[Python-ideas] Pre-conditions and post-conditions

[Jonathan Fine]

This is about a difference of opinion regarding design by contract and
static checking, that Steve D'Aprano has re-raised. Steve wrote that
Ivan Levkivskyi's opinion was that:

> contracts [are] always statically checked

This is what Ivan wrote:

> TBH, I think one of the main points of design by contract is that contracts
> are verified statically.

There's no 'always' or 'all' here. I read it to mean 'sometimes' or
'some'. And also, that static verification is a good thing.

My message of support for Ivan quoted the Eiffel docs.

> https://www.eiffel.org/doc/eiffel/ET-_Design_by_Contract_%28tm%29%2C_Assertions_and_Exceptions

> During development and testing, assertion monitoring should
> be turned on at the highest possible level. Combined with
> static typing and the immediate feedback of compilation techniques
> [...] this permits the development process  [...]
> where errors are exterminated at birth.

I then wrote:

> Based on the Eiffel docs, I find Ivan's opinion reasonable.

Steve wrote:

> Ivan said that static checking was a main point. Those Eiffel docs which
> you (Jonathon) quoted approvingly describe them as "run-time
> assertions".

I'm sorry, but I'm just not seeing that. Either in what I quoted, or
elsewhere in the page. True, my quote is from a section titled
"Run-time assertion monitoring", but the context to me makes it clear
that in Eiffel static typing IS NOT regarded as a run-time assertion.

By the way, Steve wrote

> when I asked off-list I got told to reword and post it here.

I don't think that's quite right. What I said off-list to Steve was

> If you post [...] to python-ideas, I'd be happy to respond there.

It was my intention that it was up to Steve, whether or not to
re-raise the issue. And now I'm doing my part of the bargain, by
responding happily. I'm now happy to let this particular topic rest.

-- 
Jonathan