Answer by candied_orange for How to correct a mistake in the test, after writing implementation

The absolutely critical thing is that you see the test both pass and fail.

Whether you delete the code to make the test fail then rewrite the code or sneak it off to the clipboard only to paste it back later doesn't matter. TDD never said you had to retype anything. It wants to know the test passes only when it should pass and fails only when it should fail.

Seeing the test both pass and fail is how you test the test. Never trust a test you've never seen do both.

Refactoring Against The Red Bar gives us formal steps for refactoring a working test:

• Run the test
  • Note the green bar
  • Break the code being tested
• Run the test
  • Note the red bar
  • Refactor the test
• Run the test
  • Note the red bar
  • Un-break the code being tested
• Run the test
  • Note the green bar

However, we aren't refactoring a working test. We have to transform a buggy test. One concern is code that was introduced while only this test covered it. Such code should be rolled back and reintroduced once the test is fixed.

If that isn't the case, and code coverage isn't a concern due to other tests covering the code, you can transform the test and introduce it as a green test.

Here, code is also being rolled back but just enough to cause the test to fail. If that's not enough to cover all the code introduced while only covered by the buggy test we need a bigger code roll back and more tests.

Introduce a green test

• Run the test
  • Note the green bar
  • Break the code being tested
• Run the test
  • Note the red bar
  • Un-break the code being tested
• Run the test
  • Note the green bar

Breaking the code can be commenting out code or moving it elsewhere only to paste it back later. This shows us the scope of code the test covers.

For these last two runs you're right back into the normal red green cycle. You're just pasting instead of typing to un-break the code and make the test pass. So be sure you're pasting only enough to make the test pass.

The overall pattern here is to see the color of the test change the way we expect. Note that this creates a situation where you briefly have an un-trusted green test. Be careful about getting interrupted and forgetting where you are in these steps.

My thanks to RubberDuck for the Embracing the Red Bar link.