On Sat, 07 Jun 2014 10:50:16 -0400, Antoine Pitrou <anto...@python.org> wrote:
> Le 07/06/2014 09:25, R. David Murray a écrit :
> > On Fri, 06 Jun 2014 19:50:57 +0100, Chris Withers <ch...@simplistix.co.uk>
> > wrote:
> >> I've been trying to add support for explicit comparison of namedtuples
> >> into testfixtures and hit a problem which lead me to read the source and
> >> be sad.
> >>
> >> Rather than the mixin and class assembly in the function I expected to
> >> find, I'm greeted by an exec of a string.
> >>
> >> Curious as to what lead to that implementation approach? What does it
> >> buy that couldn't have been obtained by a mixin providing the
> >> functionality?
> >>
> >> In my case, that's somewhat irrelevant, I'm looking to store a comparer
> >> in a registry that would get used for all namedtuples, but I have
> >> nothing to key that off, there are no shared bases other than object and
> >> tuple.
> >>
> >> I guess I could duck-type it based on the _fields attribute but that
> >> feels implicit and fragile.
> >>
> >> What do you guys suggest?
> >
> > I seem to remember a previous discussion that concluded that duck typing
> > based on _fields was the way to go. (It's a public API, despite the _,
> > due to name-tuple's attribute namespacing issues.)
>
> There could be many third-party classes with a _fields member, so that
> sounds rather fragile.
> There doesn't seem to be any technical reason barring the addition of a
> common base class for namedtuples.
For what it is worth, I found the discussion I was remembering:
http://bugs.python.org/issue7796
And as someone pointed out down thread, the actual check is "inherits
from tuple and has a _fields attribute".
That gets you a duck type, which is generally what you want in Python.
--David
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