[dominotw]

> Even a string length api like len(s) is not exhaustively testable;

But this has nothing do with what GP said. That isn't what gp is using TDD for. GP is using TDD for,

1. starting with api of a function vs code.

2. avoiding unused lines of code

3. reducing dependencies, its hard to tdd something that has a lots of deps so it drives your refactor it something more testable and thus more readable/maintainable also.

4. simplest set of input/output.