old_contents: from django.test import TestCase
  
  # Create your tests here.
  
new_contents: # -*- coding: utf-8 -*-
  
  from __future__ import unicode_literals
  
  from django.test import TestCase
  from djmercadopago.services import MercadoPagoService, BackUrlsBuilder,\
      CheckoutPreferenceResult
  
  
  class BackUrlsBuilderMock(BackUrlsBuilder):
      def __init__(self):
          self._success_url = 'https://www.google.com/'
          self._failure_url = 'https://www.google.com/'
          self._pending_url = 'https://www.google.com/'
  
  
  class TestMercadoPagoService(TestCase):
  
      def test(self):
          service = MercadoPagoService()
          checkout_result = service.do_checkout('', BackUrlsBuilderMock())
          assert isinstance(checkout_result, CheckoutPreferenceResult)
          checkout_result.url
  
subject: Add unittest