Jump right in to Django. You can do it, and learn as you go. Djangobook will teach you the foundations of Django to start building apps.
One of thing to keep in mind as far as Python itself goes is using PEP8 to style your code, it'll make it easier down the road when others are looking at your code, whether for help or when working on projects.
@S.Lott is right, foundations are important
However, I think it's possible to learn the foundation while building an actual project/app and having some fun/gaining some experience at the same time.
(note: such as the provided tutorials from django, like the poll app)
Edit: I'd like to point out that in the comments below, I no longer support my answer for this particular situation. If the person is experienced with programming and language fundamentals, to me they have the knowledge necessary and the language is merely the tool they use. If that is the case I don't see anything wrong with picking up some of the specifics of a language while using a framework.