text: from io import StringIO
  
  from django.apps import apps
  from django.core import management
  from django.db import migrations
  from django.db.models import signals
  from django.test import TransactionTestCase, override_settings
  
  APP_CONFIG = apps.get_app_config('migrate_signals')
  SIGNAL_ARGS = ['app_config', 'verbosity', 'interactive', 'using', 'plan', 'apps']
  MIGRATE_DATABASE = 'default'
  MIGRATE_VERBOSITY = 1
  MIGRATE_INTERACTIVE = False
  
  
  class Receiver:
      def __init__(self, signal):
          self.call_counter = 0
          self.call_args = None
          signal.connect(self, sender=APP_CONFIG)
  
      def __call__(self, signal, sender, **kwargs):
          self.call_counter += 1
          self.call_args = kwargs
  
  
  class OneTimeReceiver:
      """
      Special receiver for handle the fact that test runner calls migrate for
      several databases and several times for some of them.
      """
  
      def __init__(self, signal):
          self.signal = signal
          self.call_counter = 0
          self.call_args = None
          self.signal.connect(self, sender=APP_CONFIG)
  
      def __call__(self, signal, sender, **kwargs):
          # Although test runner calls migrate for several databases,
          # testing for only one of them is quite sufficient.
          if kwargs['using'] == MIGRATE_DATABASE:
              self.call_counter += 1
              self.call_args = kwargs
              # we need to test only one call of migrate
              self.signal.disconnect(self, sender=APP_CONFIG)
  
  
  # We connect receiver here and not in unit test code because we need to
  # connect receiver before test runner creates database.  That is, sequence of
  # actions would be:
  #
  #   1. Test runner imports this module.
  #   2. We connect receiver.
  #   3. Test runner calls migrate for create default database.
  #   4. Test runner execute our unit test code.
  pre_migrate_receiver = OneTimeReceiver(signals.pre_migrate)
  post_migrate_receiver = OneTimeReceiver(signals.post_migrate)
  
  
  class MigrateSignalTests(TransactionTestCase):
  
      available_apps = ['migrate_signals']
  
      def test_call_time(self):
          self.assertEqual(pre_migrate_receiver.call_counter, 1)
          self.assertEqual(post_migrate_receiver.call_counter, 1)
  
      def test_args(self):
          pre_migrate_receiver = Receiver(signals.pre_migrate)
          post_migrate_receiver = Receiver(signals.post_migrate)
          management.call_command(
              'migrate', database=MIGRATE_DATABASE, verbosity=MIGRATE_VERBOSITY,
              interactive=MIGRATE_INTERACTIVE, stdout=StringIO(),
          )
  
          for receiver in [pre_migrate_receiver, post_migrate_receiver]:
              args = receiver.call_args
              self.assertEqual(receiver.call_counter, 1)
              self.assertEqual(set(args), set(SIGNAL_ARGS))
              self.assertEqual(args['app_config'], APP_CONFIG)
              self.assertEqual(args['verbosity'], MIGRATE_VERBOSITY)
              self.assertEqual(args['interactive'], MIGRATE_INTERACTIVE)
              self.assertEqual(args['using'], 'default')
              self.assertEqual(args['plan'], [])
              self.assertIsInstance(args['apps'], migrations.state.StateApps)
  
      @override_settings(MIGRATION_MODULES={'migrate_signals': 'migrate_signals.custom_migrations'})
      def test_migrations_only(self):
          """
          If all apps have migrations, migration signals should be sent.
          """
          pre_migrate_receiver = Receiver(signals.pre_migrate)
          post_migrate_receiver = Receiver(signals.post_migrate)
          stdout = StringIO()
          management.call_command(
              'migrate', database=MIGRATE_DATABASE, verbosity=MIGRATE_VERBOSITY,
              interactive=MIGRATE_INTERACTIVE, stdout=stdout,
          )
          for receiver in [pre_migrate_receiver, post_migrate_receiver]:
              args = receiver.call_args
              self.assertEqual(receiver.call_counter, 1)
              self.assertEqual(set(args), set(SIGNAL_ARGS))
              self.assertEqual(args['app_config'], APP_CONFIG)
              self.assertEqual(args['verbosity'], MIGRATE_VERBOSITY)
              self.assertEqual(args['interactive'], MIGRATE_INTERACTIVE)
              self.assertEqual(args['using'], 'default')
              self.assertIsInstance(args['plan'][0][0], migrations.Migration)
              # The migration isn't applied backward.
              self.assertFalse(args['plan'][0][1])
              self.assertIsInstance(args['apps'], migrations.state.StateApps)
          self.assertEqual(pre_migrate_receiver.call_args['apps'].get_models(), [])
          self.assertEqual(
              [model._meta.label for model in post_migrate_receiver.call_args['apps'].get_models()],
              ['migrate_signals.Signal']
          )
          # Migrating with an empty plan.
          pre_migrate_receiver = Receiver(signals.pre_migrate)
          post_migrate_receiver = Receiver(signals.post_migrate)
          management.call_command(
              'migrate', database=MIGRATE_DATABASE, verbosity=MIGRATE_VERBOSITY,
              interactive=MIGRATE_INTERACTIVE, stdout=stdout,
          )
          self.assertEqual(
              [model._meta.label for model in pre_migrate_receiver.call_args['apps'].get_models()],
              ['migrate_signals.Signal']
          )
          self.assertEqual(
              [model._meta.label for model in post_migrate_receiver.call_args['apps'].get_models()],
              ['migrate_signals.Signal']
          )
  