Index: Objects/object.c
===================================================================
--- Objects/object.c	(revision 67187)
+++ Objects/object.c	(working copy)
@@ -687,6 +687,15 @@
 	PyObject *res;
 	int ok;
 
+	/* Quick result when objects are the same.
+	   Guarantees that identity implies equality. */
+	if (v == w) {
+		if (op == Py_EQ)
+			return 1;
+		else if (op == Py_NE)
+			return 0;
+	}
+
 	res = PyObject_RichCompare(v, w, op);
 	if (res == NULL)
 		return -1;
Index: Misc/NEWS
===================================================================
--- Misc/NEWS	(revision 67187)
+++ Misc/NEWS	(working copy)
@@ -13,6 +13,10 @@
 Core and Builtins
 -----------------
 
+- Issue #4296: Fix PyObject_RichCompareBool so that "x in [x]" evaluates to
+  True, even when x doesn't compare equal to itself.  This was a regression
+  from 2.6.
+
 Library
 -------
 
Index: Lib/test/test_float.py
===================================================================
--- Lib/test/test_float.py	(revision 67187)
+++ Lib/test/test_float.py	(working copy)
@@ -117,6 +117,33 @@
         self.assertRaises(OverflowError, float('-inf').as_integer_ratio)
         self.assertRaises(ValueError, float('nan').as_integer_ratio)
 
+    def test_float_containment(self):
+        floats = (INF, -INF, 0.0, 1.0, NAN)
+        for f in floats:
+            self.assert_(f in [f], "'%r' not in []" % f)
+            self.assert_(f in (f,), "'%r' not in ()" % f)
+            self.assert_(f in {f}, "'%r' not in set()" % f)
+            self.assert_(f in {f: None}, "'%r' not in {}" % f)
+            self.assertEqual([f].count(f), 1, "[].count('%r') != 1" % f)
+            self.assert_(f in floats, "'%r' not in container" % f)
+
+        for f in floats:
+            # nonidentical containers, same type, same contents
+            self.assert_([f] == [f], "[%r] != [%r]" % (f, f))
+            self.assert_((f,) == (f,), "(%r,) != (%r,)" % (f, f))
+            self.assert_({f} == {f}, "{%r} != {%r}" % (f, f))
+            self.assert_({f : None} == {f: None}, "{%r : None} != "
+                                                   "{%r : None}" % (f, f))
+
+            # identical containers
+            l, t, s, d = [f], (f,), {f}, {f: None}
+            self.assert_(l == l, "[%r] not equal to itself" % f)
+            self.assert_(t == t, "(%r,) not equal to itself" % f)
+            self.assert_(s == s, "{%r} not equal to itself" % f)
+            self.assert_(d == d, "{%r : None} not equal to itself" % f)
+
+
+
 class FormatFunctionsTestCase(unittest.TestCase):
 
     def setUp(self):