object references

[DrConti]

Dear Python developer community,
I'm quite new to Python, so perhaps my question is well known and the
answer too.

I need a variable alias ( what in other languages you would call  "a
pointer" (c) or "a reference" (perl))
I read some older mail articles and I found that the offcial position
about that was that variable referencing wasn't implemented because
it's considered bad style.
There was also a suggestion to write a real problem where referencing
is really needed.
I have one...:

I'm trying to  generate dynamically class methods which works on
predefined sets of object attributes.
one of these is the set of attributes identfying uniquely the object
(primary key).
A naïve attempt to do the job:

class ObjectClass:
""" Test primary Key assignment """

if __name__ == "__main__":

ObjectClassInstantiated=ObjectClass()
ObjectClassInstantiated.AnAttribute='First PK Elem'
ObjectClassInstantiated.AnotherOne='Second PK Elem'
ObjectClassInstantiated.Identifier=[]

ObjectClassInstantiated.Identifier.append(ObjectClassInstantiated.AnAttribute)

ObjectClassInstantiated.Identifier.append(ObjectClassInstantiated.AnotherOne)
print ObjectClassInstantiated.Identifier
ObjectClassInstantiated.AnAttribute='First PK Elem Changed'
print ObjectClassInstantiated.Identifier

leads a wrong result
>./test.py
['First PK Elem', 'Second PK Elem']
['First PK Elem', 'Second PK Elem']
--> wrong! It should write  ['First PK Elem Changed', 'Second PK Elem']


i.e. the assgnment

ObjectClassInstantiated.Identifier.append(ObjectClassInstantiated.AnAttribute)

assigns only the attribute value, not the reference.

so my question is:
is it still true that there is no possibilty to get directly object
references?
Is there a solution for the problem above ?
Thank you for any feedback and sorry for the long mail
...and the reference to perl :-)

Regs, 
Davide

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Re: object references

[DrConti]

Felipe Almeida Lessa schrieb:

> Em Sáb, 2006-03-25 às 21:33 -0800, DrConti escreveu:
> [snip]
> > There was also a suggestion to write a real problem where referencing
> > is really needed.
> > I have one...:
> [snap]
>
> There are loads of discussions about the code you wrote... but... isn't
> bad practice to put the same data in two places? Or am I missing
> something?
>
> Cheers,
>
> --
> Felipe.
Hi Felipe, surely it's bad practice to put the same data in two places.
However I don't want to put the data in the identifier list, but just
the reference to the attributes.

My general problem is to find a way to define subsets of instance
attributes (for example the identifier),  so that at later time I can
just iterate over the subset.
In the meantime I found a 90% solution to the problem through lambdas..
See now the code below: maybe you'll understand my point better.
Thanks and Regs,
Davide

class ObjectClass:
""" Test primary Key assignment
"""
def alias(self,key): return lambda: self.__dict__[key]
def __init__(self):
self.Identifier=[]

def getPK(self):
return [ GetPKValue() for GetPKValue in self.Identifier ]

if __name__ == "__main__":
ObjectClassInstantiated=ObjectClass()

ObjectClassInstantiated.AnAttribute='First PK Elem'
ObjectClassInstantiated.AnotherOne='Second PK Elem'

ObjectClassInstantiated.Identifier.append(ObjectClassInstantiated.alias('AnAttribute'))

ObjectClassInstantiated.Identifier.append(ObjectClassInstantiated.alias('AnotherOne'))
print ObjectClassInstantiated.getPK()
ObjectClassInstantiated.AnAttribute='First PK Elem Changed'
print ObjectClassInstantiated.getPK()

>./test.py
['First PK Elem', 'Second PK Elem']
['First PK Elem Changed', 'Second PK Elem']
   --> correct now!

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Re: object references

[DrConti]

Hi Bruno, hi folks!
thank you very much for your advices.
I didn't know about  the property function.
I learned also quite a lot now about "references".
Ok everything is a reference but you can't get a reference of a
reference...

I saw a lot of variations on how to solve this problem, but I find
actually that the "property approach" is the most natural of all.
Basically the idea there is that you build up this list of class
attributes not by storing a reference
to  a class attribute (which seem to be impossible), but you just store
on each element of the list one method (pardon a reference to..) to get
the associated class attribute.

Sorry for the UnPythonity. I used to be a CamelRider.
But not very longtime ago I left the Camel in the Desert, because I met
the Snake

Regs,
Davide.

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