Issue #1717, continued: remove PyObject_Compare and Py_CmpToRich declarations

from object.h; don't inherit tp_compare slot on subclasses; and raise TypeError
when initializing a type that has a nonzero tp_compare slot.  Fix up
comparison-related comments in object.c and code.h.

2 files changed, 18 insertions, 32 deletions

diff --git a/Objects/object.c b/Objects/object.c
index 85dbd28..49c9852 100644
--- a/Objects/object.c
+++ b/Objects/object.c
@@ -500,35 +500,19 @@ PyObject_Bytes(PyObject *v)
 	return PyBytes_FromObject(v);
 }
 
-/* The new comparison philosophy is: we completely separate three-way
-   comparison from rich comparison.  That is, PyObject_Compare() and
-   PyObject_Cmp() *just* use the tp_compare slot.  And PyObject_RichCompare()
-   and PyObject_RichCompareBool() *just* use the tp_richcompare slot.
+/* For Python 3.0.1 and later, the old three-way comparison has been
+   completely removed in favour of rich comparisons.  PyObject_Compare() and
+   PyObject_Cmp() are gone, and the builtin cmp function no longer exists.
+   The old tp_compare slot will be renamed to tp_reserved, and should no
+   longer be used.  Use tp_richcompare instead.
 
    See (*) below for practical amendments.
 
-   IOW, only cmp() uses tp_compare; the comparison operators (==, !=, <=, <,
-   >=, >) only use tp_richcompare.  Note that list.sort() only uses <.
+   tp_richcompare gets called with a first argument of the appropriate type
+   and a second object of an arbitrary type.  We never do any kind of
+   coercion.
 
-   (And yes, eventually we'll rip out cmp() and tp_compare.)
-
-   The calling conventions are different: tp_compare only gets called with two
-   objects of the appropriate type; tp_richcompare gets called with a first
-   argument of the appropriate type and a second object of an arbitrary type.
-   We never do any kind of coercion.
-
-   The return conventions are also different.
-
-   The tp_compare slot should return a C int, as follows:
-
-     -1 if a < b or if an exception occurred
-      0 if a == b
-     +1 if a > b
-
-   No other return values are allowed.  PyObject_Compare() has the same
-   calling convention.
-
-  The tp_richcompare slot should return an object, as follows:
+   The tp_richcompare slot should return an object, as follows:
 
     NULL if an exception occurred
     NotImplemented if the requested comparison is not implemented
@@ -544,9 +528,6 @@ PyObject_Bytes(PyObject *v)
     comparing the object pointer (i.e. falling back to the base object
     implementation).
 
-  - If three-way comparison is not implemented, it falls back on rich
-    comparison (but not the other way around!).
-
 */
 
 /* Map rich comparison operators to their swapped version, e.g. LT <--> GT */
diff --git a/Objects/typeobject.c b/Objects/typeobject.c
index 3f1df8d..c6d6fc8 100644
--- a/Objects/typeobject.c
+++ b/Objects/typeobject.c
@@ -3662,7 +3662,7 @@ inherit_slots(PyTypeObject *type, PyTypeObject *base)
 		type->tp_setattr = base->tp_setattr;
 		type->tp_setattro = base->tp_setattro;
 	}
-	/* tp_compare see tp_richcompare */
+	/* tp_compare is ignored, see tp_richcompare */
 	COPYSLOT(tp_repr);
 	/* tp_hash see tp_richcompare */
 	COPYSLOT(tp_call);
@@ -3670,12 +3670,10 @@ inherit_slots(PyTypeObject *type, PyTypeObject *base)
 	{
 		/* Copy comparison-related slots only when
 		   not overriding them anywhere */
-		if (type->tp_compare == NULL &&
-		    type->tp_richcompare == NULL &&
+		if (type->tp_richcompare == NULL &&
 		    type->tp_hash == NULL &&
 		    !overrides_hash(type))
 		{
-			type->tp_compare = base->tp_compare;
 			type->tp_richcompare = base->tp_richcompare;
 			type->tp_hash = base->tp_hash;
 		}
@@ -3888,6 +3886,13 @@ PyType_Ready(PyTypeObject *type)
 			goto error;
 	}
 
+	/* Check reserved slots */
+	if (type->tp_compare) {
+		PyErr_Format(PyExc_TypeError,
+			     "type %s has tp_compare",
+			     type->tp_name);
+	}
+
 	/* All done -- set the ready flag */
 	assert(type->tp_dict != NULL);
 	type->tp_flags =