Index: Lib/_pyio.py
===================================================================
--- Lib/_pyio.py	(révision 84451)
+++ Lib/_pyio.py	(copie de travail)
@@ -243,8 +243,13 @@
         return open(*args, **kwargs)
 
 
-class UnsupportedOperation(ValueError, IOError):
-    pass
+# In normal operation, both `UnsupportedOperation`s should be bound to the
+# same object.
+try:
+    UnsupportedOperation = io.UnsupportedOperation
+except AttributeError:
+    class UnsupportedOperation(ValueError, IOError):
+        pass
 
 
 class IOBase(metaclass=abc.ABCMeta):
@@ -362,10 +367,9 @@
         """Internal: raise an IOError if file is not seekable
         """
         if not self.seekable():
-            raise IOError("File or stream is not seekable."
-                          if msg is None else msg)
+            raise UnsupportedOperation("File or stream is not seekable."
+                                       if msg is None else msg)
 
-
     def readable(self) -> bool:
         """Return whether object was opened for reading.
 
@@ -377,8 +381,8 @@
         """Internal: raise an IOError if file is not readable
         """
         if not self.readable():
-            raise IOError("File or stream is not readable."
-                          if msg is None else msg)
+            raise UnsupportedOperation("File or stream is not readable."
+                                       if msg is None else msg)
 
     def writable(self) -> bool:
         """Return whether object was opened for writing.
@@ -391,8 +395,8 @@
         """Internal: raise an IOError if file is not writable
         """
         if not self.writable():
-            raise IOError("File or stream is not writable."
-                          if msg is None else msg)
+            raise UnsupportedOperation("File or stream is not writable."
+                                       if msg is None else msg)
 
     @property
     def closed(self):
@@ -1647,7 +1651,7 @@
 
     def tell(self):
         if not self._seekable:
-            raise IOError("underlying stream is not seekable")
+            raise UnsupportedOperation("underlying stream is not seekable")
         if not self._telling:
             raise IOError("telling position disabled by next() call")
         self.flush()
@@ -1726,17 +1730,17 @@
         if self.closed:
             raise ValueError("tell on closed file")
         if not self._seekable:
-            raise IOError("underlying stream is not seekable")
+            raise UnsupportedOperation("underlying stream is not seekable")
         if whence == 1: # seek relative to current position
             if cookie != 0:
-                raise IOError("can't do nonzero cur-relative seeks")
+                raise UnsupportedOperation("can't do nonzero cur-relative seeks")
             # Seeking to the current position should attempt to
             # sync the underlying buffer with the current position.
             whence = 0
             cookie = self.tell()
         if whence == 2: # seek relative to end of file
             if cookie != 0:
-                raise IOError("can't do nonzero end-relative seeks")
+                raise UnsupportedOperation("can't do nonzero end-relative seeks")
             self.flush()
             position = self.buffer.seek(0, 2)
             self._set_decoded_chars('')
Index: Lib/test/test_io.py
===================================================================
--- Lib/test/test_io.py	(révision 84451)
+++ Lib/test/test_io.py	(copie de travail)
@@ -179,6 +179,23 @@
     pass
 
 
+class MockUnseekableIO:
+    def seekable(self):
+        return False
+    
+    def seek(self, *args):
+        raise self.UnsupportedOperation("not seekable")
+
+    def tell(self, *args):
+        raise self.UnsupportedOperation("not seekable")
+
+class CMockUnseekableIO(MockUnseekableIO, io.BytesIO):
+    UnsupportedOperation = io.UnsupportedOperation
+
+class PyMockUnseekableIO(MockUnseekableIO, pyio.BytesIO):
+    UnsupportedOperation = pyio.UnsupportedOperation
+
+
 class MockNonBlockWriterIO:
 
     def __init__(self):
@@ -304,16 +321,26 @@
 
     def test_invalid_operations(self):
         # Try writing on a file opened in read mode and vice-versa.
+        exc = self.UnsupportedOperation
         for mode in ("w", "wb"):
             with self.open(support.TESTFN, mode) as fp:
-                self.assertRaises(IOError, fp.read)
-                self.assertRaises(IOError, fp.readline)
+                self.assertRaises(exc, fp.read)
+                self.assertRaises(exc, fp.readline)
+        with self.open(support.TESTFN, "wb", buffering=0) as fp:
+            self.assertRaises(exc, fp.read)
+            self.assertRaises(exc, fp.readline)
+        with self.open(support.TESTFN, "rb", buffering=0) as fp:
+            self.assertRaises(exc, fp.write, b"blah")
+            self.assertRaises(exc, fp.writelines, [b"blah\n"])
         with self.open(support.TESTFN, "rb") as fp:
-            self.assertRaises(IOError, fp.write, b"blah")
-            self.assertRaises(IOError, fp.writelines, [b"blah\n"])
+            self.assertRaises(exc, fp.write, b"blah")
+            self.assertRaises(exc, fp.writelines, [b"blah\n"])
         with self.open(support.TESTFN, "r") as fp:
-            self.assertRaises(IOError, fp.write, "blah")
-            self.assertRaises(IOError, fp.writelines, ["blah\n"])
+            self.assertRaises(exc, fp.write, "blah")
+            self.assertRaises(exc, fp.writelines, ["blah\n"])
+            # Non-zero seeking from current or end pos
+            self.assertRaises(exc, fp.seek, 1, self.SEEK_CUR)
+            self.assertRaises(exc, fp.seek, -1, self.SEEK_END)
 
     def test_raw_file_io(self):
         with self.open(support.TESTFN, "wb", buffering=0) as f:
@@ -670,7 +697,12 @@
         b.close()
         self.assertRaises(ValueError, b.flush)
 
