ok, you may use this:
f = open( ... )
f.seek(last_pos)
line = f.readline() # no 's' at the end of `readline()`
last_pos = f.tell()
f.close()
just remember, last_pos
is not a line number in your file, it's a byte offset from the beginning of the file -- there's no point in incrementing/decrementing it.