BrJohan wrote:
> Assume having this class hierarchy: (in principle and without details)
> class A(object):
> class B1(A):
> class B2(A):
> ...
> each of those classes have an initializer __init__(self, data): and an
> overloaded __new__(cls, data):
>
> 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
[EMAIL PROTECTED]
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