'Releases a lock using POSIX locks (``fcntl.lockf``)
Releases the lock regardless of mode. Note that read locks may
be masquerading as write locks, but this removes either.'

def _unlock(self):

    fcntl.lockf(self._file, fcntl.LOCK_UN, self._length, self._start, os.SEEK_SET)
    self._file.close()
    self._file = None
