Paulo da Silva wrote:
> Hi!
>
> Suppose I have a class A whose implementation I don't know about.
> That class A has a method f that returns a A object.
>
> class A:
> ...
> def f(self, <...>):
> ...
>
> Now I want to write B derived from A with method f1. I want f1 to return
> a B object:
>
> class B(A):
> ...
> def f1(self, <...>):
> ...
> res=f(<...>)
>
> How do I return res as a B object?
In the general case you need enough knowledge about A to create a B instance
from an A instance:
class B(A):
@classmethod
def from_A(cls, a):
b = cls(...) # or B(...)
return b
def f1(self, ...):
return self.from_A(self.f(...))
If the internal state doesn't change between A and B, and A is written in
Python changing the class of the A instance to B
class B(A):
def f1(...):
a = self.f(...)
a.__class__ = B
return a
may also work.
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