forms.py

from django import forms

from models import Pais

class MyForm(forms.Form):

    pais = forms.ModelChoiceField(queryset = Pais.objects.all())

    def __init__(self, *args, **kwargs):
        continente = kwargs.pop('continente', None)
        super(MyForm, self).__init__(*args, **kwargs)
        if continente:
            self.fields['pais'].queryset = self.fields['pais'].queryset.filter(continente=continente)

views.py

from forms import MyForm


def my_view(request):
    ...
    form = MyForm(continente='americano')
    ...