Second this, if you need a database and users, Django is better than the headache of thinking about this yourself with all security pitfalls. I've built several production APIs around Django (like, not CMS at all) and I find it a good compromise between an SDK and rolling your own, this is partly due to Python as well of course.
And I've done smaller things with Flask where it would make no sense to go with Django as well but those were things not really built around a db or CRUD API etc.
1. You're making something with 100 lines, which you're absolutely sure will stay 100 lines. Go with Flask.
2. Otherwise, Django.
Django is great at getting out of your way when you don't need its functionality, so you can just ignore most of it if you don't need it.