Restructure comparison dramatically. There is no longer a default

*ordering* between objects; there is only a default equality test
(defined by an object being equal to itself only).  Read the comment
in object.c.  The current implementation never uses a three-way
comparison to compute a rich comparison, but it does use a rich
comparison to compute a three-way comparison.  I'm not quite done
ripping out all the calls to PyObject_Compare/Cmp, or replacing
tp_compare implementations with tp_richcompare implementations;
but much of that has happened (to make most unit tests pass).

The following tests still fail, because I need help deciding
or understanding:

test_codeop -- depends on comparing code objects
test_datetime -- need Tim Peters' opinion
test_marshal -- depends on comparing code objects
test_mutants -- need help understanding it

The problem with test_codeop and test_marshal is this: these tests
compare two different code objects and expect them to be equal.
Is that still a feature we'd like to support?  I've temporarily
removed the comparison and hash code from code objects, so they
use the default (equality by pointer only) comparison.

For the other two tests, run them to see for yourself.
(There may be more failing test with "-u all".)

A general problem with getting lots of these tests to pass is
the reality that for object types that have a natural total ordering,
implementing __cmp__ is much more convenient than implementing
__eq__, __ne__, __lt__, and so on.  Should we go back to allowing
__cmp__ to provide a total ordering?  Should we provide some other
way to implement rich comparison with a single method override?
Alex proposed a __key__() method; I've considered a __richcmp__()
method.  Or perhaps __cmp__() just shouldn't be killed off...

1 files changed, 177 insertions, 375 deletions

diff --git a/Objects/object.c b/Objects/object.c
index 80111b4..9bcf08b 100644
--- a/Objects/object.c
+++ b/Objects/object.c
@@ -252,14 +252,6 @@ _PyObject_NewVar(PyTypeObject *tp, Py_ssize_t nitems)
 	return PyObject_INIT_VAR(op, tp, nitems);
 }
 
-/* for binary compatibility with 2.2 */
-#undef _PyObject_Del
-void
-_PyObject_Del(PyObject *op)
-{
-	PyObject_FREE(op);
-}
-
 /* Implementation of PyObject_Print with recursion checking */
 static int
 internal_print(PyObject *op, FILE *fp, int flags, int nesting)
@@ -513,431 +505,200 @@ PyObject_Unicode(PyObject *v)
 #endif
 
 
-/* Helper to warn about deprecated tp_compare return values.  Return:
-   -2 for an exception;
-   -1 if v <  w;
-    0 if v == w;
-    1 if v  > w.
-   (This function cannot return 2.)
-*/
-static int
-adjust_tp_compare(int c)
-{
-	if (PyErr_Occurred()) {
-		if (c != -1 && c != -2) {
-			PyObject *t, *v, *tb;
-			PyErr_Fetch(&t, &v, &tb);
-			if (PyErr_Warn(PyExc_RuntimeWarning,
-				       "tp_compare didn't return -1 or -2 "
-				       "for exception") < 0) {
-				Py_XDECREF(t);
-				Py_XDECREF(v);
-				Py_XDECREF(tb);
-			}
-			else
-				PyErr_Restore(t, v, tb);
-		}
-		return -2;
-	}
-	else if (c < -1 || c > 1) {
-		if (PyErr_Warn(PyExc_RuntimeWarning,
-			       "tp_compare didn't return -1, 0 or 1") < 0)
-			return -2;
-		else
-			return c < -1 ? -1 : 1;
-	}
-	else {
-		assert(c >= -1 && c <= 1);
-		return c;
-	}
-}
+/* 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.
+   
+   See (*) below for practical amendments.
 
+   IOW, only cmp() uses tp_compare; the comparison operators (==, !=, <=, <,
+   >=, >) only use tp_richcompare.  Note that list.sort() only uses <.
 
-/* Macro to get the tp_richcompare field of a type if defined */
-#define RICHCOMPARE(t) ((t)->tp_richcompare)
+   (And yes, eventually we'll rip out cmp() and tp_compare.)
 
-/* Map rich comparison operators to their swapped version, e.g. LT --> GT */
-int _Py_SwappedOp[] = {Py_GT, Py_GE, Py_EQ, Py_NE, Py_LT, Py_LE};
+   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.
 
-/* Try a genuine rich comparison, returning an object.  Return:
-   NULL for exception;
-   NotImplemented if this particular rich comparison is not implemented or
-     undefined;
-   some object not equal to NotImplemented if it is implemented
-     (this latter object may not be a Boolean).
-*/
-static PyObject *
-try_rich_compare(PyObject *v, PyObject *w, int op)
-{
-	richcmpfunc f;
-	PyObject *res;
+   The return conventions are also different.
 
