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Instantiating sub-class from super


On 2019-10-14 10:55 PM, DL Neil via Python-list wrote:
> Is there a technique or pattern for taking a (partially-) populated 
> instance of a class, and re-creating it as an instance of one of its 
> sub-classes?
> 
> 
> In a medically-oriented situation, we have a Person() class, and start 
> collecting information within an instance (person = Person(), etc).
> 
> During the data-collection process the person's sex may become obvious, 
> eg few males have become/been pregnant.
> 
> We could stick with Person() and implement specific methods therein, 
> rather than separate Man and Woman sub-classes, but...
> 
> It seemed better (at the design-level) to have Man( Person ) and Woman( 
> Person ) sub-classes to contain the pertinent attributes, source more 
> detailed and specific questions, and collect such data; by gender.
> 
> In coding-practice, once gender becomes apparent, how should the 
> instance of the Man() or Woman() sub-class be created - and established 
> with the ID and other attributes previously collected as a Person instance?
> 
> This attempt seems hack-y:
> 
>  ????man = Man()
>  ????man.__dict__.update( person.__dict__ )
> 
> 
> Is there a pythonic tool for such, or is the task outlined 
> fundamentally-inappropriate?
> 

Here is a link to an article entitled 'Understanding Hidden Subtypes'. 
It dates back to 2004, but I think it is still relevant. It addresses 
precisely the issues that you raise, but from a data-modelling 
perspective, not a programming one.

http://tdan.com/understanding-hidden-subtypes/5193

I found it invaluable, and applied the concepts in my own 
business/accounting application. Having created the ability to make 
subtypes visible and explicit, I found all kinds of unexpected uses for 
them.

The article seems to be missing a couple of images (Figure 1 and Figure 
2) showing the data relationships. I downloaded the original article 
onto my computer years ago, and my local copy does have the images, so 
if you would like to see them let me know and I will upload my version 
somewhere to make it accessible.

Frank Millman