diff -r 0eeffeadaa1e Lib/_pyio.py
--- a/Lib/_pyio.py	Mon Aug 06 17:53:19 2012 -0700
+++ b/Lib/_pyio.py	Tue Aug 07 03:49:56 2012 +0200
@@ -1542,6 +1542,54 @@ class TextIOWrapper(TextIOBase):
             result += " mode={0!r}".format(mode)
         return result + " encoding={0!r}>".format(self.encoding)
 
+    def set_encoding(self, encoding, errors=None):
+        if not isinstance(encoding, str):
+            raise ValueError("invalid encoding: %r" % encoding)
+
+        current_encoding = codecs.lookup(self._encoding).name
+        encoding = codecs.lookup(encoding).name
+        if (encoding == self._encoding
+        and (errors is None or errors == self._errors)):
+            # no change
+            return
+
+        # flush read buffer, may require to seek backward in the underlying
+        # file object
+        if self._decoded_chars:
+            if not self.seekable():
+                raise UnsupportedOperation(
+                    "It is not possible to set the encoding "
+                    "of a non seekable file after the first read")
+            assert self._snapshot is not None
+            dec_flags, next_input = self._snapshot
+            offset = self._decoded_chars_used - len(next_input)
+            if offset:
+                self.buffer.seek(offset, SEEK_CUR)
+
+        # flush write buffer
+        self.flush()
+
+        # reset attributes
+        self._encoding = encoding
+        if errors is not None:
+            self._errors = errors
+        self._encoder = None
+        self._decoder = None
+        self._decoded_chars = ''
+        self._decoded_chars_used = 0
+        self._snapshot = None
+        self._b2cratio = 0.0
+
+        # don't write a BOM in the middle of a file
+        if self._seekable and self.writable():
+            position = self.buffer.tell()
+            if position != 0:
+                try:
+                    self._get_encoder().setstate(0)
+                except LookupError:
+                    # Sometimes the encoder doesn't exist
+                    pass
+
     @property
     def encoding(self):
         return self._encoding
diff -r 0eeffeadaa1e Lib/test/test_io.py
--- a/Lib/test/test_io.py	Mon Aug 06 17:53:19 2012 -0700
+++ b/Lib/test/test_io.py	Tue Aug 07 03:49:56 2012 +0200
@@ -2361,7 +2361,6 @@ class TextIOWrapperTest(unittest.TestCas
         for charset in ('utf-8-sig', 'utf-16', 'utf-32'):
             with self.open(filename, 'w', encoding=charset) as f:
                 f.write('aaa')
-                pos = f.tell()
             with self.open(filename, 'rb') as f:
                 self.assertEqual(f.read(), 'aaa'.encode(charset))
 
@@ -2460,6 +2459,51 @@ class TextIOWrapperTest(unittest.TestCas
         txt.write('5')
         self.assertEqual(b''.join(raw._write_stack), b'123\n45')
 
+    def test_set_encoding(self):
+        # read
+        data = 'abc\xe9\n'.encode('latin1') + 'd\xe9f\n'.encode('utf8')
+        raw = self.BytesIO(data)
+        txt = self.TextIOWrapper(raw, encoding='latin1', newline='\n')
+        self.assertEqual(txt.readline(), 'abc\xe9\n')
+        txt.set_encoding('utf-8')
+        self.assertEqual(txt.readline(), 'd\xe9f\n')
+
+        # On non-seekable stream, set_encoding() can be called before the first
+        # read
+        data = 'abc\xe9'.encode('latin1')
+        raw = self.MockUnseekableIO(data)
+        txt = self.TextIOWrapper(raw, encoding='ascii', newline='\n')
+        txt.set_encoding('latin1')
+        self.assertEqual(txt.readline(), 'abc\xe9')
+
+        # On non-seekable stream, set_encoding() cannot be called after the
+        # first read
+        data = 'xabc\xe9\n'.encode('latin1') + 'yd\xe9f\n'.encode('utf8')
+        raw = self.MockUnseekableIO(data)
+        txt = self.TextIOWrapper(raw, encoding='latin1', newline='\n')
+        self.assertEqual(txt.readline(), 'xabc\xe9\n')
+        self.assertRaises(io.UnsupportedOperation, txt.set_encoding, 'utf-8')
+
+        # write
+        raw = self.BytesIO()
+        txt = self.TextIOWrapper(raw, encoding='latin1', newline='\n')
+        txt.write('abc\xe9\n')
+        txt.set_encoding('utf-8')
+        self.assertEqual(raw.getvalue(), b'abc\xe9\n')
+        txt.write('d\xe9f\n')
+        txt.flush()
+        self.assertEqual(raw.getvalue(), b'abc\xe9\nd\xc3\xa9f\n')
+
+        # write with BOM: don't write the BOM in the middle of the file
+        raw = self.BytesIO()
+        txt = self.TextIOWrapper(raw, encoding='ascii', newline='\n')
+        txt.write('abc\n')
+        txt.set_encoding('utf-8-sig')
+        txt.write('d\xe9f\n')
+        txt.flush()
+        self.assertEqual(raw.getvalue(), b'abc\nd\xc3\xa9f\n')
+
+
 class CTextIOWrapperTest(TextIOWrapperTest):
 
     def test_initialization(self):