Reimplement PySequence_Contains() and instance_contains(), so they work

safely together and don't duplicate logic (the common logic was factored
out into new private API function _PySequence_IterContains()).
Visible change:
    some_complex_number  in  some_instance
no longer blows up if some_instance has __getitem__ but neither
__contains__ nor __iter__.  test_iter changed to ensure that remains true.

Include/abstract.h

@@ -932,7 +932,17 @@ xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx*/
 	 expression: o.count(value).
        */
 
-     DL_IMPORT(int) PySequence_Contains(PyObject *o, PyObject *value);
+     DL_IMPORT(int) PySequence_Contains(PyObject *seq, PyObject *ob);
+       /*
+         Return -1 if error; 1 if ob in seq; 0 if ob not in seq.
+         Use __contains__ if possible, else _PySequence_IterContains().
+       */
+
+     DL_IMPORT(int) _PySequence_IterContains(PyObject *seq, PyObject *ob);
+       /*
+         Return -1 if error; 1 if ob in seq; 0 if ob not in seq.
+         Always uses the iteration protocol, and only Py_EQ comparisons.
+       */
 
 /* For DLL-level backwards compatibility */
 #undef PySequence_In

Lib/test/test_iter.py

@@ -474,24 +474,12 @@ class TestCase(unittest.TestCase):
 
     # Test iterators with 'x in y' and 'x not in y'.
     def test_in_and_not_in(self):
-        sc5 = IteratingSequenceClass(5)
-        for i in range(5):
-            self.assert_(i in sc5)
-        # CAUTION:  This test fails on 3-12j if sc5 is SequenceClass(5)
-        # instead, with:
-        #     TypeError: cannot compare complex numbers using <, <=, >, >=
-        # The trail leads back to instance_contains() in classobject.c,
-        # under comment:
-        #     /* fall back to previous behavior */
-        # IteratingSequenceClass(5) avoids the same problem only because
-        # it lacks __getitem__:  instance_contains *tries* to do a wrong
-        # thing with it too, but aborts with an AttributeError the first
-        # time it calls instance_item(); PySequence_Contains() then catches
-        # that and clears it, and tries the iterator-based "contains"
-        # instead.  But this is hanging together by a thread.
-        for i in "abc", -1, 5, 42.42, (3, 4), [], {1: 1}, 3-12j, sc5:
-            self.assert_(i not in sc5)
-        del sc5
+        for sc5 in IteratingSequenceClass(5), SequenceClass(5):
+            for i in range(5):
+                self.assert_(i in sc5)
+            for i in "abc", -1, 5, 42.42, (3, 4), [], {1: 1}, 3-12j, sc5:
+                self.assert_(i not in sc5)
+            del sc5
 
         self.assertRaises(TypeError, lambda: 3 in 12)
         self.assertRaises(TypeError, lambda: 3 not in map)

Objects/abstract.c

@@ -1381,29 +1381,14 @@ Fail:
 	return -1;
 }
 
-/* Return -1 if error; 1 if v in w; 0 if v not in w. */
+/* Return -1 if error; 1 if ob in seq; 0 if ob not in seq.
+ * Always uses the iteration protocol, and only Py_EQ comparison.
+ */
 int
-PySequence_Contains(PyObject *w, PyObject *v) /* v in w */
+_PySequence_IterContains(PyObject *seq, PyObject *ob)
 {
-	PyObject *it;  /* iter(w) */
 	int result;
-
-	if (PyType_HasFeature(w->ob_type, Py_TPFLAGS_HAVE_SEQUENCE_IN)) {
-		PySequenceMethods *sq = w->ob_type->tp_as_sequence;
-	        if (sq != NULL && sq->sq_contains != NULL) {
-			result = (*sq->sq_contains)(w, v);
-			if (result >= 0)
-				return result;
-			assert(PyErr_Occurred());
-			if (PyErr_ExceptionMatches(PyExc_AttributeError))
-				PyErr_Clear();
-			else
-				return result;
-		}
-	}
-	
-	/* Try exhaustive iteration. */
-	it = PyObject_GetIter(w);
+	PyObject *it = PyObject_GetIter(seq);
 	if (it == NULL) {
 		PyErr_SetString(PyExc_TypeError,
 			"'in' or 'not in' needs iterable right argument");
@@ -1417,7 +1402,7 @@ PySequence_Contains(PyObject *w, PyObject *v) /* v in w */
 			result = PyErr_Occurred() ? -1 : 0;
 			break;
 		}
-		cmp = PyObject_RichCompareBool(v, item, Py_EQ);
+		cmp = PyObject_RichCompareBool(ob, item, Py_EQ);
 		Py_DECREF(item);
 		if (cmp == 0)
 			continue;
@@ -1428,6 +1413,20 @@ PySequence_Contains(PyObject *w, PyObject *v) /* v in w */
 	return result;
 }
 
