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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(path, mode='r', host=None, user=None, port=DEFAULT_PORT):
 """Open a file on a remote machine over SSH. Expects authentication to be already set up via existing keys on the local machine. Parameters path: str The path to the file to open on the remote machine. mode: str, optional The mode to use for opening the file. host: str, optional The hostname of the remote machine. May not be None. user: str, optional The username to use to login to the remote machine. If None, defaults to the name of the current user. port: int, optional The port to connect to. Returns ------- A file-like object. Important --------- If you specify a previously unseen host, then its host key will be added to the local ~/.ssh/known_hosts *automatically*. """

    if not host:
        raise ValueError('you must specify the host to connect to')
    if not user:
        user = getpass.getuser()
    conn = _connect(host, user, port)
    sftp_client = conn.get_transport().open_sftp_client()
    return sftp_client.open(path, mode)