If you're going to do this, and then the dereference on the line immediately below, I think it's cleaner and clearer to say like:
```
    due_date = assessment_module.get('due', None)
    if date == 'start' and not problem_open:
        context["self_start"] = self.format_datetime_string(assessment_module["start"])
    elif due_date:
        context["self_due"] = self.format_datetime_string(due_date)
```
(and it dereferences the object one fewer time)