Sometimes. If I'm writing something where I know what the API to use it should be and the requirements are understood, yes, I'll start with tests first. This i

Sometimes. If I'm writing something where I know what the API to use it should be and the requirements are understood, yes, I'll start with tests first. This is often the case for things like utility classes: my motivation in writing the class is to scratch an itch of "wouldn't it be nice if I had X" while working on something unrelated. I know what X would look like in use because my ability to imagine and describe it is why I want it. There are times, however, where I'm not quite sure what I want or how I want to do it, and I start by reading existing code (code that I'm either going to modify or integrate with) and if something jumps out at me as a plausible design I may jump in to evaluate how I'd feel about the approach first-hand. I'm short, the more doubts or uncertainty I have about the approach to a problem (in the case of libraries, this means the API) the longer I'll defer writing tests.