The term TDD is unfortunate because API design is fundamentally an iterative process,
testability being just one (crucial) aspect.
For public APIs you simply don't have the luxury of changing the interface after release,
so you need to get it right, you need to prove the module's testability by writing and running real tests before release.
More detail on this difficult topic can be found in the "API Design Checklist" section at On Interfaces and APIs.
One bullet point from that list clarifies the iterative nature of TDD:
- "Play test" your API from different perspectives: newbie user, expert user, maintenance programmer, support analyst, tester. In the early stages, imagine the perfect interface without worrying about implementation constraints. Design iteratively.