[Python-ideas] Making it easy to prepare for PEP479

[Brett Cannon]

On Tue, May 19, 2015 at 7:30 AM Nick Coghlan <ncoghlan at gmail.com> wrote:

> On 19 May 2015 at 13:15, Steven D'Aprano <steve at pearwood.info> wrote:
> > So, I believe that the whole __future__ directive is a red herring, and
> > doesn't actually help Ram do what he wants, which is to write tests
> > which will fail if his generators call raise StopIteration regardless of
> > what version of Python he runs the test under.
>
> The essential impossibility of writing such tests is one of the
> underlying reasons *why* PEP 479 was accepted - you can't sensibly
> test for inadvertently escaping StopIteration values.
>
> However, I interpreted Ram's request slightly differently: if I'm
> understanding the request correctly, he'd like a way to write
> single-source modules such that *on Python 3.5+* they effectively run
> with "from __future__ import generator_stop", while on older Python
> versions, they run unmodified. That way, running the test suite under
> Python 3.5 will show that at least the regression tests aren't relying
> on "escaping StopIteration" in order to pass.
>
> The intended answer to Ram's request is "configure the warnings module
> to turn the otherwise silent deprecation warning into an error". From
> https://www.python.org/dev/peps/pep-0479/#transition-plan:
>
> * Python 3.5: Enable new semantics under __future__ import; silent
> deprecation warning if StopIteration bubbles out of a generator not
> under __future__ import.
>
> However, we missed the second half of that in the initial PEP
> implementation, so it doesn't currently emit the deprecation warning
> at all, which means there's no way to turn it into an error instead:
> http://bugs.python.org/issue24237
>
> Once that issue has been fixed, then "-Wall" will cause any tests
> relying on the deprecated behaviour to fail, *without* needing to
> modify the code under test to use the future import.
>

Another option is to use a custom import loader which sets the __future__
flag passed to compile() depending under what version of Python you were
running your code under; overriding
https://docs.python.org/3.5/library/importlib.html#importlib.abc.InspectLoader.source_to_code
is all that would be needed to make that happen.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-ideas/attachments/20150520/33b951b9/attachment.html>