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A problem with classes - derived type

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  A problem with classes - derived type Paulo da Silva <p_s_d_a_s_i_l_v_a_ns@netcabo.pt> - 2016-05-09 05:20 +0100
    Re: A problem with classes - derived type Yann Kaiser <kaiser.yann@gmail.com> - 2016-05-09 06:51 +0000
    Re: A problem with classes - derived type Peter Otten <__peter__@web.de> - 2016-05-09 08:59 +0200
    Re: A problem with classes - derived type Paulo da Silva <p_s_d_a_s_i_l_v_a_ns@netcabo.pt> - 2016-05-09 19:59 +0100

#108410 — A problem with classes - derived type

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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#108416

If you can't change A to use something like "type(self)(...)" to create its
return value, you could use the dark side and swap res's __class__:

    res.__class__ = B

Or:

    res.__class__ = type(self)

Do note that B.__init__ will not be run when you do this, so it is up to
you to execute any additional initialization B might require.

Alternatively if it makes sense for you you can call B's methods on an A
object like so:

    B.b_method(a_object, ...)

On Mon, 9 May 2016 at 06:26 Paulo da Silva <p_s_d_a_s_i_l_v_a_ns@netcabo.pt>
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?
>
> Thanks.
> --
> https://mail.python.org/mailman/listinfo/python-list
>
-- 
Yann Kaiser
kaiser.yann@gmail.com
yann.kaiser@efrei.net
+33 6 51 64 01 89
https://github.com/epsy

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#108417

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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#108434

Às 05:20 de 09-05-2016, Paulo da Silva escreveu:

Thank you Yann and Peter.

I really didn't know anything about those "things".
So far I have worked a lot with classes but they are written by me.
Now I needed to derive pandas.Series (for example) and it has some
methods that return pandas.Series objects. And I needed to return the
derived object, not the pandas.Series one.

The problem is now fixed.

I'll find some time to read a little more about this to improve my pyhon
knowledge.

Thank you very much.

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