On Sunday, March 29, 2015 at 9:35:25 AM UTC-7, Martin R wrote:
>
> What I'd like to achieve is that A(a) yields a, if a is an instance of A.
>
I think that means that A needs to be a factory function rather than a
normal class:
class Aclass(...):...
def A(a):
if isinstance(a,Aclass):
return a
else:
return Aclass(a)
By the time you're hitting `__init__` the object that's being created has
already been determined: it's the `self` passed to it. And indeed,
`__init__` doesn't return a meaningful value. It can only modify `self`.
Read up on metaclasses in python to see how you can integrate such
factory-function behaviour with the class itself, but those are notoriously
complicated. Using metaclasses is almost certainly the wrong solution
unless you can prove that any alternative approach is insufficient.
My initial reaction is that having such tricky behaviour is the wrong
interface choice, so I'd recommend that you revisit your choice with that
assumption and then see if you can convince yourself that you really do
need `A(a) is a` to be True.
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