Issue #1583863: An unicode subclass can now override the __str__ method

Lib/test/test_unicode.py

@@ -1193,6 +1193,17 @@ class UnicodeTest(
         self.assertRaises(MemoryError, alloc)
         self.assertRaises(MemoryError, alloc)
 
+    def test_format_subclass(self):
+        class U(unicode):
+            def __str__(self):
+                return '__str__ overridden'
+            def __unicode__(self):
+                return u'__unicode__ overridden'
+        u = U(u'xxx')
+        self.assertEquals("%s" % u, u'__unicode__ overridden')
+        self.assertEquals("{}".format(u), u'__unicode__ overridden')
+
+
 def test_main():
     test_support.run_unittest(__name__)
 

Misc/NEWS

@@ -12,6 +12,8 @@ What's New in Python 2.7 beta 1?
 Core and Builtins
 -----------------
 
+- Issue #1583863: An unicode subclass can now override the __str__ method
+
 - Issue #6474: Make error message from passing an inadequate number of keyword
   arguments to a function correct.
 

Objects/unicodeobject.c

@@ -8466,7 +8466,7 @@ PyObject *PyUnicode_Format(PyObject *format,
 
             case 's':
             case 'r':
-                if (PyUnicode_Check(v) && c == 's') {
+                if (PyUnicode_CheckExact(v) && c == 's') {
                     temp = v;
                     Py_INCREF(temp);
                 }