Index: Modules/_io/fileio.c
===================================================================
--- Modules/_io/fileio.c	(révision 73491)
+++ Modules/_io/fileio.c	(copie de travail)
@@ -736,7 +736,7 @@
 static PyObject *
 fileio_seek(fileio *self, PyObject *args)
 {
-	PyObject *posobj;
+	PyObject *posobj, *res;
 	int whence = 0;
 
 	if (self->fd < 0)
@@ -745,16 +745,25 @@
 	if (!PyArg_ParseTuple(args, "O|i", &posobj, &whence))
 		return NULL;
 
-	return portable_lseek(self->fd, posobj, whence);
+	res = portable_lseek(self->fd, posobj, whence);
+	if (res == NULL && self->seekable > 0)
+		/* Perhaps the file can't be seeked anymore */
+		self->seekable = -1;
+	return res;
 }
 
 static PyObject *
 fileio_tell(fileio *self, PyObject *args)
 {
+	PyObject *res;
 	if (self->fd < 0)
 		return err_closed();
 
-	return portable_lseek(self->fd, NULL, 1);
+	res = portable_lseek(self->fd, NULL, 1);
+	if (res == NULL && self->seekable > 0)
+		/* Perhaps the file can't be seeked anymore */
+		self->seekable = -1;
+	return res;
 }
 
 #ifdef HAVE_FTRUNCATE
Index: Modules/_io/textio.c
===================================================================
--- Modules/_io/textio.c	(révision 73491)
+++ Modules/_io/textio.c	(copie de travail)
@@ -1038,25 +1038,35 @@
         PyObject *cookieObj;
         int cmp;
 
-        self->encoding_start_of_stream = 1;
-
         cookieObj = PyObject_CallMethodObjArgs(buffer, _PyIO_str_tell, NULL);
-        if (cookieObj == NULL)
-            goto error;
-
-        cmp = PyObject_RichCompareBool(cookieObj, _PyIO_zero, Py_EQ);
-        Py_DECREF(cookieObj);
-        if (cmp < 0) {
-            goto error;
+        if (cookieObj == NULL) {
+            if (PyErr_ExceptionMatches(PyExc_EnvironmentError)) {
+                /* Perhaps the stream isn't seekable after all? */
+                PyErr_Clear();
+                res = PyObject_CallMethod(buffer, "seekable", NULL);
+                if (res == NULL)
+                    goto error;
+                self->seekable = self->telling = PyObject_IsTrue(res);
+                Py_DECREF(res);
+            }
+            else
+                goto error;
         }
-
-        if (cmp == 0) {
-            self->encoding_start_of_stream = 0;
-            res = PyObject_CallMethodObjArgs(self->encoder, _PyIO_str_setstate,
-                                             _PyIO_zero, NULL);
-            if (res == NULL)
+        else {
+            self->encoding_start_of_stream = 1;
+            cmp = PyObject_RichCompareBool(cookieObj, _PyIO_zero, Py_EQ);
+            Py_DECREF(cookieObj);
+            if (cmp < 0) {
                 goto error;
-            Py_DECREF(res);
+            }
+            if (cmp == 0) {
+                self->encoding_start_of_stream = 0;
+                res = PyObject_CallMethodObjArgs(self->encoder, _PyIO_str_setstate,
+                                                 _PyIO_zero, NULL);
+                if (res == NULL)
+                    goto error;
+                Py_DECREF(res);
+            }
         }
     }