To give a slightly contrived example, let's say I want to test that a function returns two numbers, and that the first one is smaller than the second one:def test_length():    result = my_function()    assert len(result) == 2def test_order()    a, b = my_function()    assert a < bHere, if test_length fails, then test_order will fail too. Is it a best practice to write test_length, or to skip it?EDIT: note that in this situation, both tests are mostly independent from each other, each one can be run in isolation, or they could be run in reverse order, this does not matter. So none of these former questionsHow should I test the functionality of a function that uses other functions in it?Do I need unit test if I already have integration test?How to structure tests where one test is another test's setup?How to manage success dependency between unit testsis a duplicate of the one above.
Is there any value in writing a unit test that is a subset of another test?
unit testing
There can be value, but this is a bit of a smell. Either your tests aren't well isolated (since test_order really tests two things) or you're being too dogmatic in your testing (making two tests testing the same logical thing). In the example, I would merge the two tests together. Yes, it means you have multiple asserts. Too bad. You're still testing a single thing - the result of the function. Sometimes in the real world that means doing two checks.