Is inheritance broken?

Ype Kingma ykingma at accessforall.nl
Wed Mar 28 23:58:59 CEST 2001


Jonathan,

<snip>

> 
> If I really wanted this to work, guess one thing I could do is to cheat by
> creating a 'flat' list of classes and then manually creating my class tree.
> Something like this:
> 
> <code>
> 
> # 'flat' classes
> class parallelogram_flat:
>         def angle(self): return "variable"
>         def side(self): return "variable"
> 
> class rhombus_flat:
>         def side(self): return "same"
> 
> class rectangle_flat:
>         def angle(self): return "90 degree"
> 
> class square_flat:
>         pass
> 
> # 'real' classes
> class parallelogram(parallelogram_flat):
>         pass
> class rhombus(rhombus_flat, parallelogram_flat):
>         pass
> class rectangle(rectangle_flat, parallelogram_flat):
>         pass
> class square(square_flat, rhombus_flat, rectangle_flat, parallelogram_flat):
>         pass
> 
> # usage
> 
> s = square()
> print "squares have", s.angle(), "angles"
> print "squares have", s.side(), "sides"
> 
> </code>
> 
> Ugly and error prone, no?
>

This code follows a rule: never instantiate a class that
is inherited from. (In other languages your 'flat' classes
might be called 'abstract' in order to indicate that they cannot
be instantiated.)
IIRC this is a design pattern in class hierarchies.

In this particular case you avoid having a real square that
inherits from a real rectangle. In geometry a square
is a special case of rectangle. However this special case
from geometry is not implemented well by using OO inheritance.

The 'flat' and 'real' approach in the above class hierarchy
feels a lot better. It allows more choice of where to put
eventual attributes (eg. side length).

So, no it's not ugly at all, quite the contrary.
(Being non native in English, I don't know what a rhombus is,
so I might change my mind...)

It's also not really error prone: you only need to check
right to left, there is no need to go deeper into
the inheritance hierarchy.


Regards,
Ype Kingma


-- 
email at xs4all.nl



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