How is your design affected?

For the case when the user isn't willing to give access to his/her twitter account.

The error returned is KeyError, since you are trying to access :

user = User.objects.get(username = authorized_tokens['screen_name'])

I see that you have put a try, except blocks. But you are not catching the KeyError exception. You are only making an exception when the user doesn't exist.

you should add a block : except KeyError. And figure out the logic for this particular case. (probably just redirecting the user to the home page)