-	if (v->ob_type != w->ob_type &&
-	    PyType_IsSubtype(w->ob_type, v->ob_type) &&
-	    (f = RICHCOMPARE(w->ob_type)) != NULL) {
-		res = (*f)(w, v, _Py_SwappedOp[op]);
-		if (res != Py_NotImplemented)
-			return res;
-		Py_DECREF(res);
-	}
-	if ((f = RICHCOMPARE(v->ob_type)) != NULL) {
-		res = (*f)(v, w, op);
-		if (res != Py_NotImplemented)
-			return res;
-		Py_DECREF(res);
-	}
-	if ((f = RICHCOMPARE(w->ob_type)) != NULL) {
-		return (*f)(w, v, _Py_SwappedOp[op]);
-	}
-	res = Py_NotImplemented;
-	Py_INCREF(res);
-	return res;
-}
+   The tp_compare slot should return a C int, as follows:
 
-/* Try a genuine rich comparison, returning an int.  Return:
-   -1 for exception (including the case where try_rich_compare() returns an
-      object that's not a Boolean);
-    0 if the outcome is false;
-    1 if the outcome is true;
-    2 if this particular rich comparison is not implemented or undefined.
-*/
-static int
-try_rich_compare_bool(PyObject *v, PyObject *w, int op)
-{
-	PyObject *res;
-	int ok;
+     -1 if a < b or if an exception occurred
+      0 if a == b
+     +1 if a > b
 
-	if (RICHCOMPARE(v->ob_type) == NULL && RICHCOMPARE(w->ob_type) == NULL)
-		return 2; /* Shortcut, avoid INCREF+DECREF */
-	res = try_rich_compare(v, w, op);
-	if (res == NULL)
-		return -1;
-	if (res == Py_NotImplemented) {
-		Py_DECREF(res);
-		return 2;
-	}
-	ok = PyObject_IsTrue(res);
-	Py_DECREF(res);
-	return ok;
-}
+   No other return values are allowed.  PyObject_Compare() has the same
+   calling convention.
 
-/* Try rich comparisons to determine a 3-way comparison.  Return:
-   -2 for an exception;
-   -1 if v  < w;
-    0 if v == w;
-    1 if v  > w;
-    2 if this particular rich comparison is not implemented or undefined.
-*/
-static int
-try_rich_to_3way_compare(PyObject *v, PyObject *w)
-{
-	static struct { int op; int outcome; } tries[3] = {
-		/* Try this operator, and if it is true, use this outcome: */
-		{Py_EQ, 0},
-		{Py_LT, -1},
-		{Py_GT, 1},
-	};
-	int i;
-
-	if (RICHCOMPARE(v->ob_type) == NULL && RICHCOMPARE(w->ob_type) == NULL)
-		return 2; /* Shortcut */
-
-	for (i = 0; i < 3; i++) {
-		switch (try_rich_compare_bool(v, w, tries[i].op)) {
-		case -1:
-			return -2;
-		case 1:
-			return tries[i].outcome;
-		}
-	}
+  The tp_richcompare slot should return an object, as follows:
 
-	return 2;
-}
+    NULL if an exception occurred
+    NotImplemented if the requested comparison is not implemented
+    any other false value if the requested comparison is false
+    any other true value if the requested comparison is true
 
-/* Try a 3-way comparison, returning an int.  Return:
-   -2 for an exception;
-   -1 if v <  w;
-    0 if v == w;
-    1 if v  > w;
-    2 if this particular 3-way comparison is not implemented or undefined.
-*/
-static int
-try_3way_compare(PyObject *v, PyObject *w)
-{
-	int c;
-	cmpfunc f;
+  The PyObject_RichCompare[Bool]() wrappers raise TypeError when they get
+  NotImplemented.
 
