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.

1 files changed, 13 insertions, 0 deletions

diff --git a/Modules/_fileio.c b/Modules/_fileio.c
index 2a86e07e43..2fc6de887a 100644
--- 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: