1. How to assert that a function used a fixed number of queries?

We can count number of queries for testing by using assertNumQueries() method.

def test_number_of_queries(self):
    User.objects.create(username='testuser1', first_name='Test', last_name='user1')
    # Above ORM create will run only one query.
    self.assertNumQueries(1)
    User.objects.filter(username='testuser').update(username='test1user')
    # One more query added.
    self.assertNumQueries(2)