A problem with classes - derived type

[Paulo da Silva]

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?

Thanks.
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