Issue #5008: When a file is opened in append mode with the new IO library,

antoine.pitrou · antoine.pitrou · commit e5bc49091aaa · 2009-01-21T00:45:36.000Z
do an explicit seek to the end of file (so that e.g. tell() returns the file size rather than 0). This is consistent with the behaviour of the traditional 2.x file object. git-svn-id: http://svn.python.org/projects/python/trunk@68835 6015fed2-1504-0410-9fe1-9d1591cc4771
diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
@@ -232,6 +232,17 @@ def test_with_open(self):
             else:
                 self.fail("1/0 didn't raise an exception")
 
+    # issue 5008
+    def test_append_mode_tell(self):
+        with io.open(test_support.TESTFN, "wb") as f:
+            f.write(b"xxx")
+        with io.open(test_support.TESTFN, "ab", buffering=0) as f:
+            self.assertEqual(f.tell(), 3)
+        with io.open(test_support.TESTFN, "ab") as f:
+            self.assertEqual(f.tell(), 3)
+        with io.open(test_support.TESTFN, "a") as f:
+            self.assert_(f.tell() > 0)
+
     def test_destructor(self):
         record = []
         class MyFileIO(io.FileIO):
diff --git a/Misc/NEWS b/Misc/NEWS
@@ -145,6 +145,11 @@ Core and Builtins
 Library
 -------
 
+- Issue #5008: When a file is opened in append mode with the new IO library,
+  do an explicit seek to the end of file (so that e.g. tell() returns the
+  file size rather than 0). This is consistent with the behaviour of the
+  traditional 2.x file object.
+
 - Issue #5013: Fixed a bug in FileHandler which occurred when the delay
   parameter was set.
 
diff --git a/Modules/_fileio.c b/Modules/_fileio.c
@@ -41,6 +41,9 @@ PyTypeObject PyFileIO_Type;
 
 #define PyFileIO_Check(op) (PyObject_TypeCheck((op), &PyFileIO_Type))
 
+static PyObject *
+portable_lseek(int fd, PyObject *posobj, int whence);
+
 /* Returns 0 on success, errno (which is < 0) on failure. */
 static int
 internal_close(PyFileIOObject *self)
@@ -296,6 +299,16 @@ fileio_init(PyObject *oself, PyObject *args, PyObject *kwds)
 			goto error;
 	}
 
+	if (append) {
+		/* For consistent behaviour, we explicitly seek to the
+		   end of file (otherwise, it might be done only on the
+		   first write()). */
+		PyObject *pos = portable_lseek(self->fd, NULL, 2);
+		if (pos == NULL)
+			goto error;
+		Py_DECREF(pos);
+	}
+
 	goto done;
 
  error:
