On Feb 25, 11:30 pm, Dennis Lee Bieber <[EMAIL PROTECTED]> wrote:
> On Mon, 25 Feb 2008 17:55:18 -0800 (PST), [EMAIL PROTECTED] declaimed
> the following in comp.lang.python:
>
> > I'd like to do this:
>
> > a= list( range( 5 ) )
> > assert a== [ 0, 1, 2, 3, 4 ]
> > for i in ref( a ):
> > i.ref*= 2
> > a= deref( a )
> > assert a== [ 0, 2, 4, 6, 8 ]
[snip]
> > Dictionaries, sets, and the primitives all have ways to do this, in
> > particular, to reset them: a= {} is equivalent to a.clear(), except
<em>EXCEPT THAT</em>
> > that other references to it corefer before, and in the former, don't
> > after.
No, ={} and .clear() are not equivalent.
In the case of class instances,
>>> class Instance(): pass
...
>>> obj= Instance()
>>> obj.a= {}
>>> d= obj.a
>>> obj.a[0]= object()
>>> d[0]
<object object at 0x00A37468>
>>> e= obj
>>> obj.a= {}
>>> d
{0: <object object at 0x00A37468>}
>>> e.a
{}
It works to keep a reference to the object, then access the member
again. If your only reference to the object is the member itself,
obj.a= {} breaks d, but obj.a.clear() keeps it alive.
In the case of immutables, there is no such .clear() equivalent. If
obj.a is a tuple, string, or integer (*miss any?), d is always broken
when obj.a is changed.
The generic solution involves a second level of indirection: tuple* d=
&obj.a. I can't land a clean solution to it though.
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