status of Programming by Contract (PEP 316)?

Russ uymqlp502 at
Fri Aug 31 03:06:36 CEST 2007

Ryan Ginstrom wrote:

> I tried using DBC for a time in C++ (using a library with a clever
> assembly-language hack). I personally found it neater having such code in
> unit tests, but obviously, it's a matter of preference.

I agree that it ultimately boils down to preference, but as I tried to
explain earlier, I don't
think that unit tests are equivalent to DBC (or, as I referred to it
earlier, "self-testing code").

Unit tests are not automatically executed when you run your
application. You need to write
the drivers and generate sample inputs to all the functions in your
application, which
can be very time-consuming. And if you think you can anticipate all
the weird internal data
that might come up in the execution of your application, then either
your application is very
simple or you are fooling yourself.

With self-testing code, on the other hand, all you need to do is to
get sample inputs to
your application (not internal inputs to functions) and run the entire
application. That is
much simpler and more comprehensive than unit testing.

Again, I recognize that it doesn't necessarily replace unit testing
completely. For one thing,
unit testing can test for specific outputs for specific inputs,
whereas you wouldn't want to
clutter your actual code with such specific cases. For example, a unit
test for a sorting
function could provide a specific input and test for a specific
output, but you probably
wouldn't want to clutter your code with such a case. The self-tests in
your code would be
more for general tests. PEP 316 provides an excellent of comprehensive
tests for a sorting
function. If you pass those tests, you can be sure your function
worked correctly.

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