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

[Chris Angelico]

On Wed, Jan 27, 2016 at 2:06 AM, Paul Moore <p.f.moore at gmail.com> wrote:
> Well, classes can be callable already, so how about
>
> @DBC
> class myfunction:
>     def __call__(self, args):
>         ...
>     @precondition
>     def requires(self):
>         ...
>     @postcondition
>     def ensures(self, result):
>         ...
>
> The DBC class decorator does something like
>
> def DBC(cls):
>     def wrapper(*args, **kw):
>         fn = cls()
>         fn.args = args
>         fn.kw = kw
>         for pre in fn.__preconditions__:
>             pre()
>         result = fn(*args, **kw)
>         for post in fn.__postconditions__:
>             post(result)
>     return wrapper
>
> Pre and post conditions can access the args via self.args and self.kw.
> The method decorators would let you have multiple pre- and
> post-conditions. Or you could use "magic" names and omit the
> decorators.

I'd rather use magic names - something like:

@DBC
class myfunction:
    def body(self, args):
        ...
    def requires(self):
        ...
    def ensures(self, result):
        ...

and then the DBC decorator can create a __call__ method. This still
has one nasty problem though: the requires and ensures functions can't
see function arguments. You could get around this by duplicating the
argument list onto the other two, but who wants to do that?

ChrisA