Anti-registration of various ABC methods.

- Issue #25958: Support "anti-registration" of special methods from
  various ABCs, like __hash__, __iter__ or __len__.  All these (and
  several more) can be set to None in an implementation class and the
  behavior will be as if the method is not defined at all.
  (Previously, this mechanism existed only for __hash__, to make
  mutable classes unhashable.)  Code contributed by Andrew Barnert and
  Ivan Levkivskyi.

1 files changed, 53 insertions, 7 deletions

diff --git a/Lib/test/test_collections.py b/Lib/test/test_collections.py
index c4c0a16..6e858c0 100644
--- a/Lib/test/test_collections.py
+++ b/Lib/test/test_collections.py
@@ -499,6 +499,9 @@ class ABCTestCase(unittest.TestCase):
             self.assertTrue(other.right_side,'Right side not called for %s.%s'
                             % (type(instance), name))
 
+def _test_gen():
+    yield
+
 class TestOneTrickPonyABCs(ABCTestCase):
 
     def test_Awaitable(self):
@@ -686,7 +689,7 @@ class TestOneTrickPonyABCs(ABCTestCase):
         samples = [bytes(), str(),
                    tuple(), list(), set(), frozenset(), dict(),
                    dict().keys(), dict().items(), dict().values(),
-                   (lambda: (yield))(),
+                   _test_gen(),
                    (x for x in []),
                    ]
         for x in samples:
@@ -700,6 +703,15 @@ class TestOneTrickPonyABCs(ABCTestCase):
         self.assertFalse(issubclass(str, I))
         self.validate_abstract_methods(Iterable, '__iter__')
         self.validate_isinstance(Iterable, '__iter__')
+        # Check None blocking
+        class It:
+            def __iter__(self): return iter([])
+        class ItBlocked(It):
+            __iter__ = None
+        self.assertTrue(issubclass(It, Iterable))
+        self.assertTrue(isinstance(It(), Iterable))
+        self.assertFalse(issubclass(ItBlocked, Iterable))
+        self.assertFalse(isinstance(ItBlocked(), Iterable))
 
     def test_Reversible(self):
         # Check some non-reversibles
@@ -707,8 +719,18 @@ class TestOneTrickPonyABCs(ABCTestCase):
         for x in non_samples:
             self.assertNotIsInstance(x, Reversible)
             self.assertFalse(issubclass(type(x), Reversible), repr(type(x)))
-        # Check some reversibles
-        samples = [tuple(), list()]
+        # Check some non-reversible iterables
+        non_reversibles = [dict().keys(), dict().items(), dict().values(),
+                           Counter(), Counter().keys(), Counter().items(),
+                           Counter().values(), _test_gen(),
+                           (x for x in []), iter([]), reversed([])]
+        for x in non_reversibles:
+            self.assertNotIsInstance(x, Reversible)
+            self.assertFalse(issubclass(type(x), Reversible), repr(type(x)))
+        # Check some reversible iterables
+        samples = [bytes(), str(), tuple(), list(), OrderedDict(),
+                   OrderedDict().keys(), OrderedDict().items(),
+                   OrderedDict().values()]
         for x in samples:
             self.assertIsInstance(x, Reversible)
             self.assertTrue(issubclass(type(x), Reversible), repr(type(x)))
@@ -725,6 +747,29 @@ class TestOneTrickPonyABCs(ABCTestCase):
         self.assertEqual(list(reversed(R())), [])
         self.assertFalse(issubclass(float, R))
         self.validate_abstract_methods(Reversible, '__reversed__', '__iter__')
+        # Check reversible non-iterable (which is not Reversible)
+        class RevNoIter:
+            def __reversed__(self): return reversed([])
+        class RevPlusIter(RevNoIter):
+            def __iter__(self): return iter([])
+        self.assertFalse(issubclass(RevNoIter, Reversible))
+        self.assertFalse(isinstance(RevNoIter(), Reversible))
+        self.assertTrue(issubclass(RevPlusIter, Reversible))
+        self.assertTrue(isinstance(RevPlusIter(), Reversible))
+        # Check None blocking
+        class Rev:
+            def __iter__(self): return iter([])
+            def __reversed__(self): return reversed([])
+        class RevItBlocked(Rev):
+            __iter__ = None
+        class RevRevBlocked(Rev):
+            __reversed__ = None
+        self.assertTrue(issubclass(Rev, Reversible))
+        self.assertTrue(isinstance(Rev(), Reversible))
+        self.assertFalse(issubclass(RevItBlocked, Reversible))
+        self.assertFalse(isinstance(RevItBlocked(), Reversible))
+        self.assertFalse(issubclass(RevRevBlocked, Reversible))
+        self.assertFalse(isinstance(RevRevBlocked(), Reversible))
 
     def test_Iterator(self):
         non_samples = [None, 42, 3.14, 1j, b"", "", (), [], {}, set()]
@@ -736,7 +781,7 @@ class TestOneTrickPonyABCs(ABCTestCase):
                    iter(set()), iter(frozenset()),
                    iter(dict().keys()), iter(dict().items()),
                    iter(dict().values()),
-                   (lambda: (yield))(),
+                   _test_gen(),
                    (x for x in []),
                    ]
         for x in samples:
@@ -824,7 +869,7 @@ class TestOneTrickPonyABCs(ABCTestCase):
 
     def test_Sized(self):
         non_samples = [None, 42, 3.14, 1j,
-                       (lambda: (yield))(),
+                       _test_gen(),
                        (x for x in []),
                        ]
         for x in non_samples:
@@ -842,7 +887,7 @@ class TestOneTrickPonyABCs(ABCTestCase):
 
     def test_Container(self):
         non_samples = [None, 42, 3.14, 1j,
-                       (lambda: (yield))(),
+                       _test_gen(),
                        (x for x in []),
                        ]
         for x in non_samples:
@@ -861,7 +906,7 @@ class TestOneTrickPonyABCs(ABCTestCase):
     def test_Callable(self):
         non_samples = [None, 42, 3.14, 1j,
                        "", b"", (), [], {}, set(),
-                       (lambda: (yield))(),
+                       _test_gen(),
                        (x for x in []),
                        ]
         for x in non_samples:
@@ -1276,6 +1321,7 @@ class TestCollectionABCs(ABCTestCase):
             def __iter__(self):
                 return iter(())
         self.validate_comparison(MyMapping())
+        self.assertRaises(TypeError, reversed, MyMapping())
 
     def test_MutableMapping(self):
         for sample in [dict]: