More on SF bug [#460020] bug or feature: unicode() and subclasses.

tuple(i) repaired to return a true tuple when i is an instance of a
tuple subclass.
Added PyTuple_CheckExact macro.
PySequence_Tuple():  if a tuple-like object isn't exactly a tuple, it's
not safe to return the object as-is -- make a new tuple of it instead.

Include/tupleobject.h

@@ -27,6 +27,7 @@ typedef struct {
 extern DL_IMPORT(PyTypeObject) PyTuple_Type;
 
 #define PyTuple_Check(op) PyObject_TypeCheck(op, &PyTuple_Type)
+#define PyTuple_CheckExact(op) ((op)->ob_type == &PyTuple_Type)
 
 extern DL_IMPORT(PyObject *) PyTuple_New(int size);
 extern DL_IMPORT(int) PyTuple_Size(PyObject *);

Lib/test/test_descr.py

@@ -1416,7 +1416,7 @@ def inherits():
         verify(v == t)
     a = madtuple((1,2,3,4,5))
     verify(tuple(a) == (1,2,3,4,5))
-    #XXX verify(tuple(a).__class__ is tuple)
+    verify(tuple(a).__class__ is tuple)
     a = madtuple(())
     verify(tuple(a) == ())
     #XXX verify(tuple(a).__class__ is tuple)

Objects/abstract.c

@@ -1235,7 +1235,11 @@ PySequence_Tuple(PyObject *v)
 		return null_error();
 
 	/* Special-case the common tuple and list cases, for efficiency. */
-	if (PyTuple_Check(v)) {
+	if (PyTuple_CheckExact(v)) {
+		/* Note that we can't know whether it's safe to return
+		   a tuple *subclass* instance as-is, hence the restriction
+		   to exact tuples here.  In contrasts, lists always make
+		   a copy, so there's need for exactness below. */
 		Py_INCREF(v);
 		return v;
 	}