help needed with classes/inheritance

[Bruno Desthuilliers]

Andrew Lee a écrit :
(snip)
> as a rule of thumb .. if you are using "isinstance" in a class to 
> determine what class a parameter is ... you have broken the OO contract. 

Nope.

>  Remember, every class ought to have a well defined internal state and a 
> well defined interface to its state.

I don't see how the part about the internal state relates to the problem 
here.


> If I write --
> 
> class foo (object):
>    def __init__ :
>       pass
> 
>    def some_func (self, val) :
>       if isinstance (val, "bar") :
>     ....
> 
> Then I am either doing something very wrong

If you do so in a desperate attempt to emulate static typing in Python, 
then yes, you're doing something very wrong. Else:

> or very clever 

Not necessarily. Given Python's dynamic typing, there's no builtin 
OneObviousWay(tm) way to dispatch on different functions based on 
argument's type - something often done in statically typed OOPLs using 
method overridding (ie: different methods with same name but different 
signatures). Doing a manual dispatch based on argument's type, while not 
good OO style, is sometimes the simplest working solution, and a good 
enough one for the problem at hand. Having different methods for 
different arg types has the drawback that you don't have one single 
generic function/method that you can use as a callback.

Having a real multidispatch (or rule-based dispatch etc) system either 
builtin or in the stdlib would indeed be much cleaner. Until then, there 
are a couple third-part packages solving this problem, but it can be 
overkill for simple problems (and add unneeded/unwanted dependencies).

my 2 cents.