Issue #21408: The default __ne__() now returns NotImplemented if __eq__()

returned NotImplemented.  Removed incorrect implementations of __ne__().

Lib/_collections_abc.py

@@ -224,9 +224,6 @@ class Set(Sized, Iterable, Container):
             return NotImplemented
         return len(self) == len(other) and self.__le__(other)
 
-    def __ne__(self, other):
-        return not (self == other)
-
     @classmethod
     def _from_iterable(cls, it):
         '''Construct an instance of the class from any iterable input.
@@ -451,9 +448,6 @@ class Mapping(Sized, Iterable, Container):
             return NotImplemented
         return dict(self.items()) == dict(other.items())
 
-    def __ne__(self, other):
-        return not (self == other)
-
 Mapping.register(mappingproxy)
 
 

Lib/doctest.py

@@ -481,9 +481,6 @@ class Example:
                self.options == other.options and \
                self.exc_msg == other.exc_msg
 
-    def __ne__(self, other):
-        return not self == other
-
     def __hash__(self):
         return hash((self.source, self.want, self.lineno, self.indent,
                      self.exc_msg))
@@ -547,9 +544,6 @@ class DocTest:
                self.filename == other.filename and \
                self.lineno == other.lineno
 
-    def __ne__(self, other):
-        return not self == other
-
     def __hash__(self):
         return hash((self.docstring, self.name, self.filename, self.lineno))
 
@@ -2289,9 +2283,6 @@ class DocTestCase(unittest.TestCase):
                self._dt_tearDown == other._dt_tearDown and \
                self._dt_checker == other._dt_checker
 
-    def __ne__(self, other):
-        return not self == other
-
     def __hash__(self):
         return hash((self._dt_optionflags, self._dt_setUp, self._dt_tearDown,
                      self._dt_checker))

Lib/lib2to3/pytree.py

@@ -64,16 +64,6 @@ class Base(object):
 
     __hash__ = None # For Py3 compatibility.
 
-    def __ne__(self, other):
-        """
-        Compare two nodes for inequality.
-
-        This calls the method _eq().
-        """
-        if self.__class__ is not other.__class__:
-            return NotImplemented
-        return not self._eq(other)
-
     def _eq(self, other):
         """
         Compare two nodes for equality.

Lib/numbers.py

@@ -141,11 +141,6 @@ class Complex(Number):
         """self == other"""
         raise NotImplementedError
 
-    def __ne__(self, other):
-        """self != other"""
-        # The default __ne__ doesn't negate __eq__ until 3.0.
-        return not (self == other)
-
 Complex.register(complex)
 
 

Lib/pathlib.py

@@ -665,9 +665,6 @@ class PurePath(object):
             return NotImplemented
         return self._cparts == other._cparts and self._flavour is other._flavour
 
-    def __ne__(self, other):
-        return not self == other
-
     def __hash__(self):
         try:
             return self._hash

Lib/test/test_binop.py

@@ -194,10 +194,6 @@ class Rat(object):
             return float(self) == other
         return NotImplemented
 
-    def __ne__(self, other):
-        """Compare two Rats for inequality."""
-        return not self == other
-
 class RatTestCase(unittest.TestCase):
     """Unit tests for Rat class and its support utilities."""
 

