bpo-37219: Remove erroneous optimization for differencing an empty set (GH-13965)

Lib/test/test_set.py

@@ -895,6 +895,12 @@ class TestBasicOps:
             self.assertEqual(self.set, copy,
                              "%s != %s" % (self.set, copy))
 
+    def test_issue_37219(self):
+        with self.assertRaises(TypeError):
+            set().difference(123)
+        with self.assertRaises(TypeError):
+            set().difference_update(123)
+
 #------------------------------------------------------------------------------
 
 class TestBasicOpsEmpty(TestBasicOps, unittest.TestCase):

Misc/NEWS.d/next/Core and Builtins/2019-06-10-23-18-31.bpo-37219.jPSufq.rst

+Remove errorneous optimization for empty set differences.

Objects/setobject.c

@@ -1456,10 +1456,6 @@ PyDoc_STRVAR(isdisjoint_doc,
 static int
 set_difference_update_internal(PySetObject *so, PyObject *other)
 {
-    if (PySet_GET_SIZE(so) == 0) {
-        return 0;
-    }
-
     if ((PyObject *)so == other)
         return set_clear_internal(so);
 
@@ -1534,10 +1530,6 @@ set_difference(PySetObject *so, PyObject *other)
     Py_ssize_t pos = 0, other_size;
     int rv;
 
-    if (PySet_GET_SIZE(so) == 0) {
-        return set_copy(so, NULL);
-    }
-
     if (PyAnySet_Check(other)) {
         other_size = PySet_GET_SIZE(other);
     }