[Python-ideas] DBC (Re: Explicit variable capture list)

[Michael Selik]

On Mon, Jan 25, 2016 at 6:43 PM Andrew Barnert via Python-ideas <
python-ideas at python.org> wrote:

> On Jan 25, 2016, at 15:34, Steven D'Aprano <steve at pearwood.info> wrote:
> >
> > Okay, just to satisfy your curiosity, and not as a concrete proposal at
> > this time, here is a sketch of the sort of thing Eiffel uses for Design
> > By Contract.
>
> I think it's worth explaining why this has to be an actual language
> feature, not something you just do by writing functions named "requires"
> and "ensures". Many of the benefits you cited would work just fine with a
> PyPI-library solution, but there are some problems that are much harder to
> solve:
>
> Some of these things can be shoehorned in with frame hacks and metaclasses
> and so on, but it's not fun. ... There have been a few attempts for Python,
> but most of them seem to have run into similar problems, after a lot of
> messing around with metaclasses and so on.
>

As you were writing this, I was sketching out an implementation using a
callable FunctionWithContract context manager as a decorator. As you say,
the trouble seems to be elegantly capturing the function output and passing
that to an ensure or __exit__ method. The requires side isn't so bad.

Still, I'm somewhat hopeful that someone more skilled than I might be able
to write an elegant ``Contract`` type using current Python syntax.
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