Juancarlo A$(D+P(Bez writes:
> assert cond:
> # Write some debugging info to a log.
> ...
> raise ExtType(args)
>
> I like the idea of preparing the arguments for the assertion message in a
> context that gets executed only when the assertion fails.
The more I see this elaborated, the less I like it. "assert" has a
traditional meaning in programming, which is only a slight extension
on the English meaning (that you can turn the check off for
performance).
Although the idea that assert could be the fundamental building block
for design-by-contract (DBC), to my mind this has three problems.
(1) You're coopting a keyword that has a consistent established
meaning in English, in programming languages in general, and in
Python, and changing that meaning.
(2) I doubt that "assert" alone is sufficient for DBC. In practice,
you'll want a more expressive, larger vocabulary. It's not clear
that *any* of that vocabulary is best expressed by "assert" at
this point. If not, why mess with what works?
(3) There's a subtle difference between an assertion and a contract.
An assertion is a statement of fact: it's either right or it's
wrong. But the whole point of contracts in the real world is that
they are constraints on behavior to be enforced. The assumption
is that it's possible to be in violation of the contract. The
point of *asserting* something is that you believe it's absolutely
true, there's no point in going on if it's false. You have to
start over.
A presumption that violation is possible may not be true of the
contracts in DBC, I don't know. But I would prefer not to mix the
difficulties of the semantics of "contract" with the simplicity of
the semantics of "assert".
Steve
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