test_subclass_date(): Beefed this up, to check that new instance

attributes and methods work, that new arguments can be passed to the
constructor, and that inherited methods and attrs still work.  Added
XXX comments about what to do when datetime becomes usably subclassable
too (it's not yet).

Lib/test/test_datetime.py

@@ -480,10 +480,35 @@ class TestDateOnly(unittest.TestCase):
         self.assertEqual(dt2, dt - days)
 
     def test_subclass_date(self):
+
+        # XXX When datetime becomes usably subclassable, uncomment the
+        # XXX "self.theclass" lines and move this into TestDate.
+        # class C(self.theclass):
         class C(date):
             theAnswer = 42
-        dt = C(2003, 4, 14)
-        self.assertEqual(dt.__class__, C)
+
+            def __new__(cls, *args, **kws):
+                temp = kws.copy()
+                extra = temp.pop('extra')
+                # result = self.theclass.__new__(cls, *args, **temp)
+                result = date.__new__(cls, *args, **temp)
+                result.extra = extra
+                return result
+
+            def newmeth(self, start):
+                return start + self.year + self.month
+
+        args = 2003, 4, 14
+
+        # dt1 = self.theclass(*args)
+        dt1 = date(*args)
+        dt2 = C(*args, **{'extra': 7})
+
+        self.assertEqual(dt2.__class__, C)
+        self.assertEqual(dt2.theAnswer, 42)
+        self.assertEqual(dt2.extra, 7)
+        self.assertEqual(dt1.toordinal(), dt2.toordinal())
+        self.assertEqual(dt2.newmeth(-7), dt1.year + dt1.month - 7)
 
 class TestDate(HarmlessMixedComparison):
     # Tests here should pass for both dates and datetimes, except for a
@@ -977,6 +1002,7 @@ class TestDate(HarmlessMixedComparison):
         base = cls(2000, 2, 29)
         self.assertRaises(ValueError, base.replace, year=2001)
 
+
 #############################################################################
 # datetime tests