Example of counting number of invocations for a Django QuerySet method

queryset_decorator_example.py

def counter(fn):
    """
    Adds a call counter to the given function.
    Source: http://code.activestate.com/recipes/577534-counting-decorator/
    """
    def _counted(*largs, **kargs):
        _counted.invocations += 1
        fn(*largs, **kargs)

    _counted.invocations = 0
    return _counted

class CleanHistoryTests(TestCase):
    def test_clean_history(self):
      # Original code is available at https://github.com/edx/ecommerce/blob/12f7f9f/ecommerce/core/management/commands/tests/test_clean_history.py
      initial_count = 5
      OrderFactory.create_batch(initial_count)
      cutoff_date = now() + datetime.timedelta(days=1)
      self.assertEqual(Order.history.filter(history_date__lte=cutoff_date).count(), initial_count)

      QuerySet.delete = counter(QuerySet.delete)
      call_command('clean_history', '--cutoff_date={}'.format(cutoff_date.strftime('%Y-%m-%d')), batch_size=1)
      self.assertEqual(QuerySet.delete.invocations, initial_count)
      self.assertEqual(Order.history.filter(history_date__lte=cutoff_date).count(), 0)