from django.test import TestCase

class FooTests(TestCase):
    def test_bar(self):
        self.client.get('/', HTTP_HOST='example.com')

from django.test import TestCase
from django.test.client import Client


class MyClient(Client):
    HTTP_HOST = 'example.com'

    def get(self, *args, **kwargs):
        kwargs.setdefault('HTTP_HOST', self.HTTP_HOST)
        return super(MyClient, self).get(*args, **kwargs)


class MyTestCase(TestCase):
    client_class = MyClient


class FooTests(MyTestCase):
    def test_foo(self):
        self.client.get('/')