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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def open(self, mode):
 """ Open the FileSystem target. This method returns a file-like object which can either be read from or written to depending on the specified mode. :param mode: the mode `r` opens the FileSystemTarget in read-only mode, whereas `w` will open the FileSystemTarget in write mode. Subclasses can implement additional options. :type mode: str """

        if mode == 'w':
            return self.format.pipe_writer(AtomicFtpFile(self._fs, self.path))

        elif mode == 'r':
            temp_dir = os.path.join(tempfile.gettempdir(), 'luigi-contrib-ftp')
            self.__tmp_path = temp_dir + '/' + self.path.lstrip('/') + '-luigi-tmp-%09d' % random.randrange(0, 1e10)
            # download file to local
            self._fs.get(self.path, self.__tmp_path)

            return self.format.pipe_reader(
                FileWrapper(io.BufferedReader(io.FileIO(self.__tmp_path, 'r')))
            )
        else:
            raise Exception("mode must be 'r' or 'w' (got: %s)" % mode)