diff --git a/Lib/string.py b/Lib/string.py
--- a/Lib/string.py
+++ b/Lib/string.py
@@ -299,3 +299,15 @@
                 obj = obj[i]
 
         return obj, first
+
+# Control whether str(bytes) raises an exception. Thread local.
+from contextlib import contextmanager
+@contextmanager
+def StrBytesRaises():
+  previous = _string.get_strbytes_raises()
+  _string.set_strbytes_raises(True)
+  try:
+    yield
+  finally:  
+    _string.set_strbytes_raises(previous)
+    
\ No newline at end of file
diff --git a/Objects/bytesobject.c b/Objects/bytesobject.c
--- a/Objects/bytesobject.c
+++ b/Objects/bytesobject.c
@@ -1375,6 +1375,19 @@
                          "str() on a bytes instance", 1))
             return NULL;
     }
+    {
+      PyObject *dict = PyThreadState_GetDict();
+      if (dict != NULL) {
+        PyObject *strbytes_raises = PyDict_GetItemString(dict, "strbytes_raises");
+        if (PyErr_Occurred() || strbytes_raises == NULL) {
+          PyErr_Clear();
+        } else if (strbytes_raises == Py_True) {
+          PyErr_SetString(PyExc_BytesWarning,
+              "str() on a bytes instance");
+          return NULL;
+        }
+      }
+    }
     return bytes_repr(op);
 }
 
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -15413,6 +15413,52 @@
     return copy;
 }
 
+static PyObject *
+sys_get_strbytes_raises(PyObject *self, PyObject *noargs)
+{
+    PyObject *dict, *strbytes_raises;
+
+    dict = PyThreadState_GetDict();
+    if (dict == NULL) {
+        PyErr_SetString(PyExc_RuntimeError,
+            "cannot get thread state");
+        return NULL;
+    }
+
+    strbytes_raises = PyDict_GetItemString(dict, "strbytes_raises");
+
+    if (PyErr_Occurred() || strbytes_raises == NULL) {
+      PyErr_Clear();
+      Py_INCREF(Py_False);
+      return Py_False;
+    }
+
+    return strbytes_raises;
+}
+
+static PyObject *
+sys_set_strbytes_raises(PyObject *self, PyObject *value)
+{
+  PyObject *dict;
+
+  if(!PyBool_Check(value)) {
+    PyErr_SetString(PyExc_TypeError,
+        "not a bool");
+    return NULL;
+  }
+
+  dict = PyThreadState_GetDict();
+  if (dict == NULL) {
+      PyErr_SetString(PyExc_RuntimeError,
+          "cannot get thread state");
+      return NULL;
+  }
+
+  PyDict_SetItemString(dict, "strbytes_raises", value);
+
+  Py_RETURN_NONE;
+}
+
 /* A _string module, to export formatter_parser and formatter_field_name_split
    to the string.Formatter class implemented in Python. */
 
@@ -15421,6 +15467,8 @@
      METH_O, PyDoc_STR("split the argument as a field name")},
     {"formatter_parser", (PyCFunction) formatter_parser,
      METH_O, PyDoc_STR("parse the argument as a format string")},
+    {"get_strbytes_raises", sys_get_strbytes_raises, METH_NOARGS, NULL},
+    {"set_strbytes_raises", sys_set_strbytes_raises, METH_O, NULL},
     {NULL, NULL}
 };