- On 64-bit platforms, when __len__() returns a value that cannot be

  represented as a C int, raise OverflowError.

(Forward port from 2.4.2; the patch to classobject.c was already in
but needed a correction in the error message text.)

Misc/NEWS

@@ -12,6 +12,9 @@ What's New in Python 2.5 alpha 1?
 Core and builtins
 -----------------
 
+- On 64-bit platforms, when __len__() returns a value that cannot be
+  represented as a C int, raise OverflowError.
+
 - test__locale is skipped on OS X < 10.4 (only partial locale support is
 present).
 

Objects/classobject.c

@@ -1019,7 +1019,7 @@ instance_length(PyInstanceObject *inst)
 		/* Overflow check -- range of PyInt is more than C int */
 		if (outcome != temp) {
 			PyErr_SetString(PyExc_OverflowError,
-			 "__len__() should return 0 <= outcome < 2**32");
+			 "__len__() should return 0 <= outcome < 2**31");
 			outcome = -1;
 		}
 		else

Objects/typeobject.c

@@ -4071,14 +4071,24 @@ slot_sq_length(PyObject *self)
 {
 	static PyObject *len_str;
 	PyObject *res = call_method(self, "__len__", &len_str, "()");
+	long temp;
 	int len;
 
 	if (res == NULL)
 		return -1;
-	len = (int)PyInt_AsLong(res);
+	temp = PyInt_AsLong(res);
+	len = (int)temp;
 	Py_DECREF(res);
 	if (len == -1 && PyErr_Occurred())
 		return -1;
+#if SIZEOF_INT < SIZEOF_LONG
+	/* Overflow check -- range of PyInt is more than C int */
+	if (len != temp) {
+		PyErr_SetString(PyExc_OverflowError,
+			"__len__() should return 0 <= outcome < 2**31");
+		return -1;
+	}
+#endif
 	if (len < 0) {
 		PyErr_SetString(PyExc_ValueError,
 				"__len__() should return >= 0");