Index: Modules/_fileio.c
===================================================================
--- Modules/_fileio.c	(révision 68832)
+++ Modules/_fileio.c	(copie de travail)
@@ -27,6 +27,12 @@
 #include <windows.h>
 #endif
 
+#if defined(MS_WIN64) || defined(MS_WINDOWS)
+typedef PY_LONG_LONG Py_off_t;
+#else
+typedef off_t Py_off_t;
+#endif
+
 #if BUFSIZ < (8*1024)
 #define SMALLCHUNK (8*1024)
 #elif (BUFSIZ >= (2 << 25))
@@ -315,6 +321,17 @@
 			goto error;
 	}
 
+	if (append) {
+		Py_off_t 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:
@@ -570,12 +587,6 @@
 
 /* XXX Windows support below is likely incomplete */
 
-#if defined(MS_WIN64) || defined(MS_WINDOWS)
-typedef PY_LONG_LONG Py_off_t;
-#else
-typedef off_t Py_off_t;
-#endif
-
 /* Cribbed from posix_lseek() */
 static PyObject *
 portable_lseek(int fd, PyObject *posobj, int whence)
Index: Lib/test/test_io.py
===================================================================
--- Lib/test/test_io.py	(révision 68832)
+++ 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_(f.tell() > 0)
+
     def test_destructor(self):
         record = []
         class MyFileIO(io.FileIO):