SF bug [#468061] __str__ ignored in str subclass.

object.c, PyObject_Str:  Don't try to optimize anything except exact
string objects here; in particular, let str subclasses go thru tp_str,
same as non-str objects.  This allows overrides of tp_str to take
effect.

stringobject.c:
+ string_print (str's tp_print):  If the argument isn't an exact string
  object, get one from PyObject_Str.

+ string_str (str's tp_str):  Make a genuine-string copy of the object if
  it's of a proper str subclass type.  str() applied to a str subclass
  that doesn't override __str__ ends up here.

test_descr.py:  New str_of_str_subclass() test.

Lib/test/test_descr.py

@@ -2298,6 +2298,36 @@ def buffer_inherit():
     except TypeError:
         pass
 
+def str_of_str_subclass():
+    import binascii
+    import cStringIO
+
+    if verbose:
+        print "Testing __str__ defined in subclass of str ..."
+
+    class octetstring(str):
+        def __str__(self):
+            return binascii.b2a_hex(self)
+        def __repr__(self):
+            return self + " repr"
+
+    o = octetstring('A')
+    vereq(type(o), octetstring)
+    vereq(type(str(o)), str)
+    vereq(type(repr(o)), str)
+    vereq(ord(o), 0x41)
+    vereq(str(o), '41')
+    vereq(repr(o), 'A repr')
+    vereq(o.__str__(), '41')
+    vereq(o.__repr__(), 'A repr')
+
+    capture = cStringIO.StringIO()
+    # Calling str() or not exercises different internal paths.
+    print >> capture, o
+    print >> capture, str(o)
+    vereq(capture.getvalue(), '41\n41\n')
+    capture.close()
+
 def test_main():
     class_docstrings()
     lists()
@@ -2346,6 +2376,7 @@ def test_main():
     binopoverride()
     subclasspropagation()
     buffer_inherit()
+    str_of_str_subclass()
     if verbose: print "All OK"
 
 if __name__ == "__main__":

Objects/object.c

@@ -261,12 +261,6 @@ PyObject_Str(PyObject *v)
 		Py_INCREF(v);
 		return v;
 	}
-	if (PyString_Check(v)) {
-		/* For a string subtype that's not a string, return a true
-		   string with the same string data. */
-		PyStringObject *s = (PyStringObject *)v;
-		return PyString_FromStringAndSize(s->ob_sval, s->ob_size);
-	}
 	if (v->ob_type->tp_str == NULL)
 		return PyObject_Repr(v);
 

Objects/stringobject.c

@@ -566,7 +566,18 @@ string_print(PyStringObject *op, FILE *fp, int flags)
 	int i;
 	char c;
 	int quote;
+
 	/* XXX Ought to check for interrupts when writing long strings */
+	if (! PyString_CheckExact(op)) {
+		int ret;
+		/* A str subclass may have its own __str__ method. */
+		op = (PyStringObject *) PyObject_Str((PyObject *)op);
+		if (op == NULL)
+			return -1;
+		ret = string_print(op, fp, flags);
+		Py_DECREF(op);
+		return ret;
+	}
 	if (flags & Py_PRINT_RAW) {
 		fwrite(op->ob_sval, 1, (int) op->ob_size, fp);
 		return 0;
@@ -651,8 +662,16 @@ string_repr(register PyStringObject *op)
 static PyObject *
 string_str(PyObject *s)
 {
+	assert(PyString_Check(s));
+	if (PyString_CheckExact(s)) {
 		Py_INCREF(s);
 		return s;
+	}
+	else {
+		/* Subtype -- return genuine string with the same value. */
+		PyStringObject *t = (PyStringObject *) s;
+		return PyString_FromStringAndSize(t->ob_sval, t->ob_size);
+	}
 }
 
 static int