ceval.c/do_raise(): Tighten the test to disallow raising an instance of

a str subclass.

test_descr.py/string_exceptions():  New sub-test.

For 2.3 only.  Guido doesn't want this backported.

Lib/test/test_descr.py

@@ -2974,6 +2974,31 @@ def docdescriptor():
     vereq(NewClass.__doc__, 'object=None; type=NewClass')
     vereq(NewClass().__doc__, 'object=NewClass instance; type=NewClass')
 
+def string_exceptions():
+    if verbose:
+        print "Testing string exceptions ..."
+
+    # Ensure builtin strings work OK as exceptions.
+    astring = "An exception string."
+    try:
+        raise astring
+    except astring:
+        pass
+    else:
+        raise TestFailed, "builtin string not usable as exception"
+
+    # Ensure string subclass instances do not.
+    class MyStr(str):
+        pass
+
+    newstring = MyStr("oops -- shouldn't work")
+    try:
+        raise newstring
+    except TypeError:
+        pass
+    except:
+        raise TestFailed, "string subclass allowed as exception"
+
 def test_main():
     class_docstrings()
     lists()
@@ -3039,6 +3064,7 @@ def test_main():
     funnynew()
     imulbug()
     docdescriptor()
+    string_exceptions()
     if verbose: print "All OK"
 
 if __name__ == "__main__":

Python/ceval.c

@@ -2743,7 +2743,10 @@ do_raise(PyObject *type, PyObject *value, PyObject *tb)
 		Py_DECREF(tmp);
 	}
 
-	if (PyString_Check(type))
+	if (PyString_CheckExact(type))
+		/* Raising builtin string is deprecated but still allowed --
+		 * do nothing.  Raising an instance of a new-style str
+		 * subclass is right out. */
 		;
 
 	else if (PyClass_Check(type))