title: django, examples of using a callable for choices in forms.ChoiceField
body: <p>In Django documentation, it says that the choices= parameter for forms.ChoiceField can be a callable. <a href="https://docs.djangoproject.com/en/1.10/ref/forms/fields/" rel="nofollow noreferrer">https://docs.djangoproject.com/en/1.10/ref/forms/fields/</a>
  Anyone there that can give me an example of how that would look like?
  Can't find any more about it in the official documentation.</p>