Charles Yeomans wrote:
On Aug 11, 2009, at 3:30 PM, Ethan Furman wrote:
Ethan Furman wrote:
Greetings!
I have seen posts about the assert statement and PbC (or maybe it
was DbC), and I just took a very brief look at pycontract
(http://www.wayforward.net/pycontract/ ) and now I have at least one
question: Is this basically another way of thinking about unit
testing, or is the idea of PbC more along the lines of *always*
checking the input/output of functions to ensure they are correct?
(*Contstant vigilance!* as Prof Moody would say ;)
I know asserts can be turned off, so they obviously won't work for
the latter case, and having seen the sample of pycontract it seems
it only does its thing during debugging.
So is Design (Programming) by Contract a fancy way of saying
"Document your inputs/outputs!" or is there more to it?
~Ethan~
Hmmm...
Well, from the (apparently) complete lack of interest, I shall take
away the (better?) documentation ideas and unit testing ideas, and
not worry about the rest. :)
Design by contract is complementary to unit testing (I notice that the
author of PEP 316 appears confused about this). DbC is, roughly
speaking, about explicit allocation of responsibility. Consider this
contrived example.
def foo(s):
require(s is not None)
//code
ensure(hasattr(returnValue, '__iter__'))
The require condition tells you that it's the caller's responsibility
to pass a non-nil argument to foo. The ensure condition is a guarantee
that foo will return something suitable for iteration, if the
precondition in the require condition is satisfied. These conditions
can be enforced at runtime, but may not be, for reasons of performance.
DbC is in fact about not *always* checking the input/output of
functions; on the contrary, Bertrand Meyer, the inventor of DbC, claims
that DbC allows one to eliminate such redundancy, and the resulting
overhead.
Charles Yeomans
Many thanks!
So if I understand -- Python's EAFP fits well with DbC, as DbC seems
well suited to say, "This is your responsibility, and this is mine,"
stated in programming terms (who needs comments? ;) Combined with unit
testing (which should be easier to design correctly given the DbC code),
healthy code seems more attainable.
Interesting. Thank you for the information.
~Ethan~
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