SF patch #1390657:

* set sq_repeat and sq_concat to NULL for user-defined new-style
  classes, as a way to fix a number of related problems.  See
  test_descr.notimplemented()).  One of these problems was fixed
  in r25556 and r25557 but many more existed; this is a general
  fix and thus reverts r25556-r25557.

* to avoid having PySequence_Repeat()/PySequence_Concat() failing
  on user-defined classes, they now fall back to nb_add/nb_mul if
  sq_concat/sq_repeat are not defined and the arguments appear to
  be sequences.

* added tests.

Backport candidate.

2 files changed, 60 insertions, 22 deletions

diff --git a/Objects/abstract.c b/Objects/abstract.c
index 1f8feb5..6e070a9 100644
--- a/Objects/abstract.c
+++ b/Objects/abstract.c
@@ -635,14 +635,11 @@ PyNumber_Add(PyObject *v, PyObject *w)
 	PyObject *result = binary_op1(v, w, NB_SLOT(nb_add));
 	if (result == Py_NotImplemented) {
 		PySequenceMethods *m = v->ob_type->tp_as_sequence;
+		Py_DECREF(result);
 		if (m && m->sq_concat) {
-			Py_DECREF(result);
-			result = (*m->sq_concat)(v, w);
+			return (*m->sq_concat)(v, w);
 		}
-		if (result == Py_NotImplemented) {
-			Py_DECREF(result);
-			return binop_type_error(v, w, "+");
-                }
+		result = binop_type_error(v, w, "+");
 	}
 	return result;
 }
@@ -1144,6 +1141,15 @@ PySequence_Concat(PyObject *s, PyObject *o)
 	if (m && m->sq_concat)
 		return m->sq_concat(s, o);
 
+	/* Instances of user classes defining an __add__() method only
+	   have an nb_add slot, not an sq_concat slot.  So we fall back
+	   to nb_add if both arguments appear to be sequences. */
+	if (PySequence_Check(s) && PySequence_Check(o)) {
+		PyObject *result = binary_op1(s, o, NB_SLOT(nb_add));
+		if (result != Py_NotImplemented)
+			return result;
+		Py_DECREF(result);
+	}
 	return type_error("object can't be concatenated");
 }
 
@@ -1159,6 +1165,20 @@ PySequence_Repeat(PyObject *o, int count)
 	if (m && m->sq_repeat)
 		return m->sq_repeat(o, count);
 
+	/* Instances of user classes defining a __mul__() method only
+	   have an nb_multiply slot, not an sq_repeat slot. so we fall back
+	   to nb_multiply if o appears to be a sequence. */
+	if (PySequence_Check(o)) {
+		PyObject *n, *result;
+		n = PyInt_FromLong(count);
+		if (n == NULL)
+			return NULL;
+		result = binary_op1(o, n, NB_SLOT(nb_multiply));
+		Py_DECREF(n);
+		if (result != Py_NotImplemented)
+			return result;
+		Py_DECREF(result);
+	}
 	return type_error("object can't be repeated");
 }
 
@@ -1176,6 +1196,13 @@ PySequence_InPlaceConcat(PyObject *s, PyObject *o)
 	if (m && m->sq_concat)
 		return m->sq_concat(s, o);
 
+	if (PySequence_Check(s) && PySequence_Check(o)) {
+		PyObject *result = binary_iop1(s, o, NB_SLOT(nb_inplace_add),
+					       NB_SLOT(nb_add));
+		if (result != Py_NotImplemented)
+			return result;
+		Py_DECREF(result);
+	}
 	return type_error("object can't be concatenated");
 }
 
@@ -1193,6 +1220,18 @@ PySequence_InPlaceRepeat(PyObject *o, int count)
 	if (m && m->sq_repeat)
 		return m->sq_repeat(o, count);
 