+    def test_unseekable(self):
+        bufio = self.tp(self.MockUnseekableIO(b"A" * 10))
+        self.assertRaises(self.UnsupportedOperation, bufio.tell)
+        self.assertRaises(self.UnsupportedOperation, bufio.seek, 0)
 
+
 class BufferedReaderTest(unittest.TestCase, CommonBufferedTests):
     read_mode = "rb"
 
@@ -1433,6 +1465,9 @@
         BufferedReaderTest.test_misbehaved_io(self)
         BufferedWriterTest.test_misbehaved_io(self)
 
+    # You can't construct a BufferedRandom over a non-seekable stream.
+    test_unseekable = None
+
 class CBufferedRandomTest(BufferedRandomTest):
     tp = io.BufferedRandom
 
@@ -2176,6 +2211,11 @@
         txt.close()
         txt.close()
         self.assertRaises(ValueError, txt.flush)
+    
+    def test_unseekable(self):
+        txt = self.TextIOWrapper(self.MockUnseekableIO(self.testdata))
+        self.assertRaises(self.UnsupportedOperation, txt.tell)
+        self.assertRaises(self.UnsupportedOperation, txt.seek, 0)
 
 class CTextIOWrapperTest(TextIOWrapperTest):
 
@@ -2550,7 +2590,7 @@
     # Put the namespaces of the IO module we are testing and some useful mock
     # classes in the __dict__ of each test.
     mocks = (MockRawIO, MisbehavedRawIO, MockFileIO, CloseFailureIO,
-             MockNonBlockWriterIO)
+             MockNonBlockWriterIO, MockUnseekableIO)
     all_members = io.__all__ + ["IncrementalNewlineDecoder"]
     c_io_ns = {name : getattr(io, name) for name in all_members}
     py_io_ns = {name : getattr(pyio, name) for name in all_members}
Index: Modules/_io/iobase.c
===================================================================
--- Modules/_io/iobase.c	(révision 84451)
+++ Modules/_io/iobase.c	(copie de travail)
@@ -317,7 +317,7 @@
         return NULL;
     if (res != Py_True) {
         Py_CLEAR(res);
-        PyErr_SetString(PyExc_IOError, "File or stream is not seekable.");
+        iobase_unsupported("File or stream is not seekable.");
         return NULL;
     }
     if (args == Py_True) {
@@ -346,7 +346,7 @@
         return NULL;
     if (res != Py_True) {
         Py_CLEAR(res);
-        PyErr_SetString(PyExc_IOError, "File or stream is not readable.");
+        iobase_unsupported("File or stream is not readable.");
         return NULL;
     }
     if (args == Py_True) {
@@ -375,7 +375,7 @@
         return NULL;
     if (res != Py_True) {
         Py_CLEAR(res);
-        PyErr_SetString(PyExc_IOError, "File or stream is not writable.");
+        iobase_unsupported("File or stream is not writable.");
         return NULL;
     }
     if (args == Py_True) {
Index: Modules/_io/fileio.c
===================================================================
--- Modules/_io/fileio.c	(révision 84451)
+++ Modules/_io/fileio.c	(copie de travail)
@@ -417,7 +417,8 @@
 static PyObject *
 err_mode(char *action)
 {
-    PyErr_Format(PyExc_ValueError, "File not open for %s", action);
+    PyErr_Format(IO_STATE->unsupported_operation,
+                 "File not open for %s", action);
     return NULL;
 }
 
Index: Modules/_io/textio.c
===================================================================
--- Modules/_io/textio.c	(révision 84451)
+++ Modules/_io/textio.c	(copie de travail)
@@ -1259,10 +1259,8 @@
 
     CHECK_CLOSED(self);
 
-    if (self->encoder == NULL) {
-        PyErr_SetString(PyExc_IOError, "not writable");
-        return NULL;
-    }
+    if (self->encoder == NULL)
+        return _unsupported("not writable");
 
     Py_INCREF(text);
 
@@ -1399,7 +1397,7 @@
      */
 
     if (self->decoder == NULL) {
-        PyErr_SetString(PyExc_IOError, "not readable");
+        _unsupported("not readable");
         return -1;
     }
 
@@ -1489,10 +1487,8 @@
 
     CHECK_CLOSED(self);
 
-    if (self->decoder == NULL) {
-        PyErr_SetString(PyExc_IOError, "not readable");
-        return NULL;
-    }
+    if (self->decoder == NULL)
+        return _unsupported("not readable");
 
     if (_textiowrapper_writeflush(self) < 0)
         return NULL;
@@ -1983,8 +1979,7 @@
     Py_INCREF(cookieObj);
 
     if (!self->seekable) {
-        PyErr_SetString(PyExc_IOError,
-                        "underlying stream is not seekable");
+        _unsupported("underlying stream is not seekable");
         goto fail;
     }
 
@@ -1995,8 +1990,7 @@
             goto fail;
 
         if (cmp == 0) {
-            PyErr_SetString(PyExc_IOError,
-                            "can't do nonzero cur-relative seeks");
+            _unsupported("can't do nonzero cur-relative seeks");
             goto fail;
         }
 
@@ -2016,8 +2010,7 @@
             goto fail;
 
         if (cmp == 0) {
-            PyErr_SetString(PyExc_IOError,
-                            "can't do nonzero end-relative seeks");
+            _unsupported("can't do nonzero end-relative seeks");
             goto fail;
         }
 
@@ -2151,8 +2144,7 @@
     CHECK_CLOSED(self);
 
     if (!self->seekable) {
-        PyErr_SetString(PyExc_IOError,
-                        "underlying stream is not seekable");
+        _unsupported("underlying stream is not seekable");
         goto fail;
     }
     if (!self->telling) {