[Python-ideas] generic Liftable abc-mixin breaks at MRO

[Stephan Sahm]

Thanks for asking, I am glad to explain the idea behind Liftable:

My first implementation of *lift* just checked whether the class has an
initialize method (and not being of type Liftable. However then B must look
like so:

class B(A):
    def __init__(self, b, a):
        super(B, self).__init__(a)
        self.__initialize__(b)

    def __initialize__(self, b):
        self.b = b

this *__init__* method is generic for all Liftable-types in my sense, so I
wanted to factorize it out, where the first guess I had was to use an
abc-mixin.

So one main point is that I want to make further initializations to my
lifted instance which make it a truly valid instance of the new type.

best,
Stephan

On 10 December 2015 at 10:44, M.-A. Lemburg <mal at egenix.com> wrote:

> On 10.12.2015 10:35, Stephan Sahm wrote:
> > thanks for the fast response
> > in fact, I do use this already, however more generically:
> >
> > def lift(self, new_class, **kwargs):
> > ​    ​
> > ...
> > ​    ​
> > self.__class__ = new_class
> >     ...
>
> Hmm, then I don't understand why you need a meta class
> for this. Could you explain the requirements that led up to
> needing a meta class ?
>
> If all you want to do is let the lift() know about
> a certain property of a class to do it's thing, an
> attribute or perhaps an empty base class would do the
> trick, e.g.
>
> class A:
>     __liftable__ = True
>
> or perhaps:
>
> class A:
>     def lift(self, ...):
>         # lift operation goes here
>
> > On 10 December 2015 at 10:33, M.-A. Lemburg <mal at egenix.com> wrote:
> >
> >> On 10.12.2015 09:58, Stephan Sahm wrote:
> >>> Dear all,
> >>>
> >>> I think I found a crucial usecase where the standard MRO does not work
> >> out.
> >>> I would appreciate your help to still solve this usecase, or might MRO
> >> even
> >>> be adapted?
> >>>
> >>> The idea is to build a generic Lift-type which I call this way because
> >> the
> >>> derived classes should be able to easily lift from subclasses. So for
> >>> example if I have an instance *a* from *class A* and a *class B(A)* I
> >> want
> >>> to make *a* an instance of *B* in a straightforward way.
> >>
> >> Why don't you use:
> >>
> >> a.__class__ = B
> >>
> >> ?
> >>
> >>>>> class A: pass
> >> ...
> >>>>> class B(A): pass
> >> ...
> >>>>> a = A()
> >>>>> a
> >> <__main__.A instance at 0x7f0bf5ccdc68>
> >>>>> a.__class__ = B
> >>>>> a
> >> <__main__.B instance at 0x7f0bf5ccdc68>
> >>>>>
> >>
> >> --
> >> Marc-Andre Lemburg
> >> eGenix.com
> >>
> >> Professional Python Services directly from the Experts (#1, Dec 10 2015)
> >>>>> Python Projects, Coaching and Consulting ...  http://www.egenix.com/
> >>>>> Python Database Interfaces ...           http://products.egenix.com/
> >>>>> Plone/Zope Database Interfaces ...           http://zope.egenix.com/
> >> ________________________________________________________________________
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> >>
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> --
> Marc-Andre Lemburg
> eGenix.com
>
> Professional Python Services directly from the Experts (#1, Dec 10 2015)
> >>> Python Projects, Coaching and Consulting ...  http://www.egenix.com/
> >>> Python Database Interfaces ...           http://products.egenix.com/
> >>> Plone/Zope Database Interfaces ...           http://zope.egenix.com/
> ________________________________________________________________________
>
> ::: We implement business ideas - efficiently in both time and costs :::
>
>    eGenix.com Software, Skills and Services GmbH  Pastor-Loeh-Str.48
>     D-40764 Langenfeld, Germany. CEO Dipl.-Math. Marc-Andre Lemburg
>            Registered at Amtsgericht Duesseldorf: HRB 46611
>                http://www.egenix.com/company/contact/
>                       http://www.malemburg.com/
>
>
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