Make sure filter() never returns tuple, str or unicode

subclasses. (Discussed in SF patch #665835)

Lib/test/test_builtin.py

@@ -408,6 +408,29 @@ class BuiltinTest(unittest.TestCase):
                 unicode("345")
             )
 
+    def test_filter_subclasses(self):
+        # test, that filter() never returns tuple, str or unicode subclasses
+        funcs = (None, lambda x: True)
+        class tuple2(tuple):
+            pass
+        class str2(str):
+            pass
+        inputs = {
+            tuple2: [(), (1,2,3)],
+            str2:   ["", "123"]
+        }
+        if have_unicode:
+            class unicode2(unicode):
+                pass
+            inputs[unicode2] = [unicode(), unicode("123")]
+
+        for func in funcs:
+            for (cls, inps) in inputs.iteritems():
+                for inp in inps:
+                    out = filter(func, cls(inp))
+                    self.assertEqual(inp, out)
+                    self.assert_(not isinstance(out, cls))
+
     def test_float(self):
         self.assertEqual(float(3.14), 3.14)
         self.assertEqual(float(314), 314.0)

Python/bltinmodule.c

@@ -1838,7 +1838,10 @@ filtertuple(PyObject *func, PyObject *tuple)
 	int len = PyTuple_Size(tuple);
 
 	if (len == 0) {
-		Py_INCREF(tuple);
+		if (PyTuple_CheckExact(tuple))
+			Py_INCREF(tuple);
+		else
+			tuple = PyTuple_New(0);
 		return tuple;
 	}
 
@@ -1895,8 +1898,15 @@ filterstring(PyObject *func, PyObject *strobj)
 	int outlen = len;
 
 	if (func == Py_None) {
-		/* No character is ever false -- share input string */
-		Py_INCREF(strobj);
+		/* No character is ever false -- share input string
+		 * (if it's not a subclass) */
+		if (PyString_CheckExact(strobj))
+			Py_INCREF(strobj);
+		else
+			strobj = PyString_FromStringAndSize(
+				PyString_AS_STRING(strobj),
+				len
+			);
 		return strobj;
 	}
 	if ((result = PyString_FromStringAndSize(NULL, len)) == NULL)
@@ -1980,8 +1990,15 @@ filterunicode(PyObject *func, PyObject *strobj)
 	int outlen = len;
 
 	if (func == Py_None) {
-		/* No character is ever false -- share input string */
-		Py_INCREF(strobj);
+		/* No character is ever false -- share input string
+		 * (it if's not a subclass) */
+		if (PyUnicode_CheckExact(strobj))
+			Py_INCREF(strobj);
+		else
+			strobj = PyUnicode_FromUnicode(
+				PyUnicode_AS_UNICODE(strobj),
+				len
+			);
 		return strobj;
 	}
 	if ((result = PyUnicode_FromUnicode(NULL, len)) == NULL)