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Why does changing 1 list affect the other?

Louis Pecora pecora at anvil.nrl.navy.mil
Fri Nov 7 15:58:01 EST 2003 
In article <vqo0rpcn6lj2dd at corp.supernews.com>,
 "Francis Avila" <francisgavila at yahoo.com> wrote:

> "Louis Pecora" <pecora at anvil.nrl.navy.mil> wrote in message
> news:pecora-9C8FEE.14325507112003 at ra.nrl.navy.mil...
> > So the action is on the binding end?  Only one 5 is created, but the x=4
> > following statement UNbinds x and rebinds it to '4', rather than
> > changing the object. y remains bound to '5'.  Did that come out right?
> >
> > Then for lists, when we do   x[0]=4 we are NOT unbinding x, but rather
> > just changing the object?
> >
> > Sounds like if we try harder we will get to the _binding_ actions of the
> > '=' operator.  Let me try harder right here (off the top of my head).
> >
> > So, let's see, we view '=' as a binding operation.
> >
> > x=list
> >
> > binds x to the list, but each list element is bound to something else (I
> > refer to this as a second level binding).
> >
> > For lists we are allowed to change that second level binding.  This
> > second level action does NOT affect the first level y=x binding.
> >
> > Now, does that sound right?  Anyone?
> >
> > -- Lou Pecora
> 
> This "first order, second order" binding is just confusing things, I think.
> 
> Try this:
> 
> There are two kinds of "things" in Python--names and objects.
> 
> 1) Names point to objects, and only to objects
> 2) Names are not objects.
> 
> It follows from this that names never point to names, AND that objects can
> CONTAIN names.  So, names are THINGS, but not OBJECTS.  Names are like
> arrows that you can hold and point at things with, but that can't be pointed
> to.
> 
> If you say something like "y = x", you're saying, "whatever object x points
> to, make y point to the *same* object." (NOT a *copy* of that object!)
> 
> Further, some names are explicit, some implicit (or "anonymous," if you
> don't mind the slight contradiction that entails....).  Container objects
> (dictionaries, lists, tuples) contain "anonymous names" which point to
> objects.  These implicit names are accessed indirectly only by an index or
> key, not directly by a name in a namespace.
> 
> If the container is "mutable", then the implicit names contained within the
> container object can be made to point to different objects after the
> container object's creation.

I see what you mean.  But I don't see that this is any simpler than what 
I said, above.  Maybe just style.  I think both 'explanations' are 
stating essentially that container objects point 'further' to a 
collection of objects (my second level, your implicit names).

I think it helped to see another take on this.  Thanks.

-- Lou Pecora
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