object references

Bruno Desthuilliers bdesth.quelquechose at free.quelquepart.fr
Mon Mar 27 00:13:53 CEST 2006

DrConti a écrit :
> 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))

Well, that's the only kind of "variable"[1] in Python.

[1] the correct name in Python is 'binding', since it's about 'binding' 
a reference to a name, not labelling an in-memory address and storing 
data there.

> 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...:

You *think* you 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
> ['First PK Elem', 'Second PK Elem']
> ['First PK Elem', 'Second PK Elem']
> --> wrong! It should write  ['First PK Elem Changed', 'Second PK Elem']

Nope, it's exactly what you asked for !-)

> i.e. the assgnment
> ObjectClassInstantiated.Identifier.append(ObjectClassInstantiated.AnAttribute)
> assigns only the attribute value, not the reference.

1/ it's not an assignement
2/ it does not store the attribute "value", it stores the reference to 
the object the attribute is bound to. When you later rebind the 
attribute, it only impact this binding - there's no reason it should 
impact other bindings.

> so my question is:
> is it still true that there is no possibilty to get directly object
> references?

But object references *are* what you have.

> Is there a solution for the problem above ?

Yes : keep a reference to the attribute name, not to the value bound to 
that name. There are many ways to do it, here's one:

ObjectClass.Identifier = property(
   fget=lambda self: [self.AnAttribute, self.AnotherOne]

and here's another one:

ObjectClassInstantiated._identifier_parts = []
# note the use of strings, not symbols

ObjectClass.Identifier = property(
   fget=lambda self: [getattr(self, name) \
                        for name in self._identifier_parts]

> Thank you for any feedback 

May I add some ? Your naming conventions are highly unpythonic. We 
usually use CamelCase for classes names, and (in order of preference) 
all_lower_with_underscores or mixedCaps for 
variables/attributes/functions etc.


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