Django表单ChoiceField依赖于另一个ChoiceField

I have the following form:

我有以下表格：

class MyForm(forms.Form):
    country = forms.ChoiceField(
        widget = forms.Select()
    )

    region = forms.CharField(
        widget = forms.Select()
    )

    def update_region(self):
        self['region'].field.widget.choices = get_choices_for_region(
            self['region'].field.initial)
class My