Get a value before submitting a form in Django

Question

class StudentRegistrationForm(forms.Form):
    end_year = forms.IntegerField(widget = forms.TextInput(
        attrs={
            'id': 'end-year'
        }
    ))
    grade_letter = forms.ChoiceField(
        choices= get_grades(get_year())
    )

To get the list of grade_letters for choice I need to know the end_year value. How can I get the value of end_year field before the submit (when the user wants to choose grade_letter)?