expressed as bytes rather than strings.

Misc/NEWS.d/next/Core and Builtins/2017-09-15-09-13-07.bpo-31478.o06iKD.rst

+Fix an assertion failure in `_random.Random.seed()` in case the argument has a
+bad ``__abs__()`` method. Patch by Oren Milman.

Modules/_randommodule.c

@@ -259,8 +259,11 @@ random_seed(RandomObject *self, PyObject *args)
      * So: if the arg is a PyLong, use its absolute value.
      * Otherwise use its hash value, cast to unsigned.
      */
-    if (PyLong_Check(arg))
-        n = PyNumber_Absolute(arg);
+    if (PyLong_Check(arg)) {
+        /* Calling int.__abs__() prevents calling arg.__abs__(), which might
+           return an invalid value. See issue #31478. */
+        n = PyLong_Type.tp_as_number->nb_absolute(arg);
+    }
     else {
         Py_hash_t hash = PyObject_Hash(arg);
         if (hash == -1)