Re: [TFUI] the simplest test that could possibly fail

TFUIers:

I figured out a good answer.

(This is a legitimate revelation: I did not set the first post up as an 
"exam question" or "guru of the week" deal...)

> This test fails if my built-in server cannot serve the graphic of the 
> mascot
> that goes in the upper left corner of the Wiki:
>
>    def test_getPenBird()
>     response = Response::get( "http://localhost:8001/images/penBird.gif" )
>     assert_no_match /Internal Server Error/, response.body
>    end
>
> This test is the simplest that can fail. Some TDD references say "write a
> test that can only pass if you have the feature you want." That sounds 
> hard!

Writing simplest tests that can fail is acceptable (and occassionally 
passing them with an obvious implementation, then retrofitting more tests 
that enforce that implementation is acceptable too...).

But the point of theory is this: A test case should not state in the 
negative what it does not want to happen. My test that detects an error is 
fine, and we should use it to prevent errors. Don't use many such tests to 
drive development.

To drive development, write simple tests that state positively what you _do_ 
want.

Here's my next test. With more research, it could have been my _first_ test. 
It is the simplest thing that could possibly fail, but which states 
positively what it _does_ want, not negatively what it does _not_ want:

>     response = Response::get( "http://localhost:8001/images/penBird.gif" )
>     assert_match /^GIF89aB/, response.body

The difference is trivial to spot: We upgraded assert_no_match() to 
assert_match(). We assert that we _do_ want a GIF file in the HTTP pipe.

This is the cheapest _positive_ test that could possibly fail. The test does 
not then go into the GIF and detect, using image analysis, that it indeed 
portrays the mythical PenBird.

Because other tests will continue to tighten the constraints around this 
bird, around GIFs, and around the image serving system in general, this test 
is sufficient to drive development.

-- 
  Phlip
  http://www.greencheese.org/ZeekLand  <-- NOT a blog!!! 



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