When you work with legacy code, it’s easy to fall into the trap of thinking that because you can’t atomically test it that you should refactor it and add tests. This is a mistake. You should write integration tests, then refactor it, then write atomic tests. Even when the refactoring seems simple, it’s still possible to stuff it up.

Most code can have integration tests written for it, even when it wasn’t designed for testing. In some cases the tests may not be able to cover everything but should give you at least a foundation to work from. Just make sure that as you write the tests you note down the things you couldn’t write an automated test to cover and make sure you do add a test for them after your refactoring.

See also: Refactoring Without Tests