Newbie inheritance problem

[Alex Martelli]

"zzzzz" <zzizz_ at notmail.com> wrote in message
news:17ogct0jjvuhnb8g8ji9uqh011egdn0l2b at 4ax.com...
> Hi all, I have overwridden the __add__ method for a class, then built
> a class based on the above class; however, the object I'm returning is
> of the base class not the inherited class. If that doesn't make sense
> see below:
>
> class A:
>     def __init__(self,value=0.0)
>         self._value=float(value)
>     def __add__(self,other)
>         return A(self._value+other._value)
>
> class B(A):
>     pass
>
> x=B(3)
> y=B(2)
> z=x+y
>
> The way I've done it z becomes an instance of A not B!!! How do I do

Right, it does -- it is constructed (by A's method __add__) with
a hardwired 'A'.


> it correctly? Do I have to override all the __add__, __sub__, etc...
> methods?

If I understand your desire correctly, changing the definition of
A's method __add__ to:

    def __add__(self,other)
        return self.__class__(self._value+other._value)

might accomplish a part of it.  Of course, this may not be quite
enough -- what if the LH operand is an A but the RH one is a B,
for example -- choosing what class to use to generate the needed
result may be very complex, but I hope this gets you started.


Alex