-	/* Comparisons involving instances are given to instance_compare,
-	   which has the same return conventions as this function. */
-
-	f = v->ob_type->tp_compare;
-
-	/* If both have the same (non-NULL) tp_compare, use it. */
-	if (f != NULL && f == w->ob_type->tp_compare) {
-		c = (*f)(v, w);
-		return adjust_tp_compare(c);
-	}
-
-	/* 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);
-
-	/* If we're here, v and w,
-	    a) are not instances;
-	    b) have different types or a type without tp_compare; and
-	    c) don't have a user-defined tp_compare.
-	   tp_compare implementations in C assume that both arguments
-	   have their type, so we give up if the coercion fails.
-	*/
-	c = PyNumber_CoerceEx(&v, &w);
-	if (c < 0)
-		return -2;
-	if (c > 0)
-		return 2;
-	f = v->ob_type->tp_compare;
-	if (f != NULL && f == w->ob_type->tp_compare) {
-		c = (*f)(v, w);
-		Py_DECREF(v);
-		Py_DECREF(w);
-		return adjust_tp_compare(c);
-	}
+  (*) Practical amendments:
 
-	/* No comparison defined */
-	Py_DECREF(v);
-	Py_DECREF(w);
-	return 2;
-}
+  - If rich comparison returns NotImplemented, == and != are decided by
+    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!).
 
-/* Final fallback 3-way comparison, returning an int.  Return:
-   -2 if an error occurred;
-   -1 if v <  w;
-    0 if v == w;
-    1 if v >  w.
 */
-static int
-default_3way_compare(PyObject *v, PyObject *w)
-{
-	int c;
-	const char *vname, *wname;
 
-	if (v->ob_type == w->ob_type) {
-		/* When comparing these pointers, they must be cast to
-		 * integer types (i.e. Py_uintptr_t, our spelling of C9X's
-		 * uintptr_t).  ANSI specifies that pointer compares other
-		 * than == and != to non-related structures are undefined.
-		 */
-		Py_uintptr_t vv = (Py_uintptr_t)v;
-		Py_uintptr_t ww = (Py_uintptr_t)w;
-		return (vv < ww) ? -1 : (vv > ww) ? 1 : 0;
-	}
+/* Forward */
+static PyObject *do_richcompare(PyObject *v, PyObject *w, int op);
 
-	/* None is smaller than anything */
-	if (v == Py_None)
-		return -1;
-	if (w == Py_None)
-		return 1;
-
-	/* different type: compare type names; numbers are smaller */
-	if (PyNumber_Check(v))
-		vname = "";
-	else
-		vname = v->ob_type->tp_name;
-	if (PyNumber_Check(w))
-		wname = "";
-	else
-		wname = w->ob_type->tp_name;
-	c = strcmp(vname, wname);
-	if (c < 0)
-		return -1;
-	if (c > 0)
-		return 1;
-	/* Same type name, or (more likely) incomparable numeric types */
-	return ((Py_uintptr_t)(v->ob_type) < (
-		Py_uintptr_t)(w->ob_type)) ? -1 : 1;
-}
-
-/* Do a 3-way comparison, by hook or by crook.  Return:
-   -2 for an exception (but see below);
-   -1 if v <  w;
-    0 if v == w;
-    1 if v >  w;
-   BUT: if the object implements a tp_compare function, it returns
-   whatever this function returns (whether with an exception or not).
-*/
+/* Perform a three-way comparison, raising TypeError if three-way comparison
+   is not supported.  */
 static int
-do_cmp(PyObject *v, PyObject *w)
+do_compare(PyObject *v, PyObject *w)
 {
-	int c;
 	cmpfunc f;
+	int ok;
 
-	if (v->ob_type == w->ob_type
-	    && (f = v->ob_type->tp_compare) != NULL) {
-		c = (*f)(v, w);
-		return adjust_tp_compare(c);
-	}
-	/* We only get here if one of the following is true:
-	   a) v and w have different types
-	   b) v and w have the same type, which doesn't have tp_compare
-	   c) v and w are instances, and either __cmp__ is not defined or
-	      __cmp__ returns NotImplemented
-	*/
-	c = try_rich_to_3way_compare(v, w);
-	if (c < 2)
-		return c;
-	c = try_3way_compare(v, w);
-	if (c < 2)
-		return c;
-	return default_3way_compare(v, w);
-}
-
-/* Compare v to w.  Return
-   -1 if v <  w or exception (PyErr_Occurred() true in latter case).
-    0 if v == w.
-    1 if v > w.
-   XXX The docs (C API manual) say the return value is undefined in case
-   XXX of error.
-*/
+	if (v->ob_type == w->ob_type && 
+	    (f = v->ob_type->tp_compare) != NULL) {
+		return (*f)(v, w);
+	}
+
+	/* Now try three-way compare before giving up.  This is intentionally
+	   elaborate; if you have a it will raise TypeError if it detects two
+	   objects that aren't ordered with respect to each other. */
+	ok = PyObject_RichCompareBool(v, w, Py_LT);
+	if (ok < 0)
+		return -1; /* Error */
+	if (ok)
+		return -1; /* Less than */
+	ok = PyObject_RichCompareBool(v, w, Py_GT);
+	if (ok < 0)
+		return -1; /* Error */
+	if (ok)
+		return 1; /* Greater than */
+	ok = PyObject_RichCompareBool(v, w, Py_EQ);
+	if (ok < 0)
+		return -1; /* Error */
+	if (ok)
+		return 0; /* Equal */
+
+	/* Give up */
+	PyErr_Format(PyExc_TypeError,
+		     "unorderable types: '%.100s' <> '%.100s'",
+		     v->ob_type->tp_name,
+		     w->ob_type->tp_name);
+	return -1;
+}
+
+/* Perform a three-way comparison.  This wraps do_compare() with a check for
+   NULL arguments and a recursion check. */
 int
 PyObject_Compare(PyObject *v, PyObject *w)
 {
-	int result;
+	int res;
 
