Can you add a test for where only part of the passed files are missing? You can use https://github.com/pytest-dev/pytest-html/blob/master/testing/test_pytest_html.py#L321 as an example for creating adhoc files in tests.

Also, please use @pytest.mark.parametrize to combine your tests with the test above. It'll be a lot cleaner than having 2-3 separate tests for the same feature ( reporting of missing css files )

Here is an example of how we parametrize in this file

@gnikonorov See 8ccc104
Have I used @pytest.mark.parametrize correctly ?

Almost @prakhargurunani! Please see my last review

You can merge this test with the test above, just create the valid css file in the body of the test.

This is correct, but you could make it even more clean by parametrizing with a list of files instead of passing file1 and file2. For example:

$ cat test_parametrize.py
import pytest

@pytest.mark.parametrize("arg", ["arg1", "arg2", ["arg3.a", "arg3.b"]])
def test_list(arg):
     assert arg is not None
$

In the example above the test is run 3 times. First with "arg1", then "arg2", and finally with the list ["arg3.a", "arg3.b"]

This way your test can be expanded on in the future without changing its signature every time, and it becomes easier to reason about

Is this debug code? files should never be None based on your parametrization

Sorry I missed this, but this doesn't do what you expect. You aren't actually checking for files[0]. This just checks whether or not files[0] is truthy and executes files[1] in result.stderr.str() if it's not. For example:

>>> l = [1,2,3]
>>> assert 0, 2 in l
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AssertionError: True
>>> assert 44, 2 in l