[kazinator]

Even a string length api like len(s) is not exhaustively testable; you can't prove it correct with blackbox tests.

TDD has refactoring steps which only have to preserve passing tests; refactoring can easly be the vector that introduces dead code as well as changes behavior for untested input combinations.

I suspect that a lot of code developed TDD is actually deployed on input combinations that are not covered in the TDD test suite.

A string length function developed by TDD will still work on len("supercalifragilisticexpealidocious") even though that exact string never appeared as a test case, and the consumers of that function will use it on all sorts of untested inputs all the time.

[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.