what the need of simple tests in python django

I was reading about tests and came accross an example as below.

import pytestimport pytest


@pytest.fixture
def customer():
    customer = Customer(first_name="Cosmo", last_name="Kramer")
    return customer


def test_customer_sale(customer):
    assert customer.first_name == "Cosmo"
    assert customer.last_name == "Kramer"
    assert isinstance(customer, Customer)

I am wondering why to test the below three if we we already know how we are creating the object

customer.first_name == "Cosmo"
customer.last_name == "Kramer"
isinstance(customer, Customer)

As a basic implementation test, it verifies that the code does what it's supposed to do. If you made a careless change in the Customer implementation, maybe it would lose the first name in some circumstances, for example. This helps avoid more complex failures down the line from tests which assume that the basic functionality is working as expected.

You wouldn’t. If those were meant to be real unit tests they would be bad unit tests. But most likely they were just meant to demonstrate how to use pytest and not how to write good unit tests.

Why is it a bad unit test? Because it’s a test for one method that is effectively doing nothing more that testing 1) the constructor (another method), and 2) that the assignment feature of Python works. If you want to test the constructor (not a bad idea if it has non-trivial logic), you should write tests specifically for it. But unit tests should only test the method they’re written for, not methods called by the method under test (those should be mocked out). And they should test non-trivial functionality. Functions that are short and simple enough to be easy to validate by just looking at them (e.g. functions that just assign a value) don’t need unit tests. Trivial unit tests waste development time and give a false sense of security.