diff -r 1f31bf3f76f5 Lib/_pyio.py
--- a/Lib/_pyio.py	Fri Dec 02 08:42:43 2016 +0200
+++ b/Lib/_pyio.py	Wed Dec 07 12:50:03 2016 +0200
@@ -276,7 +276,7 @@ class OpenWrapper:
 try:
     UnsupportedOperation = io.UnsupportedOperation
 except AttributeError:
-    class UnsupportedOperation(ValueError, OSError):
+    class UnsupportedOperation(OSError, ValueError):
         pass
 
 
diff -r 1f31bf3f76f5 Modules/_io/_iomodule.c
--- a/Modules/_io/_iomodule.c	Fri Dec 02 08:42:43 2016 +0200
+++ b/Modules/_io/_iomodule.c	Wed Dec 07 12:50:03 2016 +0200
@@ -654,7 +654,7 @@ PyInit__io(void)
     /* UnsupportedOperation inherits from ValueError and IOError */
     state->unsupported_operation = PyObject_CallFunction(
         (PyObject *)&PyType_Type, "s(OO){}",
-        "UnsupportedOperation", PyExc_ValueError, PyExc_IOError);
+        "UnsupportedOperation", PyExc_OSError, PyExc_ValueError);
     if (state->unsupported_operation == NULL)
         goto fail;
     Py_INCREF(state->unsupported_operation);