My first real dynamic-typing bug in 6 years of doing python and Django. Luckily, we caught it in code review.

test.py

# This test passes, even though the existing-username validation is completely broken
class UsernameUpdateCSVTestCase(TestCase):    
    def test_new_username_already_exists_validation_error(self):
        # If the new_username already exists on the same home site, we throw a
        # validation error
        site_admin_username, _ = split_username(self.site_admin_user.username)
        UserFactory(
            username='existing_username',
            tenant=self.tenant,
        )
        # HINT
        UserFactory(
            username='another_existing_username',
            tenant=self.tenant,
        )

        data = tablib.Dataset(headers=self.header_row)
        test_rows = [
            (
                site_admin_username,
                'existing_username',
            ),
        ]
        data.extend(test_rows)

        num_updated, errors = self._run_updater(
            data,
            self.tenant,
        )
        self.assertEqual(num_updated, 0)
        self.assertEqual(len(errors), 1, "Errors: %s" % errors
        
        
# Now read username_csv_updater.py and come back

# Spot the bug?

# Here's a hint:

'existing_username' in 'another_existing_username'

'existing_username' in ['existing_username', 'another_existing_username']

# It's the mythical dynamic typing bug! 

username_csv_updater.py

# Take in a CSV file and validate it for valid bulk username changes from the current to a new username.

def _get_usernames_by_tenant(users):
    usernames_by_tenant = defaultdict(list)

    for user in users:
        username, _ = split_username(user['username'])
        # Do you see the bug?
        usernames_by_tenant[user['profile__tenant']] = username

    return usernames_by_tenant
    
current_users  # A values_list QuerySet of users

usernames_by_tenant = _get_usernames_by_tenant(current_users)

def validate_user_cant_exist_on_tenant(row):
    current_tenants = tenants_for_username[row[current_username_f]]
    if len(current_tenants) != 1:
        # This is already an ambiguous username, No need to pile on
        # with another error.
        return
    current_tenant = current_tenants[0]
    if row[new_username_f] in usernames_by_tenant[current_tenant]:
        raise RecordError(
            message=VALIDATION_ERRORS['new_username_exists'],
        )
        
# ... snip Plugging in the validation