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.

5 files changed, 91 insertions, 84 deletions

diff --git a/Doc/library/datetime.rst b/Doc/library/datetime.rst
index 930af6c..a46eed3 100644
--- a/Doc/library/datetime.rst
+++ b/Doc/library/datetime.rst
@@ -619,11 +619,27 @@ Notes:
 (4)
    :class:`date` objects are equal if they represent the same date.
 
+   :class:`!date` objects that are not also :class:`.datetime` instances
+   are never equal to :class:`!datetime` objects, even if they represent
+   the same date.
+
 (5)
    *date1* is considered less than *date2* when *date1* precedes *date2* in time.
    In other words, ``date1 < date2`` if and only if ``date1.toordinal() <
    date2.toordinal()``.
 
+   Order comparison between a :class:`!date` object that is not also a
+   :class:`.datetime` instance and a :class:`!datetime` object raises
+   :exc:`TypeError`.
+
+.. versionchanged:: 3.13
+   Comparison between :class:`.datetime` object and an instance of
+   the :class:`date` subclass that is not a :class:`!datetime` subclass
+   no longer coverts the latter to :class:`!date`, ignoring the time part
+   and the time zone.
+   The default behavior can be changed by overriding the special comparison
+   methods in subclasses.
+
 In Boolean contexts, all :class:`date` objects are considered to be true.
 
 Instance methods:
@@ -1192,9 +1208,6 @@ Supported operations:
    and time, taking into account the time zone.
 
    Naive and aware :class:`!datetime` objects are never equal.
-   :class:`!datetime` objects are never equal to :class:`date` objects
-   that are not also :class:`!datetime` instances, even if they represent
-   the same date.
 
    If both comparands are aware and have different :attr:`~.datetime.tzinfo`
    attributes, the comparison acts as comparands were first converted to UTC
@@ -1206,9 +1219,8 @@ Supported operations:
    *datetime1* is considered less than *datetime2* when *datetime1* precedes
    *datetime2* in time, taking into account the time zone.
 
-   Order comparison between naive and aware :class:`.datetime` objects,
-   as well as a :class:`!datetime` object and a :class:`!date` object
-   that is not also a :class:`!datetime` instance, raises :exc:`TypeError`.
+   Order comparison between naive and aware :class:`.datetime` objects
+   raises :exc:`TypeError`.
 
    If both comparands are aware and have different :attr:`~.datetime.tzinfo`
    attributes, the comparison acts as comparands were first converted to UTC
@@ -1218,6 +1230,14 @@ Supported operations:
    Equality comparisons between aware and naive :class:`.datetime`
    instances don't raise :exc:`TypeError`.
 
+.. versionchanged:: 3.13
+   Comparison between :class:`.datetime` object and an instance of
+   the :class:`date` subclass that is not a :class:`!datetime` subclass
+   no longer coverts the latter to :class:`!date`, ignoring the time part
+   and the time zone.
+   The default behavior can be changed by overriding the special comparison
+   methods in subclasses.
+
 Instance methods:
 
 .. method:: datetime.date()
diff --git a/Lib/_pydatetime.py b/Lib/_pydatetime.py
index 54c12d3..b7d569c 100644
--- a/Lib/_pydatetime.py
+++ b/Lib/_pydatetime.py
@@ -556,10 +556,6 @@ def _check_tzinfo_arg(tz):
     if tz is not None and not isinstance(tz, tzinfo):
         raise TypeError("tzinfo argument must be None or of a tzinfo subclass")
 
-def _cmperror(x, y):
-    raise TypeError("can't compare '%s' to '%s'" % (
-                    type(x).__name__, type(y).__name__))
-
 def _divide_and_round(a, b):
     """divide a by b and round result to the nearest integer
 
@@ -1113,32 +1109,33 @@ class date:
     # Comparisons of date objects with other.
 
     def __eq__(self, other):
-        if isinstance(other, date):
+        if isinstance(other, date) and not isinstance(other, datetime):
             return self._cmp(other) == 0
         return NotImplemented
 
     def __le__(self, other):
-        if isinstance(other, date):
+        if isinstance(other, date) and not isinstance(other, datetime):
             return self._cmp(other) <= 0
         return NotImplemented
 
     def __lt__(self, other):
-        if isinstance(other, date):
+        if isinstance(other, date) and not isinstance(other, datetime):
             return self._cmp(other) < 0
         return NotImplemented
 
     def __ge__(self, other):
-        if isinstance(other, date):
+        if isinstance(other, date) and not isinstance(other, datetime):
             return self._cmp(other) >= 0
         return NotImplemented
 
     def __gt__(self, other):
-        if isinstance(other, date):
+        if isinstance(other, date) and not isinstance(other, datetime):
             return self._cmp(other) > 0
         return NotImplemented
 
