Re: A simple unit test framework

James Kanze <>
5 May 2007 12:51:27 -0700
On May 5, 2:31 am, Ian Collins <> wrote:

Pete Becker wrote:

Ian Collins wrote:

Pete Becker wrote:

If you apply TDD correctly, you only write code to pass tests, so all =


your code is covered.

Suppose you're writing test cases for the function log, which calculates
the logarithm of its argument. Internally, it will use different
techniques for various ranges of argument values. But the specification
for log, of course, doesn't tell you this, so your test cases aren't
likely to hit each of those ranges, and certainly won't make careful
probes near their boundaries. It's only by looking at the code that you
can write these tests.

Pete, I think you are missing the point of TDD.

It's easy for those unfamiliar with the process to focus on the "T" and
ignore the "DD". TDD is a tool for delivering better code, the tests
drive the design, they are not driven by it.

Which, of course, is entirely backwards.

So if I were tasked with
writing he function log, I'd start with a simple test, say log(10) and
then add more tests to cover the full range of inputs.

That is, of course, one solution. It's theoretically possible
for log, but it will take several hundred centuries of CPU time,
which means that it's not very practical. In practice, the way
you verify that a log function is correct is with code review,
with testing of the border cases (which implies the what Pete is
calling white box testing), to back up the review.

These tests
would specify the behavior and drive the internals of the function.

In this case, I think that the behavior is specified before
hand. It is a mathematical function, after all, and we can know
the precise result for every possible input. In practice, of
course, it isn't at all possible to test it, at least not

Remember, if code isn't required to pass a test, it doesn't get written.

So your log function only has to produce correct results for the
limited set of values you use to test it? I hope I never have
to use a library you wrote.

James Kanze (Gabi Software) email:
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