Index: Lib/_pyio.py
===================================================================
--- Lib/_pyio.py	(révision 70474)
+++ Lib/_pyio.py	(copie de travail)
@@ -1390,6 +1390,12 @@
         self._snapshot = None  # info for reconstructing decoder state
         self._seekable = self._telling = self.buffer.seekable()
 
+        if self._seekable and self.writable():
+            position = self.buffer.tell()
+            if position != 0:
+                self._encoder = self._get_encoder()
+                self._encoder.setstate(0)
+
     # self._snapshot is either None, or a tuple (dec_flags, next_input)
     # where dec_flags is the second (integer) item of the decoder state
     # and next_input is the chunk of input bytes that comes next after the
Index: Lib/test/test_io.py
===================================================================
--- Lib/test/test_io.py	(révision 70474)
+++ Lib/test/test_io.py	(copie de travail)
@@ -1797,6 +1797,20 @@
 
         self.assertEqual(buffer.seekable(), txt.seekable())
 
+    def test_append_bom(self):
+        filename = support.TESTFN
+        for charset in ('utf-8-sig', 'utf16', 'utf-32'):
+            with self.open(filename, 'w', encoding=charset) as f:
+                f.write('aaa')
+            with self.open(filename, 'r', encoding=charset) as f:
+                self.assertEquals(f.read(), 'aaa')
+
+            with self.open(filename, 'a', encoding=charset) as f:
+                f.write('xxx')
+            with self.open(filename, 'r', encoding=charset) as f:
+                self.assertEquals(f.read(), 'aaaxxx')
+
+
 class CTextIOWrapperTest(TextIOWrapperTest):
 
     def test_initialization(self):
Index: Modules/_textio.c
===================================================================
--- Modules/_textio.c	(révision 70474)
+++ Modules/_textio.c	(copie de travail)
@@ -754,6 +754,7 @@
     char *newline = NULL;
     int line_buffering = 0;
     _PyIO_State *state = IO_STATE;
+    static PyObject *zero = NULL;
 
     PyObject *res;
     int r;
@@ -963,9 +964,40 @@
     Py_DECREF(res);
 
     self->ok = 1;
+
+    if (self->seekable && self->encoder) {
+        PyObject *cookieObj;
+        int cmp;
+
+        if (zero == NULL) {
+            zero = PyLong_FromLong(0L);
+            if (zero == NULL)
+                goto error;
+        }
+
+        cookieObj = PyObject_CallMethod(buffer, "tell", NULL);
+        if (cookieObj == NULL)
+            goto error;
+
+        cmp = PyObject_RichCompareBool(cookieObj, zero, Py_EQ);
+        Py_DECREF(cookieObj);
+        if (cmp < 0) {
+            goto error;
+        }
+
+        if (cmp == 0) {
+            res = PyObject_CallMethod(self->encoder, "setstate", "O", zero);
+            if (res == NULL)
+                goto error;
+            Py_DECREF(res);
+
+            self->encodefunc = NULL;
+        }
+    }
     return 0;
 
   error:
+    self->ok = 0;
     return -1;
 }