image.picker : how to initialize edit form with corresponding image?

With the folliwing code I get this wrong result : nose.proxy.AssertionError: 302 != 200 : Couldn't retrieve redirection page '/mes_dossiers/': response code was 302 (expected 200)

what is wrong with my code ?

from django.test import TestCase, RequestFactory, Client
from ..models import *
from ..views import *
from django.core.management import call_command

class Cas(TestCase):

    def setUp(self):
        call_command('loaddata', 'fixture_users.json', verbosity=1)
        call_command('loaddata', 'dclicapp_tests_appareils_isoles.yaml', 
        verbosity=1)

    def test_dossier_duplicate(self) :
        request = self.factory.get('/dossier/3/copier/', follow = True)
        request.user = User.objects.get(id=3)
        pk = 3
        response = dossier_duplicate(request, pk)
        response.client = Client()
        self.assertRedirects(response,'/mes_dossiers/',status_code=302, 
        target_status_code=200)