Index: Modules/_fileio.c
===================================================================
--- Modules/_fileio.c	(révision 68826)
+++ Modules/_fileio.c	(copie de travail)
@@ -53,6 +53,8 @@
 
 PyTypeObject PyFileIO_Type;
 
+static PyObject *portable_lseek(int fd, PyObject *posobj, int whence);
+
 #define PyFileIO_Check(op) (PyObject_TypeCheck((op), &PyFileIO_Type))
 
 /* Returns 0 on success, -1 with exception set on failure. */
@@ -315,6 +317,17 @@
 			goto error;
 	}
 
+	if (append) {
+		long pos;
+		Py_BEGIN_ALLOW_THREADS
+		pos = lseek(self->fd, 0, SEEK_END);
+		Py_END_ALLOW_THREADS
+		if (pos < 0) {
+			PyErr_SetFromErrno(PyExc_IOError);
+			goto error;
+		}
+	}
+
 	goto done;
 
  error:
Index: Lib/test/test_io.py
===================================================================
--- Lib/test/test_io.py	(révision 68826)
+++ Lib/test/test_io.py	(copie de travail)
@@ -233,6 +233,17 @@
             else:
                 self.fail("1/0 didn't raise an exception")
 
+    # issue 5008
+    def test_append(self):
+        with open(support.TESTFN, "wb") as f:
+            f.write(b"xxx")
+        with open(support.TESTFN, "ab") as f:
+            self.assertEqual(f.tell(), 3)
+        with open(support.TESTFN, "ab", buffering=0) as f:
+            self.assertEqual(f.tell(), 3)
+        with open(support.TESTFN, "a") as f:
+            self.assert_(0 < f.tell())
+
     def test_destructor(self):
         record = []
         class MyFileIO(io.FileIO):