[Python-Dev] subclassing builtin data structures

[Neil Girdhar]

Interesting:
http://stackoverflow.com/questions/5490824/should-constructors-comply-with-the-liskov-substitution-principle

On Fri, Feb 13, 2015 at 3:37 PM, Isaac Schwabacher <ischwabacher at wisc.edu>
wrote:

> On 15-02-13, Guido van Rossum  wrote:
> > Are you willing to wait 10 days for an answer? I'm out of round
> tuits for a while.
>
> IIUC, the argument is that the Liskov Substitution Principle is a
> statement about how objects of a subtype behave relative to objects of a
> supertype, and it doesn't apply to constructors because they aren't
> behaviors of existing objects. So other overriding methods *should* be able
> to handle the same inputs that the respective overridden methods do, but
> constructors don't need to. Even though __init__ is written as an instance
> method, it seems like it's "morally" a part of the class method __new__
> that's only split off for convenience.
>
> If this message is unclear, it's because I don't really understand this
> myself and I'm trying to articulate my best understanding of what's been
> said on this thread and those it links to.
>
> ijs
>
> > On Fri, Feb 13, 2015 at 10:22 AM, Alexander Belopolsky <
> alexander.belopolsky at gmail.com(javascript:main.compose()> wrote:
> >
> > >
> > > On Fri, Feb 13, 2015 at 1:19 PM, Alexander Belopolsky <
> alexander.belopolsky at gmail.com(javascript:main.compose()> wrote:
> > > >>
> > > >> FWIW you're wrong when you claim that "a constructor is no
> different from any other method". Someone else should probably explain this
> (it's an old argument that's been thoroughly settled).
> > > >
> > > >
> > > > Well, the best answer I've got in the past [1] was "ask on
> python-dev since Guido called the operator overriding expectation." :-)
> > >
> > >
> > > And let me repost this bit of history [1]:
> > >
> > > Here is the annotated pre-r82065 code:
> > >
> > > 39876 gvanrossum def __add__(self, other):
> > > 39876 gvanrossum if isinstance(other, timedelta):
> > > 39928 gvanrossum return self.__class__(self.__days + other.__days,
> > > 39876 gvanrossum self.__seconds + other.__seconds,
> > > 39876 gvanrossum self.__microseconds + other.__microseconds)
> > > 40207 tim_one return NotImplemented
> > > 39876 gvanrossum
> > >
> > >
> > >
> > > [1] http://bugs.python.org/issue2267#msg125979
> > >
> > >
> >
> >
> >
> >
> > --
> > --Guido van Rossum (python.org/~guido(http://python.org/~guido))
> _______________________________________________
> Python-Dev mailing list
> Python-Dev at python.org
> https://mail.python.org/mailman/listinfo/python-dev
> Unsubscribe:
> https://mail.python.org/mailman/options/python-dev/mistersheik%40gmail.com
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-dev/attachments/20150213/6d609a87/attachment.html>