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From: Peter Otten <__peter__@web.de>
Newsgroups: comp.lang.python
Subject: Re: A problem with classes - derived type
Date: Mon, 09 May 2016 08:59:20 +0200
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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.