[Python-Dev] subclassing builtin data structures

[Neil Girdhar]

I think it works as Isaac explained if __make_me__ is an instance method
that also accepts the calling class type.

On Fri, Feb 13, 2015 at 8:12 PM, Ethan Furman <ethan at stoneleaf.us> wrote:

> On 02/13/2015 02:31 PM, Serhiy Storchaka wrote:
> > On 13.02.15 05:41, Ethan Furman wrote:
> >> So there are basically two choices:
> >>
> >> 1) always use the type of the most-base class when creating new
> instances
> >>
> >>     pros:
> >>       - easy
> >>       - speedy code
> >>       - no possible tracebacks on new object instantiation
> >>
> >>     cons:
> >>       - a subclass that needs/wants to maintain itself must override all
> >>         methods that create new instances, even if the only change is to
> >>         the type of object returned
> >>
> >> 2) always use the type of self when creating new instances
> >>
> >>     pros:
> >>       - subclasses automatically maintain type
> >>       - much less code in the simple cases [1]
> >>
> >>     cons:
> >>       - if constructor signatures change, must override all methods
> which
> >>         create new objects
> >
> > And switching to (2) would break existing code which uses subclasses
> with constructors with different signature (e.g.
> > defaultdict).
>
> I don't think defaultdict is a good example -- I don't see any methods on
> it that return a new dict, default or
> otherwise. So if this change happened, defaultdict would have to have its
> own __add__ and not rely on dict's __add__.
>
>
> > The third choice is to use different specially designed constructor.
> >
> > class A(int):
> >
> > --> class A(int):
> > ...     def __add__(self, other):
> > ...         return self.__make_me__(int(self) + int(other))
> >
> > ...     def __repr__(self):
> > ...         return 'A(%d)' % self
>
> How would this help in the case of defaultdict?  __make_me__ is a class
> method, but it needs instance info to properly
> create a new dict with the same default factory.
>
> --
> ~Ethan~
>
>
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