 	if (v == NULL || w == NULL) {
-		PyErr_BadInternalCall();
+		if (!PyErr_Occurred())
+			PyErr_BadInternalCall();
 		return -1;
 	}
-	if (v == w)
-		return 0;
 	if (Py_EnterRecursiveCall(" in cmp"))
 		return -1;
-	result = do_cmp(v, w);
+	res = do_compare(v, w);
 	Py_LeaveRecursiveCall();
-	return result < 0 ? -1 : result;
+	return res < 0 ? -1 : res;
 }
 
-/* Return (new reference to) Py_True or Py_False. */
-static PyObject *
-convert_3way_to_object(int op, int c)
-{
-	PyObject *result;
-	switch (op) {
-	case Py_LT: c = c <  0; break;
-	case Py_LE: c = c <= 0; break;
-	case Py_EQ: c = c == 0; break;
-	case Py_NE: c = c != 0; break;
-	case Py_GT: c = c >  0; break;
-	case Py_GE: c = c >= 0; break;
-	}
-	result = c ? Py_True : Py_False;
-	Py_INCREF(result);
-	return result;
-}
-
-/* We want a rich comparison but don't have one.  Try a 3-way cmp instead.
-   Return
-   NULL      if error
-   Py_True   if v op w
-   Py_False  if not (v op w)
-*/
-static PyObject *
-try_3way_to_rich_compare(PyObject *v, PyObject *w, int op)
-{
-	int c;
+/* Map rich comparison operators to their swapped version, e.g. LT <--> GT */
+int _Py_SwappedOp[] = {Py_GT, Py_GE, Py_EQ, Py_NE, Py_LT, Py_LE};
 
-	c = try_3way_compare(v, w);
-	if (c >= 2)
-		c = default_3way_compare(v, w);
-	if (c <= -2)
-		return NULL;
-	return convert_3way_to_object(op, c);
-}
+static char *opstrings[] = {">", ">=", "==", "!=", "<", "<="};
 
-/* Do rich comparison on v and w.  Return
-   NULL      if error
-   Else a new reference to an object other than Py_NotImplemented, usually(?):
-   Py_True   if v op w
-   Py_False  if not (v op w)
-*/
+/* Perform a rich comparison, raising TypeError when the requested comparison
+   operator is not supported. */
 static PyObject *
-do_richcmp(PyObject *v, PyObject *w, int op)
+do_richcompare(PyObject *v, PyObject *w, int op)
 {
+	richcmpfunc f;
 	PyObject *res;
 
-	res = try_rich_compare(v, w, op);
-	if (res != Py_NotImplemented)
-		return res;
-	Py_DECREF(res);
-
-	return try_3way_to_rich_compare(v, w, op);
+	if (v->ob_type != w->ob_type &&
+	    PyType_IsSubtype(w->ob_type, v->ob_type) &&
+	    (f = w->ob_type->tp_richcompare) != NULL) {
+		res = (*f)(w, v, _Py_SwappedOp[op]);
+		if (res != Py_NotImplemented)
+			return res;
+		Py_DECREF(res);
+	}
+	if ((f = v->ob_type->tp_richcompare) != NULL) {
+		res = (*f)(v, w, op);
+		if (res != Py_NotImplemented)
+			return res;
+		Py_DECREF(res);
+	}
+	if ((f = w->ob_type->tp_richcompare) != NULL) {
+		res = (*f)(w, v, _Py_SwappedOp[op]);
+		if (res != Py_NotImplemented)
+			return res;
+		Py_DECREF(res);
+	}
+	/* If neither object implements it, provide a sensible default
+	   for == and !=, but raise an exception for ordering. */
+	switch (op) {
+	case Py_EQ:
+		res = (v == w) ? Py_True : Py_False;
+		break;
+	case Py_NE:
+		res = (v != w) ? Py_True : Py_False;
+		break;
+	default:
+		PyErr_Format(PyExc_TypeError,
+			     "unorderable types: %.100s() %s %.100s()",
+			     v->ob_type->tp_name,
+			     opstrings[op],
+			     w->ob_type->tp_name);
+		return NULL;
+	}
+	Py_INCREF(res);
+	return res;
 }
 
