Sure, but I usually write tests when I have a good confidence in the API of my class, even if that API isn't implemented. I tend to have more biases and assumpt

Sure, but I usually write tests when I have a good confidence in the API of my class, even if that API isn't implemented. I tend to have more biases and assumptions if I write the tests later, so I find this works better for me. So, unit testing is often a subset of TDD.