Re: [myhdl-list] beginning with unittests

[Jan D. <ja...@ja...>]

Martín Gaitán wrote:
> On Thu, Nov 4, 2010 at 6:26 PM, Jan Decaluwe <ja...@ja... 
> <mailto:ja...@ja...>> wrote:
> 
>     Perhaps because 0b0010 != 3 ?
> 
>     If this is the reason, why use all these different representations
>     for the same thing?
> 
>     Python 2.6 has bit representation for integer values. Any representation
>     should work with intbv's and in conversion.
> 
>     Jan
> 
> 
> right, and the best example of usefulness of unit test ;-)

Yes, interesting isn't it :-) Your original test bench that "worked"
also revealed the bug, but you overlooked it. It's the formal test
that revealed it.

Let me add a few comments here.

Your example demonstrates one feature of unit tests: use a formal
check instead of "inspection" to verify correctness.

This works best if there is some high-level way to specify the
expected result. In your case I would probably set up a dict
with operators from module operator, or your own function
if required:

spec = {0b0001 : operator.add,
        0b0110 : operator.sub,

       }

and then randomly generate operands and operator control sequences.

The alu operation would come down do: spec[op](*operands).

More importantly, unit tests work best if you write the test
before the implementation. A unit that has never failed
is not very useful. The idea is to start with something
that fails, and then starts working as the implemenation
progresses.

When your are done, you know that your design works according
to the unit test spec, but also that it can fail when you
change the implementation and introduce a bug!

Personally, I find unittest way too heavy. I simply
write regular python tests that use assert statements to
verify results. You can run this with a regular python
interpreter, but it is more useful to use a framework
like py.test (there are others) for this: this looks
up all your tests automatically, runs them, and uses
some magic to report issues in a clearer way.

Jan

-- 
Jan Decaluwe - Resources bvba - http://www.jandecaluwe.com
    Python as a HDL: http://www.myhdl.org
    VHDL development, the modern way: http://www.sigasi.com
    Analog design automation: http://www.mephisto-da.com
    World-class digital design: http://www.easics.com