gh-49766: Make date-datetime comparison more symmetric and flexible (GH-114760)

Now the special comparison methods like `__eq__` and `__lt__` return
NotImplemented if one of comparands is date and other is datetime
instead of ignoring the time part and the time zone or forcefully
return "not equal" or raise TypeError.

It makes comparison of date and datetime subclasses more symmetric
and allows to change the default behavior by overriding
the special comparison methods in subclasses.

It is now the same as if date and datetime was independent classes.

1 files changed, 36 insertions, 28 deletions

diff --git a/Lib/test/datetimetester.py b/Lib/test/datetimetester.py
index 53ad5e5..980a8e6 100644
--- a/Lib/test/datetimetester.py
+++ b/Lib/test/datetimetester.py
@@ -5435,42 +5435,50 @@ class TestTimezoneConversions(unittest.TestCase):
 
 class Oddballs(unittest.TestCase):
 
-    def test_bug_1028306(self):
+    def test_date_datetime_comparison(self):
+        # bpo-1028306, bpo-5516 (gh-49766)
         # Trying to compare a date to a datetime should act like a mixed-
         # type comparison, despite that datetime is a subclass of date.
         as_date = date.today()
         as_datetime = datetime.combine(as_date, time())
-        self.assertTrue(as_date != as_datetime)
-        self.assertTrue(as_datetime != as_date)
-        self.assertFalse(as_date == as_datetime)
-        self.assertFalse(as_datetime == as_date)
-        self.assertRaises(TypeError, lambda: as_date < as_datetime)
-        self.assertRaises(TypeError, lambda: as_datetime < as_date)
-        self.assertRaises(TypeError, lambda: as_date <= as_datetime)
-        self.assertRaises(TypeError, lambda: as_datetime <= as_date)
-        self.assertRaises(TypeError, lambda: as_date > as_datetime)
-        self.assertRaises(TypeError, lambda: as_datetime > as_date)
-        self.assertRaises(TypeError, lambda: as_date >= as_datetime)
-        self.assertRaises(TypeError, lambda: as_datetime >= as_date)
-
-        # Nevertheless, comparison should work with the base-class (date)
-        # projection if use of a date method is forced.
-        self.assertEqual(as_date.__eq__(as_datetime), True)
-        different_day = (as_date.day + 1) % 20 + 1
-        as_different = as_datetime.replace(day= different_day)
-        self.assertEqual(as_date.__eq__(as_different), False)
+        date_sc = SubclassDate(as_date.year, as_date.month, as_date.day)
+        datetime_sc = SubclassDatetime(as_date.year, as_date.month,
+                                       as_date.day, 0, 0, 0)
+        for d in (as_date, date_sc):
+            for dt in (as_datetime, datetime_sc):
+                for x, y in (d, dt), (dt, d):
+                    self.assertTrue(x != y)
+                    self.assertFalse(x == y)
+                    self.assertRaises(TypeError, lambda: x < y)
+                    self.assertRaises(TypeError, lambda: x <= y)
+                    self.assertRaises(TypeError, lambda: x > y)
+                    self.assertRaises(TypeError, lambda: x >= y)
 
         # And date should compare with other subclasses of date.  If a
         # subclass wants to stop this, it's up to the subclass to do so.
-        date_sc = SubclassDate(as_date.year, as_date.month, as_date.day)
-        self.assertEqual(as_date, date_sc)
-        self.assertEqual(date_sc, as_date)
-
         # Ditto for datetimes.
-        datetime_sc = SubclassDatetime(as_datetime.year, as_datetime.month,
-                                       as_date.day, 0, 0, 0)
-        self.assertEqual(as_datetime, datetime_sc)
-        self.assertEqual(datetime_sc, as_datetime)
+        for x, y in ((as_date, date_sc),
+                     (date_sc, as_date),
+                     (as_datetime, datetime_sc),
+                     (datetime_sc, as_datetime)):
+            self.assertTrue(x == y)
+            self.assertFalse(x != y)
+            self.assertFalse(x < y)
+            self.assertFalse(x > y)
+            self.assertTrue(x <= y)
+            self.assertTrue(x >= y)
+
+        # Nevertheless, comparison should work if other object is an instance
+        # of date or datetime class with overridden comparison operators.
+        # So special methods should return NotImplemented, as if
+        # date and datetime were independent classes.
+        for x, y in (as_date, as_datetime), (as_datetime, as_date):
+            self.assertEqual(x.__eq__(y), NotImplemented)
+            self.assertEqual(x.__ne__(y), NotImplemented)
+            self.assertEqual(x.__lt__(y), NotImplemented)
+            self.assertEqual(x.__gt__(y), NotImplemented)
+            self.assertEqual(x.__gt__(y), NotImplemented)
+            self.assertEqual(x.__ge__(y), NotImplemented)
 
     def test_extra_attributes(self):
         with self.assertWarns(DeprecationWarning):