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

[David Mertz]

Just curious, Michael, what would you like the Python syntax version to
look like if you *can* do whatever metaclass or stack hackery that's
needed?  I'm a little confused when you mention a decorator and a context
manager in the same sentence since those would seem like different
approaches.  E.g.:

@Contract(pre=my_pre, post=my_post)
def my_fun(...): ...


Versus:

with contract(pre=my_pre, post=my_post):

    def my_fun(...): ...


Versus:

def my_fun(...):

    with contract(pre=my_pre, post=my_post):
        <suite>


I'm sure lots of other variations are possible too (if any can be made
fully to work).


On Mon, Jan 25, 2016 at 5:01 PM, Michael Selik <mike at selik.org> wrote:

> 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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