[Python-Dev] Iterable String Redux (aka String ABC)

[Steven D'Aprano]

On Sat, 31 May 2008 12:48:41 am Armin Ronacher wrote:
> Greg Ewing <greg.ewing <at> canterbury.ac.nz> writes:
> > Well, I'm skeptical about the whole ABC thing in the
> > first place -- it all seems very unpythonic to me.
>
> I think it's very pythonic and the very best solution to interfaces
> *and* duck typing.  Not only does it extend duck-typing in a very,
> very cool way 

I'm with Greg on this one: despite the assertions made in the PEP, I 
don't see how ABC can fail to be anything but anti-duck-typing. 

How does it extend duck-typing? Can you give an example?


> but also does it provide a very cool way to get custom 
> sets or lists going with few extra work.  Subclassing builtins was
> always very painful in the past 

"Always" very painful?

class ListWithClear(list):
    def clear(self):
        self[:] = self.__class__()

Not so very painful to me. Maybe I just have more pain-tolerance than 
some people.


> and many used the User* objects which 
> however often broke because some code did something like
> isinstance(x, (tuple, list)).  Of course one could argue that
> instance checking is the root of all evil 

Perhaps not the root of *all* evil but it is certainly the root of much 
evil, and the treatment of delegation-based classes like UserString as 
second-class objects is a good example of why isinstance checking 
should be avoided as much as possible.



-- 
Steven