     def _cmp(self, other):
         assert isinstance(other, date)
+        assert not isinstance(other, datetime)
         y, m, d = self._year, self._month, self._day
         y2, m2, d2 = other._year, other._month, other._day
         return _cmp((y, m, d), (y2, m2, d2))
@@ -2137,42 +2134,32 @@ class datetime(date):
     def __eq__(self, other):
         if isinstance(other, datetime):
             return self._cmp(other, allow_mixed=True) == 0
-        elif not isinstance(other, date):
-            return NotImplemented
         else:
-            return False
+            return NotImplemented
 
     def __le__(self, other):
         if isinstance(other, datetime):
             return self._cmp(other) <= 0
-        elif not isinstance(other, date):
-            return NotImplemented
         else:
-            _cmperror(self, other)
+            return NotImplemented
 
     def __lt__(self, other):
         if isinstance(other, datetime):
             return self._cmp(other) < 0
-        elif not isinstance(other, date):
-            return NotImplemented
         else:
-            _cmperror(self, other)
+            return NotImplemented
 
     def __ge__(self, other):
         if isinstance(other, datetime):
             return self._cmp(other) >= 0
-        elif not isinstance(other, date):
-            return NotImplemented
         else:
-            _cmperror(self, other)
+            return NotImplemented
 
     def __gt__(self, other):
         if isinstance(other, datetime):
             return self._cmp(other) > 0
-        elif not isinstance(other, date):
-            return NotImplemented
         else:
-            _cmperror(self, other)
+            return NotImplemented
 
     def _cmp(self, other, allow_mixed=False):
         assert isinstance(other, datetime)
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):
diff --git a/Misc/NEWS.d/next/Library/2024-01-30-22-10-50.gh-issue-49766.yulJL_.rst b/Misc/NEWS.d/next/Library/2024-01-30-22-10-50.gh-issue-49766.yulJL_.rst
new file mode 100644
index 0000000..eaaa3ba
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2024-01-30-22-10-50.gh-issue-49766.yulJL_.rst
@@ -0,0 +1,8 @@
+Fix :class:`~datetime.date`-:class:`~datetime.datetime` comparison. Now the
+special comparison methods like ``__eq__`` and ``__lt__`` return
+:data:`NotImplemented` if one of comparands is :class:`!date` and other is
+:class:`!datetime` instead of ignoring the time part and the time zone or
+forcefully return "not equal" or raise :exc:`TypeError`. It makes comparison
+of :class:`!date` and :class:`!datetime` subclasses more symmetric and
+allows to change the default behavior by overriding the special comparison
+methods in subclasses.
diff --git a/Modules/_datetimemodule.c b/Modules/_datetimemodule.c
index 9b8e0a7..b984ea6 100644
--- a/Modules/_datetimemodule.c
+++ b/Modules/_datetimemodule.c
@@ -1816,16 +1816,6 @@ diff_to_bool(int diff, int op)
     Py_RETURN_RICHCOMPARE(diff, 0, op);
 }
 
-/* Raises a "can't compare" TypeError and returns NULL. */
-static PyObject *
-cmperror(PyObject *a, PyObject *b)
-{
-    PyErr_Format(PyExc_TypeError,
-                 "can't compare %s to %s",
-                 Py_TYPE(a)->tp_name, Py_TYPE(b)->tp_name);
-    return NULL;
-}
-
 /* ---------------------------------------------------------------------------
  * Class implementations.
  */
@@ -3448,7 +3438,15 @@ date_isocalendar(PyDateTime_Date *self, PyObject *Py_UNUSED(ignored))
 static PyObject *
 date_richcompare(PyObject *self, PyObject *other, int op)
 {
-    if (PyDate_Check(other)) {
+    /* Since DateTime is a subclass of Date, if the other object is
+     * a DateTime, it would compute an equality testing or an ordering
+     * based on the date part alone, and we don't want that.
+     * So return NotImplemented here in that case.
+     * If a subclass wants to change this, it's up to the subclass to do so.
+     * The behavior is the same as if Date and DateTime were independent
+     * classes.
+     */
+    if (PyDate_Check(other) && !PyDateTime_Check(other)) {
         int diff = memcmp(((PyDateTime_Date *)self)->data,
                           ((PyDateTime_Date *)other)->data,
                           _PyDateTime_DATE_DATASIZE);
@@ -5880,21 +5878,7 @@ datetime_richcompare(PyObject *self, PyObject *other, int op)
     PyObject *offset1, *offset2;
     int diff;
 
-    if (! PyDateTime_Check(other)) {
-        if (PyDate_Check(other)) {
-            /* Prevent invocation of date_richcompare.  We want to
-               return NotImplemented here to give the other object
-               a chance.  But since DateTime is a subclass of
-               Date, if the other object is a Date, it would
-               compute an ordering based on the date part alone,
-               and we don't want that.  So force unequal or
-               uncomparable here in that case. */
-            if (op == Py_EQ)
-                Py_RETURN_FALSE;
-            if (op == Py_NE)
-                Py_RETURN_TRUE;
-            return cmperror(self, other);
-        }
+    if (!PyDateTime_Check(other)) {
         Py_RETURN_NOTIMPLEMENTED;
     }