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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 run_download_media(filename=None):
 """ Downloads the media dump from the server into your local machine. In order to import the downloaded media dump, run ``fab import_media`` Usage:: fab prod run_download_media fab prod run_download_media:filename=foobar.tar.gz """

    if not filename:
        filename = settings.MEDIA_DUMP_FILENAME
    if env.key_filename:
        ssh = settings.PROJECT_NAME
    else:
        ssh = '{0}@{1}'.format(env.user, env.host_string)
    local('scp {0}:{1}{2} .'.format(
        ssh, settings.FAB_SETTING('SERVER_MEDIA_BACKUP_DIR'), filename))