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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 sftp(self):
 """ Return a `~paramiko.sftp_client.SFTPClient` object. If called more than one time, memoizes the first result; thus, any given `.Connection` instance will only ever have a single SFTP client, and state (such as that managed by `~paramiko.sftp_client.SFTPClient.chdir`) will be preserved. .. versionadded:: 2.0 """

        if self._sftp is None:
            self._sftp = self.client.open_sftp()
        return self._sftp