bpo-27275: Change popitem() and pop() methods of collections.OrderedDict (GH-27530)

* Unify the C and Python implementations of OrderedDict.popitem().

The C implementation no longer calls ``__getitem__`` and ``__delitem__``
methods of the OrderedDict subclasses.

* Change popitem() and pop() methods of collections.OrderedDict

For consistency with dict both implementations (pure Python and C)
of these methods in OrderedDict no longer call __getitem__ and
__delitem__ methods of the OrderedDict subclasses.

Previously only the Python implementation of popitem() did not
call them.

Lib/test/test_ordered_dict.py

@@ -896,5 +896,88 @@ class CPythonSubclassMappingTests(mapping_tests.BasicTestMappingProtocol):
         self.assertRaises(KeyError, d.popitem)
 
 
+class SimpleLRUCache:
+
+    def __init__(self, size):
+        super().__init__()
+        self.size = size
+        self.counts = dict.fromkeys(('get', 'set', 'del'), 0)
+
+    def __getitem__(self, item):
+        self.counts['get'] += 1
+        value = super().__getitem__(item)
+        self.move_to_end(item)
+        return value
+
+    def __setitem__(self, key, value):
+        self.counts['set'] += 1
+        while key not in self and len(self) >= self.size:
+            self.popitem(last=False)
+        super().__setitem__(key, value)
+        self.move_to_end(key)
+
+    def __delitem__(self, key):
+        self.counts['del'] += 1
+        super().__delitem__(key)
+
+
+class SimpleLRUCacheTests:
+
+    def test_add_after_full(self):
+        c = self.type2test(2)
+        c['t1'] = 1
+        c['t2'] = 2
+        c['t3'] = 3
+        self.assertEqual(c.counts, {'get': 0, 'set': 3, 'del': 0})
+        self.assertEqual(list(c), ['t2', 't3'])
+        self.assertEqual(c.counts, {'get': 0, 'set': 3, 'del': 0})
+
+    def test_popitem(self):
+        c = self.type2test(3)
+        for i in range(1, 4):
+            c[i] = i
+        self.assertEqual(c.popitem(last=False), (1, 1))
+        self.assertEqual(c.popitem(last=True), (3, 3))
+        self.assertEqual(c.counts, {'get': 0, 'set': 3, 'del': 0})
+
+    def test_pop(self):
+        c = self.type2test(3)
+        for i in range(1, 4):
+            c[i] = i
+        self.assertEqual(c.counts, {'get': 0, 'set': 3, 'del': 0})
+        self.assertEqual(c.pop(2), 2)
+        self.assertEqual(c.counts, {'get': 0, 'set': 3, 'del': 0})
+        self.assertEqual(c.pop(4, 0), 0)
+        self.assertEqual(c.counts, {'get': 0, 'set': 3, 'del': 0})
+        self.assertRaises(KeyError, c.pop, 4)
+        self.assertEqual(c.counts, {'get': 0, 'set': 3, 'del': 0})
+
+    def test_change_order_on_get(self):
+        c = self.type2test(3)
+        for i in range(1, 4):
+            c[i] = i
+        self.assertEqual(list(c), list(range(1, 4)))
+        self.assertEqual(c.counts, {'get': 0, 'set': 3, 'del': 0})
+        self.assertEqual(c[2], 2)
+        self.assertEqual(c.counts, {'get': 1, 'set': 3, 'del': 0})
+        self.assertEqual(list(c), [1, 3, 2])
+
+
+class PySimpleLRUCacheTests(SimpleLRUCacheTests, unittest.TestCase):
+
+    class type2test(SimpleLRUCache, py_coll.OrderedDict):
+        pass
+
+
+@unittest.skipUnless(c_coll, 'requires the C version of the collections module')
+class CSimpleLRUCacheTests(SimpleLRUCacheTests, unittest.TestCase):
+
+    @classmethod
+    def setUpClass(cls):
+        class type2test(SimpleLRUCache, c_coll.OrderedDict):
+            pass
+        cls.type2test = type2test
+
+
 if __name__ == "__main__":
     unittest.main()