On Thu, Feb 12, 2015 at 06:14:22PM -0800, Ethan Furman wrote:
> On 02/12/2015 05:46 PM, MRAB wrote:
> > On 2015-02-13 00:55, Guido van Rossum wrote:
> >> Actually, the problem is that the base class (e.g. int) doesn't know how
> >> to construct an instance of the subclass -- there is no reason (in
> >> general) why the signature of a subclass constructor should match the
> >> base class constructor, and it often doesn't.
> >>
> >> So this is pretty much a no-go. It's not unique to Python -- it's a
> >> basic issue with OO.
> >>
> > Really?
>
> What I was asking about, and Guido responded to, was not having to
> specifically override __add__, __mul__, __sub__, and
> all the others; if we do override them then there is no problem.
I think you have misunderstood MRAB's comment. My interpretation is
that MRAB is suggesting that methods in the base classes should use
type(self) rather than hard-coding their own type.
E.g. if int were written in pure Python, it might look something like
this:
class int(object):
def __new__(cls, arg):
...
def __add__(self, other):
return int(self, other)
(figuratively, rather than literally). But if it looked like this:
def __add__(self, other):
return type(self)(self, other)
then sub-classing would "just work" without the sub-class having to
override each and every method.
--
Steve
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