Dima Dorfman's patch for coercion/comparison of C types (patch #995939), with

a minor change after the coercion, to accept two objects not necessarily of
the same type but with the same tp_compare.

Lib/test/test_coercion.py

@@ -125,9 +125,45 @@ def do_prefix_binops():
                 else:
                     print '=', format_result(x)
 
+# New-style class version of CoerceNumber
+class CoerceTo(object):
+    def __init__(self, arg):
+        self.arg = arg
+    def __coerce__(self, other):
+        if isinstance(other, CoerceTo):
+            return self.arg, other.arg
+        else:
+            return self.arg, other
+
+def assert_(expr, msg=None):
+    if not expr:
+        raise AssertionError, msg
+
+def do_cmptypes():
+    # Built-in tp_compare slots expect their arguments to have the
+    # same type, but a user-defined __coerce__ doesn't have to obey.
+    # SF #980352
+    evil_coercer = CoerceTo(42)
+    # Make sure these don't crash any more
+    assert_(cmp(u'fish', evil_coercer) != 0)
+    assert_(cmp(slice(1), evil_coercer) != 0)
+    # ...but that this still works
+    class WackyComparer(object):
+        def __cmp__(self, other):
+            assert_(other == 42, 'expected evil_coercer, got %r' % other)
+            return 0
+    assert_(cmp(WackyComparer(), evil_coercer) == 0)
+    # ...and classic classes too, since that code path is a little different
+    class ClassicWackyComparer:
+        def __cmp__(self, other):
+            assert_(other == 42, 'expected evil_coercer, got %r' % other)
+            return 0
+    assert_(cmp(ClassicWackyComparer(), evil_coercer) == 0)
+
 warnings.filterwarnings("ignore",
                         r'complex divmod\(\), // and % are deprecated',
                         DeprecationWarning,
                         r'test.test_coercion$')
 do_infix_binops()
 do_prefix_binops()
+do_cmptypes()

Objects/object.c

@@ -606,33 +606,28 @@ try_3way_compare(PyObject *v, PyObject *w)
 	    w->ob_type->tp_compare == _PyObject_SlotCompare)
 		return _PyObject_SlotCompare(v, w);
 
-	/* Try coercion; if it fails, give up */
+	/* 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 or if
+	   it yields types which are still incompatible (which can
+	   happen with a user-defined nb_coerce).
+	*/
 	c = PyNumber_CoerceEx(&v, &w);
 	if (c < 0)
 		return -2;
 	if (c > 0)
 		return 2;
-
-	/* Try v's comparison, if defined */
-	if ((f = v->ob_type->tp_compare) != NULL) {
+	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);
 	}
 
-	/* Try w's comparison, if defined */
-	if ((f = w->ob_type->tp_compare) != NULL) {
-		c = (*f)(w, v); /* swapped! */
-		Py_DECREF(v);
-		Py_DECREF(w);
-		c = adjust_tp_compare(c);
-		if (c >= -1)
-			return -c; /* Swapped! */
-		else
-			return c;
-	}
-
 	/* No comparison defined */
 	Py_DECREF(v);
 	Py_DECREF(w);