bpo-37579: Improve equality behavior for pure Python datetime and time (GH-14726)

Returns NotImplemented for timedelta and time in __eq__ for different types in Python implementation, which matches the C implementation.

This also adds tests to enforce that these objects will fall back to the right hand side's __eq__ and/or __ne__ implementation.

bpo-37579

Author: tirkarthi

Lib/datetime.py

@@ -733,7 +733,7 @@ def __eq__(self, other):
         if isinstance(other, timedelta):
             return self._cmp(other) == 0
         else:
-            return False
+            return NotImplemented
 
     def __le__(self, other):
         if isinstance(other, timedelta):
@@ -1310,7 +1310,7 @@ def __eq__(self, other):
         if isinstance(other, time):
             return self._cmp(other, allow_mixed=True) == 0
         else:
-            return False
+            return NotImplemented
 
     def __le__(self, other):
         if isinstance(other, time):

Lib/test/datetimetester.py

@@ -53,6 +53,19 @@
 INF = float("inf")
 NAN = float("nan")
 
+
+class ComparesEqualClass(object):
+    """
+    A class that is always equal to whatever you compare it to.
+    """
+
+    def __eq__(self, other):
+        return True
+
+    def __ne__(self, other):
+        return False
+
+
 #############################################################################
 # module tests
 
@@ -399,6 +412,13 @@ def test_harmless_mixed_comparison(self):
         self.assertIn(me, [1, 20, [], me])
         self.assertIn([], [me, 1, 20, []])
 
+        # Comparison to objects of unsupported types should return
+        # NotImplemented which falls back to the right hand side's __eq__
+        # method. In this case, ComparesEqualClass.__eq__ always returns True.
+        # ComparesEqualClass.__ne__ always returns False.
+        self.assertTrue(me == ComparesEqualClass())
+        self.assertFalse(me != ComparesEqualClass())
+
     def test_harmful_mixed_comparison(self):
         me = self.theclass(1, 1, 1)
 

Misc/NEWS.d/next/Library/2019-07-13-10-59-43.bpo-37579.B1Tq9i.rst

@@ -0,0 +1,4 @@
+Return :exc:`NotImplemented` in Python implementation of ``__eq__`` for
+:class:`~datetime.timedelta` and :class:`~datetime.time` when the other
+object being compared is not of the same type to match C implementation.
+Patch by Karthikeyan Singaravelan.