``` Python 3.6.9 (default, Nov 23 2019, 06:41:34) [GCC 8.3.0] on linux Type "help", "copyright", "credits" or "license" for more information. >>> from crontab import CronTab >>> cron = CronTab() >>> job = cron.new() >>> job.is_valid() False >>> job.set_command("foo") >>> job.is_valid() False >>> job = cron.new(command="foo") >>> job.is_valid() True ``` Now valid set in constructor only. But not when setter used.