-/* Return:
-   NULL for exception;
-   some object not equal to NotImplemented if it is implemented
-     (this latter object may not be a Boolean).
-*/
+/* Perform a rich comparison with object result.  This wraps do_richcompare()
+   with a check for NULL arguments and a recursion check. */
+
 PyObject *
 PyObject_RichCompare(PyObject *v, PyObject *w, int op)
 {
 	PyObject *res;
 
 	assert(Py_LT <= op && op <= Py_GE);
-	if (Py_EnterRecursiveCall(" in cmp"))
+	if (v == NULL || w == NULL) {
+		if (!PyErr_Occurred())
+			PyErr_BadInternalCall();
 		return NULL;
-
-	/* If the types are equal, and not old-style instances, try to
-	   get out cheap (don't bother with coercions etc.). */
-	if (v->ob_type == w->ob_type) {
-		cmpfunc fcmp;
-		richcmpfunc frich = RICHCOMPARE(v->ob_type);
-		/* If the type has richcmp, try it first.  try_rich_compare
-		   tries it two-sided, which is not needed since we've a
-		   single type only. */
-		if (frich != NULL) {
-			res = (*frich)(v, w, op);
-			if (res != Py_NotImplemented)
-				goto Done;
-			Py_DECREF(res);
-		}
-		/* No richcmp, or this particular richmp not implemented.
-		   Try 3-way cmp. */
-		fcmp = v->ob_type->tp_compare;
-		if (fcmp != NULL) {
-			int c = (*fcmp)(v, w);
-			c = adjust_tp_compare(c);
-			if (c == -2) {
-				res = NULL;
-				goto Done;
-			}
-			res = convert_3way_to_object(op, c);
-			goto Done;
-		}
 	}
-
-	/* Fast path not taken, or couldn't deliver a useful result. */
-	res = do_richcmp(v, w, op);
-Done:
+	if (Py_EnterRecursiveCall(" in cmp"))
+		return NULL;
+	res = do_richcompare(v, w, op);
 	Py_LeaveRecursiveCall();
 	return res;
 }
 
-/* Return -1 if error; 1 if v op w; 0 if not (v op w). */
+/* Perform a rich comparison with integer result.  This wraps
+   PyObject_RichCompare(), returning -1 for error, 0 for false, 1 for true. */
 int
 PyObject_RichCompareBool(PyObject *v, PyObject *w, int op)
 {
 	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;
@@ -949,6 +710,44 @@ PyObject_RichCompareBool(PyObject *v, PyObject *w, int op)
 	return ok;
 }
 
+/* Turn the result of a three-way comparison into the result expected by a
+   rich comparison. */
+PyObject *
+Py_CmpToRich(int op, int cmp)
+{
+	PyObject *res;
+	int ok;
+
+	if (PyErr_Occurred())
+		return NULL;
+	switch (op) {
+	case Py_LT:
+		ok = cmp <  0; 
+		break;
+	case Py_LE:
+		ok = cmp <= 0; 
+		break;
+	case Py_EQ:
+		ok = cmp == 0; 
+		break;
+	case Py_NE:
+		ok = cmp != 0; 
+		break;
+	case Py_GT: 
+		ok = cmp >  0; 
+		break;
+	case Py_GE:
+		ok = cmp >= 0; 
+		break;
+	default:
+		PyErr_BadArgument(); 
+		return NULL;
+	}
+	res = ok ? Py_True : Py_False;
+	Py_INCREF(res);
+	return res;
+}
+
 /* Set of hash utility functions to help maintaining the invariant that
 	if a==b then hash(a)==hash(b)
 
@@ -1832,6 +1631,9 @@ _Py_ReadyTypes(void)
 
 	if (PyType_Ready(&PyNotImplemented_Type) < 0)
 		Py_FatalError("Can't initialize type(NotImplemented)");
+
+	if (PyType_Ready(&PyCode_Type) < 0)
+		Py_FatalError("Can't initialize 'code'");
 }