Select dependency (Django)?

Question

Django

How to make dependent items of the SELECT field. not using a model?
There is a form
class Coutry(forms.Form):
coutries=((1,("USA)),(2,("Russia")))
counry_select = forms.ChoiceField(choices = countries)
city = ChoiceField(choices=?) Here you need to make items dependent on country_select
How can I implement this?Thank you very much for your help.