Unpythonic? Impossible??

[BrJohan]

"Scott David Daniels" <scott.daniels at acm.org> skrev i meddelandet 
news:441d8a74$1 at nntp0.pdx.net...
> BrJohan wrote:
...
>> is it then possible to have this call:
>>     obj = A(data)
>> return an instance of that particular class (e.g. class C3)  in the 
>> hierarchy that - as decided by the __new__ functions - is the 'correct' 
>> one?
>>
>> A.__new__ could select between A, B1 and B2, while B1.__new__ could 
>> choose from B1, C3 and C4.
>>
>> I know how to use a class factory - and could work around using such a 
>> mechanism. However I am interested to know if I could let the classes do 
>> the work by themselves.
>
> Yes, it can be done.  Yes, it is unclear (and hence UnPythonic).
> The class factory _is_ the straightforward way to do this.  The
> following is the workaround (if you have to maintain A(...)):
>
>
>     class A(object):
>         def __new__(class_, *args, **kwargs):
>             if class_ is A:
>                 if want_a_B1(*args, **kwargs):
>                     return B1(*args, **kwargs)
>                 elif want_a_B2(*args, **kwargs):
>                     return B2(*args, **kwargs)
>             return object.__new__(class_) # Use *a,... except for object
>
>     class B1(A):
>         def __new__(class_, *args, **kwargs):
>             if class_ is B1:
>                 if want_a_B1(*args, **kwargs):
>                     return B1(*args, **kwargs)
>                 elif want_a_B2(*args, **kwargs):
>                     return B2(*args, **kwargs)
>             return super(B1, class_).__new__(class_, *args, **kwargs)
>
>
> --Scott David Daniels
> scott.daniels at acm.org

Agreed that the class factory method most often (maybe always) is the best 
one. For certain reasons, and in this particular case, I prefer the 
UnPythonic way. Sometimes it's good to have "more than one way to do it".

It was the "return object.__new__(class_) " that I did not came to think of 
myself, that did it. Thank you for yor helpfulness.

BrJohan