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Re: Class hierarchy problem

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On Tue, Aug 6, 2013 at 12:36 PM, BrJohan <brjohan@gmail.com> wrote:
> On 06/08/2013 16:02, Chris Angelico wrote:
>
>>> My classhierarchy is like a multilevel tree where each non-leaf node
>>> (class)
>>> is given knowledge about its nearest subclasses and their 'capacities'.
>>>
>>> So, my idea is to let the 'upper' class recursively choose which of its
>>> nearest subclasses is the 'correct' one, until approaching a 'leaf' class
>>> from which the instance should be created. And, given my knowledge that a
>>> solution along the lines of this idea has been designed and was working,
>>> I'm
>>> still hopeful ... (or I'll have to investigate all those old backup-DVDs)
>>
>>
>> [ responding on-list - I hope it was mere oversight that had this come
>> privately to me alone ]
>>
>> This is code smell; this recursive search for the "right" class seems
>> likely to be wrong. Can you have the classes perhaps register
>> themselves in some way? On what basis is a superclass to determine
>> that one of its subclasses should handle this request?
>>
>> ChrisA
>>
>
>
> Consider a botanical classification system (somewhat analogous to my
> 'problem' as it effectively is related to classification of entities):
>
> A Domain should know about its Kingdoms,
> a Kingdom should know about its Phylums,
> ...
> a Genus should know about its Species.
>
> Of course it is possible to implement such a decision tree as a 'factory'.
> However, I would rather prefer to encapsulate those decisions at the class
> level where they 'belong'.
>
> BrJohan
> --

I think it's a "has a" vs a "is a" problem. A Domain has a Kingdom, a Kingdom is
not a Domain, so it shouldn't actually inherit Domain. In this case
you should use
composition instead of inheritance.

When you say that a A Domain should know about it's Kingdons note that you're
talking about a specific Domain and it's specific Kingdons, which
means, a Domain
instance and various Kingdom instances.

JoeS

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Class hierarchy problem BrJohan <brjohan@gmail.com> - 2013-08-06 11:10 +0200
  Re: Class hierarchy problem Chris Angelico <rosuav@gmail.com> - 2013-08-06 10:30 +0100
    Re: Class hierarchy problem Chris Angelico <rosuav@gmail.com> - 2013-08-06 15:02 +0100
      Re: Class hierarchy problem BrJohan <brjohan@gmail.com> - 2013-08-06 17:36 +0200
        Re: Class hierarchy problem Joe Junior <joe.fbs.junior@gmail.com> - 2013-08-06 13:23 -0300
        Re: Class hierarchy problem Jordi Riera <kender.jr@gmail.com> - 2013-08-06 18:13 +0200
        Re: Class hierarchy problem Chris Angelico <rosuav@gmail.com> - 2013-08-06 18:12 +0100
        Re: Class hierarchy problem Terry Reedy <tjreedy@udel.edu> - 2013-08-06 19:13 -0400
        Re: Class hierarchy problem Ben Finney <ben+python@benfinney.id.au> - 2013-08-07 11:58 +1000
  Re: Class hierarchy problem Peter Otten <__peter__@web.de> - 2013-08-06 11:34 +0200
  Re: Class hierarchy problem Steven D'Aprano <steve@pearwood.info> - 2013-08-06 09:36 +0000
  pexpect,  loading an entry field inq1ltd <inq1ltd@inqvista.com> - 2013-08-06 11:05 -0400
  Re: pexpect, loading an entry field "Lakshmipathi.G" <lakshmipathi.g@gmail.com> - 2013-08-06 22:04 +0530
  Re: pexpect, loading an entry field inq1ltd <inq1ltd@inqvista.com> - 2013-08-06 14:03 -0400
  Re: pexpect, loading an entry field "Lakshmipathi.G" <lakshmipathi.g@gmail.com> - 2013-08-07 13:40 +0530

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