+	if (PySequence_Check(o)) {
+		PyObject *n, *result;
+		n = PyInt_FromLong(count);
+		if (n == NULL)
+			return NULL;
+		result = binary_iop1(o, n, NB_SLOT(nb_inplace_multiply),
+				     NB_SLOT(nb_multiply));
+		Py_DECREF(n);
+		if (result != Py_NotImplemented)
+			return result;
+		Py_DECREF(result);
+	}
 	return type_error("object can't be repeated");
 }
 
diff --git a/Objects/typeobject.c b/Objects/typeobject.c
index 7c36ba4..b74fa1a 100644
--- a/Objects/typeobject.c
+++ b/Objects/typeobject.c
@@ -4095,9 +4095,6 @@ slot_sq_length(PyObject *self)
 	return len;
 }
 
-SLOT1(slot_sq_concat, "__add__", PyObject *, "O")
-SLOT1(slot_sq_repeat, "__mul__", int, "i")
-
 /* Super-optimized version of slot_sq_item.
    Other slots could do the same... */
 static PyObject *
@@ -4211,9 +4208,6 @@ slot_sq_contains(PyObject *self, PyObject *value)
 	return result;
 }
 
-SLOT1(slot_sq_inplace_concat, "__iadd__", PyObject *, "O")
-SLOT1(slot_sq_inplace_repeat, "__imul__", int, "i")
-
 #define slot_mp_length slot_sq_length
 
 SLOT1(slot_mp_subscript, "__getitem__", PyObject *, "O")
@@ -4926,12 +4920,17 @@ typedef struct wrapperbase slotdef;
 static slotdef slotdefs[] = {
 	SQSLOT("__len__", sq_length, slot_sq_length, wrap_inquiry,
 	       "x.__len__() <==> len(x)"),
-	SQSLOT("__add__", sq_concat, slot_sq_concat, wrap_binaryfunc,
-	       "x.__add__(y) <==> x+y"),
-	SQSLOT("__mul__", sq_repeat, slot_sq_repeat, wrap_intargfunc,
-	       "x.__mul__(n) <==> x*n"),
-	SQSLOT("__rmul__", sq_repeat, slot_sq_repeat, wrap_intargfunc,
-	       "x.__rmul__(n) <==> n*x"),
+	/* Heap types defining __add__/__mul__ have sq_concat/sq_repeat == NULL.
+	   The logic in abstract.c always falls back to nb_add/nb_multiply in
+	   this case.  Defining both the nb_* and the sq_* slots to call the
+	   user-defined methods has unexpected side-effects, as shown by
+	   test_descr.notimplemented() */
+	SQSLOT("__add__", sq_concat, NULL, wrap_binaryfunc,
+          "x.__add__(y) <==> x+y"),
+	SQSLOT("__mul__", sq_repeat, NULL, wrap_intargfunc,
+          "x.__mul__(n) <==> x*n"),
+	SQSLOT("__rmul__", sq_repeat, NULL, wrap_intargfunc,
+          "x.__rmul__(n) <==> n*x"),
 	SQSLOT("__getitem__", sq_item, slot_sq_item, wrap_sq_item,
 	       "x.__getitem__(y) <==> x[y]"),
 	SQSLOT("__getslice__", sq_slice, slot_sq_slice, wrap_intintargfunc,
@@ -4953,10 +4952,10 @@ static slotdef slotdefs[] = {
                Use of negative indices is not supported."),
 	SQSLOT("__contains__", sq_contains, slot_sq_contains, wrap_objobjproc,
 	       "x.__contains__(y) <==> y in x"),
-	SQSLOT("__iadd__", sq_inplace_concat, slot_sq_inplace_concat,
-	       wrap_binaryfunc, "x.__iadd__(y) <==> x+=y"),
-	SQSLOT("__imul__", sq_inplace_repeat, slot_sq_inplace_repeat,
-	       wrap_intargfunc, "x.__imul__(y) <==> x*=y"),
+	SQSLOT("__iadd__", sq_inplace_concat, NULL,
+          wrap_binaryfunc, "x.__iadd__(y) <==> x+=y"),
+	SQSLOT("__imul__", sq_inplace_repeat, NULL,
+          wrap_intargfunc, "x.__imul__(y) <==> x*=y"),
 
 	MPSLOT("__len__", mp_length, slot_mp_length, wrap_inquiry,
 	       "x.__len__() <==> len(x)"),