Lib/test/test_compare.py

@@ -48,8 +48,69 @@ class ComparisonTest(unittest.TestCase):
     def test_ne_defaults_to_not_eq(self):
         a = Cmp(1)
         b = Cmp(1)
-        self.assertTrue(a == b)
-        self.assertFalse(a != b)
+        c = Cmp(2)
+        self.assertIs(a == b, True)
+        self.assertIs(a != b, False)
+        self.assertIs(a != c, True)
+
+    def test_ne_high_priority(self):
+        """object.__ne__() should allow reflected __ne__() to be tried"""
+        calls = []
+        class Left:
+            # Inherits object.__ne__()
+            def __eq__(*args):
+                calls.append('Left.__eq__')
+                return NotImplemented
+        class Right:
+            def __eq__(*args):
+                calls.append('Right.__eq__')
+                return NotImplemented
+            def __ne__(*args):
+                calls.append('Right.__ne__')
+                return NotImplemented
+        Left() != Right()
+        self.assertSequenceEqual(calls, ['Left.__eq__', 'Right.__ne__'])
+
+    def test_ne_low_priority(self):
+        """object.__ne__() should not invoke reflected __eq__()"""
+        calls = []
+        class Base:
+            # Inherits object.__ne__()
+            def __eq__(*args):
+                calls.append('Base.__eq__')
+                return NotImplemented
+        class Derived(Base):  # Subclassing forces higher priority
+            def __eq__(*args):
+                calls.append('Derived.__eq__')
+                return NotImplemented
+            def __ne__(*args):
+                calls.append('Derived.__ne__')
+                return NotImplemented
+        Base() != Derived()
+        self.assertSequenceEqual(calls, ['Derived.__ne__', 'Base.__eq__'])
+
+    def test_other_delegation(self):
+        """No default delegation between operations except __ne__()"""
+        ops = (
+            ('__eq__', lambda a, b: a == b),
+            ('__lt__', lambda a, b: a < b),
+            ('__le__', lambda a, b: a <= b),
+            ('__gt__', lambda a, b: a > b),
+            ('__ge__', lambda a, b: a >= b),
+        )
+        for name, func in ops:
+            with self.subTest(name):
+                def unexpected(*args):
+                    self.fail('Unexpected operator method called')
+                class C:
+                    __ne__ = unexpected
+                for other, _ in ops:
+                    if other != name:
+                        setattr(C, other, unexpected)
+                if name == '__eq__':
+                    self.assertIs(func(C(), object()), False)
+                else:
+                    self.assertRaises(TypeError, func, C(), object())
 
     def test_issue_1393(self):
         x = lambda: None

Lib/unittest/case.py

@@ -1342,9 +1342,6 @@ class FunctionTestCase(TestCase):
                self._testFunc == other._testFunc and \
                self._description == other._description
 
-    def __ne__(self, other):
-        return not self == other
-
     def __hash__(self):
         return hash((type(self), self._setUpFunc, self._tearDownFunc,
                      self._testFunc, self._description))

Lib/unittest/suite.py

@@ -31,9 +31,6 @@ class BaseTestSuite(object):
             return NotImplemented
         return list(self) == list(other)
 
-    def __ne__(self, other):
-        return not self == other
-
     def __iter__(self):
         return iter(self._tests)
 

Misc/NEWS

@@ -11,6 +11,9 @@ Release date: TBA
 Core and Builtins
 -----------------
 
+- Issue #21408: The default __ne__() now returns NotImplemented if __eq__()
+  returned NotImplemented.
+
 - Issue #23321: Fixed a crash in str.decode() when error handler returned
   replacment string longer than mailformed input data.
 
@@ -47,6 +50,10 @@ Core and Builtins
 Library
 -------
 
+- Issue #21408: Removed incorrect implementations of __ne__() which didn't
+  returned NotImplemented if __eq__() returned NotImplemented.  The default
+  __ne__() now works correctly.
+
 - Issue #19996: :class:`email.feedparser.FeedParser` now handles (malformed)
   headers with no key rather than amusing the body has started.
 

Objects/typeobject.c

@@ -3348,9 +3348,14 @@ object_richcompare(PyObject *self, PyObject *other, int op)
         break;
 
     case Py_NE:
-        /* By default, != returns the opposite of ==,
+        /* By default, __ne__() delegates to __eq__() and inverts the result,
            unless the latter returns NotImplemented. */
-        res = PyObject_RichCompare(self, other, Py_EQ);
+        if (self->ob_type->tp_richcompare == NULL) {
+            res = Py_NotImplemented;
+            Py_INCREF(res);
+            break;
+        }
+        res = (*self->ob_type->tp_richcompare)(self, other, Py_EQ);
         if (res != NULL && res != Py_NotImplemented) {
             int ok = PyObject_IsTrue(res);
             Py_DECREF(res);