Index: Python/pythonrun.c
===================================================================
--- Python/pythonrun.c	(Revision 58857)
+++ Python/pythonrun.c	(Arbeitskopie)
@@ -75,6 +75,7 @@
 int Py_InteractiveFlag; /* Needed by Py_FdIsInteractive() below */
 int Py_InspectFlag; /* Needed to determine whether to exit at SystemError */
 int Py_NoSiteFlag; /* Suppress 'import site' */
+int Py_StrBytesWarningFlag; /* Warn on str(bytes) and str(buffer) */
 int Py_UseClassExceptionsFlag = 1; /* Needed by bltinmodule.c: deprecated */
 int Py_FrozenFlag; /* Needed by getpath.c */
 int Py_IgnoreEnvironmentFlag; /* e.g. PYTHONPATH, PYTHONHOME */
Index: Include/pydebug.h
===================================================================
--- Include/pydebug.h	(Revision 58857)
+++ Include/pydebug.h	(Arbeitskopie)
@@ -11,6 +11,7 @@
 PyAPI_DATA(int) Py_InspectFlag;
 PyAPI_DATA(int) Py_OptimizeFlag;
 PyAPI_DATA(int) Py_NoSiteFlag;
+PyAPI_DATA(int) Py_StrBytesWarningFlag;
 PyAPI_DATA(int) Py_UseClassExceptionsFlag;
 PyAPI_DATA(int) Py_FrozenFlag;
 PyAPI_DATA(int) Py_TabcheckFlag;
Index: Objects/bytesobject.c
===================================================================
--- Objects/bytesobject.c	(Revision 58857)
+++ Objects/bytesobject.c	(Arbeitskopie)
@@ -923,6 +923,23 @@
 }
 
 static PyObject *
+bytes_str(PyObject *op)
+{
+	if (Py_StrBytesWarningFlag == 1) {
+		if (PyErr_WarnEx(PyExc_Warning,
+				 "Calling str() on a bytes instance.",
+				 1))
+			return NULL;
+	}
+	if (Py_StrBytesWarningFlag > 1) {
+		PyErr_SetString(PyExc_TypeError,
+				"Calling str() on a bytes instance.");
+		return NULL;
+	}
+	return bytes_repr((PyBytesObject*)op);
+}
+
+static PyObject *
 bytes_richcompare(PyObject *self, PyObject *other, int op)
 {
     Py_ssize_t self_size, other_size;
@@ -3063,7 +3080,7 @@
     &bytes_as_mapping,                  /* tp_as_mapping */
     0,                                  /* tp_hash */
     0,                                  /* tp_call */
-    (reprfunc)bytes_repr,               /* tp_str */
+    bytes_str,                          /* tp_str */
     PyObject_GenericGetAttr,            /* tp_getattro */
     0,                                  /* tp_setattro */
     &bytes_as_buffer,                   /* tp_as_buffer */
Index: Objects/stringobject.c
===================================================================
--- Objects/stringobject.c	(Revision 58857)
+++ Objects/stringobject.c	(Arbeitskopie)
@@ -679,6 +679,23 @@
 	return PyString_Repr(op, 1);
 }
 
+static PyObject *
+string_str(PyObject *op)
+{
+	if (Py_StrBytesWarningFlag == 1) {
+		if (PyErr_WarnEx(PyExc_Warning,
+				 "Calling str() on a bytes instance.",
+				 1))
+			return NULL;
+	}
+	if (Py_StrBytesWarningFlag > 1) {
+		PyErr_SetString(PyExc_TypeError,
+				"Calling str() on a bytes instance.");
+			return NULL;
+	}
+	return string_repr(op);
+}
+
 static Py_ssize_t
 string_length(PyStringObject *a)
 {
@@ -3090,13 +3107,13 @@
 	0,					/* tp_getattr */
 	0,					/* tp_setattr */
 	0,					/* tp_compare */
-	string_repr, 				/* tp_repr */
+	(reprfunc)string_repr, 			/* tp_repr */
 	0,					/* tp_as_number */
 	&string_as_sequence,			/* tp_as_sequence */
 	&string_as_mapping,			/* tp_as_mapping */
 	(hashfunc)string_hash, 			/* tp_hash */
 	0,					/* tp_call */
-	string_repr,				/* tp_str */
+	string_str,				/* tp_str */
 	PyObject_GenericGetAttr,		/* tp_getattro */
 	0,					/* tp_setattro */
 	&string_as_buffer,			/* tp_as_buffer */
Index: Modules/main.c
===================================================================
--- Modules/main.c	(Revision 58857)
+++ Modules/main.c	(Arbeitskopie)
@@ -44,7 +44,7 @@
 static int  orig_argc;
 
 /* command line options */
-#define BASE_OPTS "c:dEhim:OStuvVW:xX?"
+#define BASE_OPTS "bc:dEhim:OStuvVW:xX?"
 
 #define PROGRAM_OPTS BASE_OPTS
 
@@ -55,32 +55,34 @@
 /* Long usage message, split into parts < 512 bytes */
 static char *usage_1 = "\
 Options and arguments (and corresponding environment variables):\n\
+-b     : issue warnings about str(bytes_instance) and str(buffer_instance)\n\
+         (-bb: issue errors)\n\
 -c cmd : program passed in as string (terminates option list)\n\
 -d     : debug output from parser; also PYTHONDEBUG=x\n\
 -E     : ignore environment variables (such as PYTHONPATH)\n\
 -h     : print this help message and exit (also --help)\n\
+";
+static char *usage_2 = "\
 -i     : inspect interactively after running script; forces a prompt even\n\
          if stdin does not appear to be a terminal; also PYTHONINSPECT=x\n\
-";
-static char *usage_2 = "\
 -m mod : run library module as a script (terminates option list)\n\
 -O     : optimize generated bytecode slightly; also PYTHONOPTIMIZE=x\n\
 -OO    : remove doc-strings in addition to the -O optimizations\n\
 -S     : don't imply 'import site' on initialization\n\
 -t     : issue warnings about inconsistent tab usage (-tt: issue errors)\n\
--u     : unbuffered binary stdout and stderr; also PYTHONUNBUFFERED=x\n\
 ";
 static char *usage_3 = "\
+-u     : unbuffered binary stdout and stderr; also PYTHONUNBUFFERED=x\n\
          see man page for details on internal buffering relating to '-u'\n\
 -v     : verbose (trace import statements); also PYTHONVERBOSE=x\n\
          can be supplied multiple times to increase verbosity\n\
 -V     : print the Python version number and exit (also --version)\n\
 -W arg : warning control; arg is action:message:category:module:lineno\n\
 -x     : skip first line of source, allowing use of non-Unix forms of #!cmd\n\
+";
+static char *usage_4 = "\
 file   : program read from script file\n\
 -      : program read from stdin (default; interactive mode if a tty)\n\
-";
-static char *usage_4 = "\
 arg ...: arguments passed to program in sys.argv[1:]\n\n\
 Other environment variables:\n\
 PYTHONSTARTUP: file executed on interactive startup (no default)\n\
@@ -252,6 +254,9 @@
 		}
 
 		switch (c) {
+		case 'b':
+			Py_StrBytesWarningFlag++;
+			break;
 
 		case 'd':
 			Py_DebugFlag++;