diff -r 82b58807f481 Include/pydebug.h
--- a/Include/pydebug.h	Sat Nov 16 19:10:57 2013 +0200
+++ b/Include/pydebug.h	Mon Jan 20 15:13:58 2014 -0600
@@ -13,6 +13,7 @@
 PyAPI_DATA(int) Py_OptimizeFlag;
 PyAPI_DATA(int) Py_NoSiteFlag;
 PyAPI_DATA(int) Py_BytesWarningFlag;
+PyAPI_DATA(int) Py_BytesComplatibleFormatFlag;
 PyAPI_DATA(int) Py_UseClassExceptionsFlag;
 PyAPI_DATA(int) Py_FrozenFlag;
 PyAPI_DATA(int) Py_IgnoreEnvironmentFlag;

diff -r 82b58807f481 Modules/main.c
--- a/Modules/main.c	Sat Nov 16 19:10:57 2013 +0200
+++ b/Modules/main.c	Mon Jan 20 15:13:58 2014 -0600
@@ -43,7 +43,7 @@
 static int  orig_argc;
 
 /* command line options */
-#define BASE_OPTS L"bBc:dEhiIJm:OqRsStuvVW:xX:?"
+#define BASE_OPTS L"bBc:dEhiIJm:OqRsStuvVW:xX:2?"
 
 #define PROGRAM_OPTS BASE_OPTS
 
@@ -86,6 +86,7 @@
 -X opt : set implementation-specific option\n\
 ";
 static char *usage_4 = "\
+-2     : enable extra Python 2.x backwards compatibility features\n\
 file   : program read from script file\n\
 -      : program read from stdin (default; interactive mode if a tty)\n\
 arg ...: arguments passed to program in sys.argv[1:]\n\n\
@@ -414,6 +415,12 @@
         }
 
         switch (c) {
+	case '2':
+            /* features we want disabled by default but ease porting
+               of Python 2.x code */
+	    Py_BytesComplatibleFormatFlag++;
+	    break;
+
         case 'b':
             Py_BytesWarningFlag++;
             break;
diff -r 82b58807f481 Python/pythonrun.c
--- a/Python/pythonrun.c	Sat Nov 16 19:10:57 2013 +0200
+++ b/Python/pythonrun.c	Mon Jan 20 15:13:58 2014 -0600
@@ -116,15 +116,16 @@
 int Py_InteractiveFlag; /* Needed by Py_FdIsInteractive() below */
 int Py_InspectFlag; /* Needed to determine whether to exit at SystemExit */
 int Py_NoSiteFlag; /* Suppress 'import site' */
+int Py_BytesComplatibleFormatFlag; /* enable %s and %r */
 int Py_BytesWarningFlag; /* Warn on str(bytes) and str(buffer) */
 int Py_DontWriteBytecodeFlag; /* Suppress writing bytecode files (*.py[co]) */
 int Py_UseClassExceptionsFlag = 1; /* Needed by bltinmodule.c: deprecated */
 int Py_FrozenFlag; /* Needed by getpath.c */
 int Py_IgnoreEnvironmentFlag; /* e.g. PYTHONPATH, PYTHONHOME */
-int Py_NoUserSiteDirectory = 0; /* for -s and site.py */
-int Py_UnbufferedStdioFlag = 0; /* Unbuffered binary std{in,out,err} */
-int Py_HashRandomizationFlag = 0; /* for -R and PYTHONHASHSEED */
-int Py_IsolatedFlag = 0; /* for -I, isolate from user's env */
+int Py_NoUserSiteDirectory; /* for -s and site.py */
+int Py_UnbufferedStdioFlag; /* Unbuffered binary std{in,out,err} */
+int Py_HashRandomizationFlag; /* for -R and PYTHONHASHSEED */
+int Py_IsolatedFlag; /* for -I, isolate from user's env */
 
 PyThreadState *_Py_Finalizing = NULL;
 
diff --git a/Objects/bytesobject.c b/Objects/bytesobject.c
--- b/Objects/bytesobject.c
+++ a/Objects/bytesobject.c~
@@ -517,6 +517,14 @@
         }
         return result;
     }
+    if (Py_BytesComplatibleFormatFlag) {
+        PyObject *temp = PyObject_Str(v);
+        if (temp == NULL)
+            return NULL;
+        result = _PyUnicode_AsASCIIString(temp, "strict");
+        Py_DECREF(temp);
+        return result;
+    }
     PyErr_Format(PyExc_TypeError,
                  "Type '%.100s' doesn't define __bytes__, encoding required?",
                  Py_TYPE(v)->tp_name);
@@ -748,17 +756,27 @@
                 len = 1;
                 break;
             case 'a':
+	    case 'r':
+                if (c != 'r' || Py_BytesComplatibleFormatFlag) {
+                    temp2 = PyObject_ASCII(v);
+                    if (temp2 == NULL)
+                        goto error;
+                    temp = PyUnicode_AsLatin1String(temp2);
+                    Py_DECREF(temp2);
+                    if (temp == NULL)
+                        goto error;
+                    pbuf = PyBytes_AS_STRING(temp);
+                    len = PyBytes_GET_SIZE(temp);
+                    if (prec >= 0 && len > prec)
+                        len = prec;
+                }
+                else {
+                    PyErr_Format(PyExc_ValueError,
+                      "bytes does not support format character '%c' (0x%x) "
+                      "at index %zd",
+                      c, c, (Py_ssize_t)(fmt - 1 - PyBytes_AsString(format)));
-                temp2 = PyObject_ASCII(v);
-                if (temp2 == NULL)
                     goto error;
+                }
-                temp = PyUnicode_AsLatin1String(temp2);
-                Py_DECREF(temp2);
-                if (temp == NULL)
-                    goto error;
-                pbuf = PyBytes_AS_STRING(temp);
-                len = PyBytes_GET_SIZE(temp);
-                if (prec >= 0 && len > prec)
-                    len = prec;
                 break;
             case 's':
                 if (_getbuffer(v, &buf) < 0) {