def METHOD_NAME(self):
    if os.path.exists(TESTFN):
        mode = 'r+b'
    else:
        mode = 'w+b'
    with self.open(TESTFN, mode) as f:
        current_size = os.fstat(f.fileno())[stat.ST_SIZE]
        if current_size == size+1:
            return
        if current_size == 0:
            f.write(b'z')
        f.seek(0)
        f.seek(size)
        f.write(b'a')
        f.flush()
        self.assertEqual(os.fstat(f.fileno())[stat.ST_SIZE], size+1)