[Tutor] list of references to object properties

[Jose Amoreira]

Thanks, Peter.
I was trying to avoid the regenerate step for updating. But maybe I can
restructure my code and objects to make this simpler. Before that I'll try
this view approach.
Thanks again
Ze


On Fri, Jan 18, 2013 at 11:37 AM, Peter Otten <__peter__ at web.de> wrote:

> Jose Amoreira wrote:
>
> > Hello
> > Suppose I have a list l_obj of similar objects. Is there any way I can
> > generate a list l_prp of references to a given property of those objects
> > in such a way that, if change the value of one element in l_prp, the
> > corresponding object in l_obj gets its property updated, and vice-versa?
> > Let give an example of what I have in mind.
> >
> > In [1]: class MyCls(object):
> >
> >    ...: def __init__(self,a):
> >
> >    ...: self.prp = a
> >
> >    ...:
> >
> > In [2]: l_obj = [MyCls(float(i)) for i in range(3)]
> >
> > In [3]: l_prp = [item.prp for item in l_obj]
> >
> > In [4]: for ob in l_obj:
> >
> >    ...: print ob.prp,
> >
> >    ...:
> >
> > 0.0 1.0 2.0
> >
> > In [5]: l_prp
> >
> > Out[5]: [0.0, 1.0, 2.0]
> >
> > In [6]: l_prp[1]=5.
> >
> > In [7]: l_obj[1].prp
> >
> > Out[7]: 1.0
> >
> > As expected, changes in l_prp do not change the properties of the
> elements
> > in l_obj, neither do changes in l_obj's element's properties change the
> > values in l_prp.
> >
> > Is there a simple way to implement such connections?
>
> No. You'd need something like the observer pattern (listeners in Java),
> where the class owning the property (MyCls) has to cooperate. The
> administrative overhead is relatively high.
>
> The pythonic way is to regenerate the l_prp list every time you need an up-
> to-date overview of the current values.
>
> An intermediate approach is to turn l_prp into a view on the l_obj list:
>
> from collections import Sequence
>
> class A(object):
>     def __init__(self, attrib):
>         self.attrib = attrib
>
> class AttribView(Sequence):
>     def __init__(self, items):
>         self._items = items
>     def __getitem__(self, index):
>         return self._items[index].attrib
>     def __len__(self):
>         return len(self._items)
>     def __repr__(self):
>         return repr(list(self))
>
> items = [A(c) for c in "abcde"]
> attribs = AttribView(items)
> print attribs
> items[1].attrib = 42
> print attribs
>
>
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