[antris]

If the code is that bad, unit tests aren't going to help you. Messy spaghetti code with massive structural issues will break every unit test every time you change anything in the code, in ways that make no sense and provide you with little useful information. You will fix things faster if you just let the tests fail, fix the code and then rewrite the tests. Which makes the unit tests useless.

Also, have you ever seen a project where the code is a mess but the unit tests are perfect? Even if somehow you could write unit tests that would cover for super bad code (which you can't), it is extremely unlikely that your unit tests would be that amazing.

[mcv]

What do you mean with "perfect" unit tests? For this case, I wrote the unit tests to document the current functionality. That's basically what unit tests do: they document functionality and enable you to preserve that functionality of that piece of code. Of course once you realise that the functionality is wrong, you should change it and the unit test. And you can't fix the code if nobody knows what it's meant to do.

There's quite a lot you can refactor without breaking unit tests. If you've got a single 200-line function full of nested loops with cryptic variable names, modern IDEs make it really easy to extract those loops to their own functions. Figure out what they're meant to do, give them a descriptive name, and you already improved the code a bit without breaking any unit tests. If your IDE does this well, you could even do this without unit tests, but you really will need those unit tests once you start reorganising the code making use of the excessive number of parameters those extracted functions invariably end up with.

Of course you can write unit tests for super bad code. If it's a function that returns something, it's trivial to unit test, no matter how badly written that function is. If it calls other code, you have to mock those calls and test that those mocks get called under the same conditions. If they mess with global variables, that's terrible, but even that can be mocked.

If the code uses gotos to code outside the module, somebody needs to get shot, and I guess you need a unit testing framework that can mock those gotos. I've never seen one, because nobodoy uses gotos anymore.

[CraigJPerry]

Of course they can help, you don’t have to use the existing tests, you can add your own as you learn the system and what its supposed to do