REGR: NaT.__richmp__(dateobj) (pandas-dev#39196)

Author: jbrockmendel

doc/source/whatsnew/v1.2.1.rst

@@ -31,7 +31,7 @@ Fixed regressions
 - Fixed regression that raised ``AttributeError`` with PyArrow versions [0.16.0, 1.0.0) (:issue:`38801`)
 - Fixed regression in :meth:`DataFrame.groupby` when aggregating an :class:`ExtensionDType` that could fail for non-numeric values (:issue:`38980`)
 - Fixed regression in :meth:`DataFrame.loc.__setitem__` raising ``KeyError`` with :class:`MultiIndex` and list-like columns indexer enlarging :class:`DataFrame` (:issue:`39147`)
--
+- Fixed regression in comparisons between ``NaT`` and ``datetime.date`` objects incorrectly returning ``True`` (:issue:`39151`)
 
 .. ---------------------------------------------------------------------------
 

pandas/_libs/tslibs/nattype.pyx

@@ -1,4 +1,7 @@
+import warnings
+
 from cpython.datetime cimport (
+    PyDate_Check,
     PyDateTime_Check,
     PyDateTime_IMPORT,
     PyDelta_Check,
@@ -125,6 +128,21 @@ cdef class _NaT(datetime):
                 return NotImplemented
             return result
 
+        elif PyDate_Check(other):
+            # GH#39151 don't defer to datetime.date object
+            if op == Py_EQ:
+                return False
+            if op == Py_NE:
+                return True
+            warnings.warn(
+                "Comparison of NaT with datetime.date is deprecated in "
+                "order to match the standard library behavior.  "
+                "In a future version these will be considered non-comparable.",
+                FutureWarning,
+                stacklevel=1,
+            )
+            return False
+
         return NotImplemented
 
     def __add__(self, other):

pandas/tests/scalar/test_nat.py

@@ -584,6 +584,40 @@ def test_nat_comparisons_invalid(other_and_type, symbol_and_op):
         op(other, NaT)
 
 
+def test_compare_date():
+    # GH#39151 comparing NaT with date object is deprecated
+    # See also: tests.scalar.timestamps.test_comparisons::test_compare_date
+
+    dt = Timestamp.now().to_pydatetime().date()
+
+    for left, right in [(NaT, dt), (dt, NaT)]:
+        assert not left == right
+        assert left != right
+
+        with tm.assert_produces_warning(FutureWarning):
+            assert not left < right
+        with tm.assert_produces_warning(FutureWarning):
+            assert not left <= right
+        with tm.assert_produces_warning(FutureWarning):
+            assert not left > right
+        with tm.assert_produces_warning(FutureWarning):
+            assert not left >= right
+
+    # Once the deprecation is enforced, the following assertions
+    #  can be enabled:
+    #    assert not left == right
+    #    assert left != right
+    #
+    #    with pytest.raises(TypeError):
+    #        left < right
+    #    with pytest.raises(TypeError):
+    #        left <= right
+    #    with pytest.raises(TypeError):
+    #        left > right
+    #    with pytest.raises(TypeError):
+    #        left >= right
+
+
 @pytest.mark.parametrize(
     "obj",
     [