Hopefully fix 3-way comparisons. This unfortunately adds yet another

hack, and it's even more disgusting than a PyInstance_Check() call.
If the tp_compare slot is the slot used for overrides in Python,
it's always called.

Add some tests that show what should work too.

Include/object.h

@@ -346,6 +346,10 @@ extern DL_IMPORT(int) PyNumber_CoerceEx(PyObject **, PyObject **);
 
 extern DL_IMPORT(void) (*PyObject_ClearWeakRefs)(PyObject *);
 
+/* A slot function whose address we need to compare */
+extern int _PyObject_SlotCompare(PyObject *, PyObject *);
+
+
 /* PyObject_Dir(obj) acts like Python __builtin__.dir(obj), returning a
    list of strings.  PyObject_Dir(NULL) is like __builtin__.dir(),
    returning the names of the current locals.  In this case, if there are

Lib/test/test_descr.py

@@ -1831,6 +1831,33 @@ def str_subclass_as_dict_key():
     verify(cistr('ONe') in d)
     verify(d.get(cistr('thrEE')) == 3)
 
+def classic_comparisons():
+    if verbose: print "Testing classic comparisons..."
+    for base in (int, object):
+        if verbose: print "        (base = %s)" % base
+        class C(base):
+            def __init__(self, value):
+                self.value = int(value)
+            def __cmp__(self, other):
+                if isinstance(other, C):
+                    return cmp(self.value, other.value)
+                if isinstance(other, int) or isinstance(other, long):
+                    return cmp(self.value, other)
+                return NotImplemented
+        c1 = C(1)
+        c2 = C(2)
+        c3 = C(3)
+        verify(c1 == 1)
+        c = {1: c1, 2: c2, 3: c3}
+        for x in 1, 2, 3:
+            for y in 1, 2, 3:
+                verify(cmp(c[x], c[y]) == cmp(x, y), "x=%d, y=%d" % (x, y))
+                for op in "<", "<=", "==", "!=", ">", ">=":
+                    verify(eval("c[x] %s c[y]" % op) == eval("x %s y" % op),
+                           "x=%d, y=%d" % (x, y))
+                verify(cmp(c[x], y) == cmp(x, y), "x=%d, y=%d" % (x, y))
+                verify(cmp(x, c[y]) == cmp(x, y), "x=%d, y=%d" % (x, y))
+
 
 def all():
     lists()
@@ -1869,6 +1896,7 @@ def all():
     keywords()
     restricted()
     str_subclass_as_dict_key()
+    classic_comparisons()
 
 all()
 

Objects/object.c

@@ -455,11 +455,25 @@ try_3way_compare(PyObject *v, PyObject *w)
 	/* Comparisons involving instances are given to instance_compare,
 	   which has the same return conventions as this function. */
 
+	f = v->ob_type->tp_compare;
 	if (PyInstance_Check(v))
-		return (*v->ob_type->tp_compare)(v, w);
+		return (*f)(v, w);
 	if (PyInstance_Check(w))
 		return (*w->ob_type->tp_compare)(v, w);
 
+	/* If both have the same (non-NULL) tp_compare, use it. */
+	if (f != NULL && f == w->ob_type->tp_compare) {
+		c = (*f)(v, w);
+		if (c < 0 && PyErr_Occurred())
+			return -1;
+		return c < 0 ? -1 : c > 0 ? 1 : 0;
+	}
+
+	/* If either tp_compare is _PyObject_SlotCompare, that's safe. */
+	if (f == _PyObject_SlotCompare ||
+	    w->ob_type->tp_compare == _PyObject_SlotCompare)
+		return _PyObject_SlotCompare(v, w);
+
 	/* Try coercion; if it fails, give up */
 	c = PyNumber_CoerceEx(&v, &w);
 	if (c < 0)

Objects/typeobject.c

@@ -2761,17 +2761,18 @@ half_compare(PyObject *self, PyObject *other)
 	return 2;
 }
 
-static int
-slot_tp_compare(PyObject *self, PyObject *other)
+/* This slot is published for the benefit of try_3way_compare in object.c */
+int
+_PyObject_SlotCompare(PyObject *self, PyObject *other)
 {
 	int c;
 
-	if (self->ob_type->tp_compare == slot_tp_compare) {
+	if (self->ob_type->tp_compare == _PyObject_SlotCompare) {
 		c = half_compare(self, other);
 		if (c <= 1)
 			return c;
 	}
-	if (other->ob_type->tp_compare == slot_tp_compare) {
+	if (other->ob_type->tp_compare == _PyObject_SlotCompare) {
 		c = half_compare(other, self);
 		if (c < -1)
 			return -2;
@@ -3190,7 +3191,7 @@ override_slots(PyTypeObject *type, PyObject *dict)
 	    PyDict_GetItemString(dict, "__repr__"))
 		type->tp_print = NULL;
 
-	TPSLOT("__cmp__", tp_compare, slot_tp_compare);
+	TPSLOT("__cmp__", tp_compare, _PyObject_SlotCompare);
 	TPSLOT("__repr__", tp_repr, slot_tp_repr);
 	TPSLOT("__hash__", tp_hash, slot_tp_hash);
 	TPSLOT("__call__", tp_call, slot_tp_call);