+/* Return -1 if error; 1 if ob in seq; 0 if ob not in seq.
+ * Use sq_contains if possible, else defer to _PySequence_IterContains().
+ */
+int
+PySequence_Contains(PyObject *seq, PyObject *ob)
+{
+	if (PyType_HasFeature(seq->ob_type, Py_TPFLAGS_HAVE_SEQUENCE_IN)) {
+		PySequenceMethods *sqm = seq->ob_type->tp_as_sequence;
+	        if (sqm != NULL && sqm->sq_contains != NULL)
+			return (*sqm->sq_contains)(seq, ob);
+	}
+	return _PySequence_IterContains(seq, ob);
+}
+
 /* Backwards compatibility */
 #undef PySequence_In
 int

Objects/classobject.c

@@ -1131,11 +1131,15 @@ instance_ass_slice(PyInstanceObject *inst, int i, int j, PyObject *value)
 	return 0;
 }
 
-static int instance_contains(PyInstanceObject *inst, PyObject *member)
+static int
+instance_contains(PyInstanceObject *inst, PyObject *member)
 {
 	static PyObject *__contains__;
-	PyObject *func, *arg, *res;
-	int ret;
+	PyObject *func;
+
+	/* Try __contains__ first.
+	 * If that can't be done, try iterator-based searching.
+	 */
 
 	if(__contains__ == NULL) {
 		__contains__ = PyString_InternFromString("__contains__");
@@ -1143,45 +1147,34 @@ static int instance_contains(PyInstanceObject *inst, PyObject *member)
 			return -1;
 	}
 	func = instance_getattr(inst, __contains__);
-	if(func == NULL) {
-		/* fall back to previous behavior */
-		int i, cmp_res;
-
-		if(!PyErr_ExceptionMatches(PyExc_AttributeError))
+	if (func) {
+		PyObject *res;
+		int ret;
+		PyObject *arg = Py_BuildValue("(O)", member);
+		if(arg == NULL) {
+			Py_DECREF(func);
 			return -1;
-		PyErr_Clear();
-		for(i=0;;i++) {
-			PyObject *obj = instance_item(inst, i);
-			int ret = 0;
-
-			if(obj == NULL) {
-				if(!PyErr_ExceptionMatches(PyExc_IndexError))
-					return -1;
-				PyErr_Clear();
-				return 0;
-			}
-			if(PyObject_Cmp(obj, member, &cmp_res) == -1)
-				ret = -1;
-			if(cmp_res == 0) 
-				ret = 1;
-			Py_DECREF(obj);
-			if(ret)
-				return ret;
 		}
-	}
-	arg = Py_BuildValue("(O)", member);
-	if(arg == NULL) {
+		res = PyEval_CallObject(func, arg);
 		Py_DECREF(func);
-		return -1;
+		Py_DECREF(arg);
+		if(res == NULL) 
+			return -1;
+		ret = PyObject_IsTrue(res);
+		Py_DECREF(res);
+		return ret;
 	}
-	res = PyEval_CallObject(func, arg);
-	Py_DECREF(func);
-	Py_DECREF(arg);
-	if(res == NULL) 
+
+	/* Couldn't find __contains__. */
+	if (PyErr_ExceptionMatches(PyExc_AttributeError)) {
+		/* Assume the failure was simply due to that there is no
+		 * __contains__ attribute, and try iterating instead.
+		 */
+		PyErr_Clear();
+		return _PySequence_IterContains((PyObject *)inst, member);
+	}
+	else
 		return -1;
-	ret = PyObject_IsTrue(res);
-	Py_DECREF(res);
-	return ret;
 }
 
 static PySequenceMethods