Finding the instance reference of an object [long and probably boring]

[Arnaud Delobelle]

Joe Strout <joe at strout.net> writes:

> So.  How about this for a summary?
>
> "Python uses call-by-sharing.  That's a special case of call-by-value
> where the variables are references to objects; it is these references
> that are copied to the parameters, not the objects themselves.  For
> users of other languages, this is the same semantics used for objects
> in Java, RB/VB.NET, and C++ when dealing with objects."

Here's a story about call by sharing:

One day a black cat came strolling into our garden.  It seemed quite
hungry so we gave it some milk and food remains.  The cat drank the milk
and ate the food, stayed for a bit then walked away.  A couple of days
later it came back, miaowing, so we fed it again.  It started coming to
see us almost every day, sometimes sleeping in the house, sometimes
disappearing for a day or two.  We started thinking of it as our cat and
we *named* it Napoleon.  Napoleon became very popular and stayed at home
more and more often, and very young son was very fond of it, calling it
something like 'Polion'.

Unfortunately Napoleon got infested with fleas and we had to take him to
the vet.  The vet scanned Napoleon with a little machine and discovered
that it had an ID chip.  This revealed that Napoleon was really called
Nelson (ironically!) and belonged to a house down our road.  We
contacted them and they were happy to 'share' the cat with us.  By this
time the cat answered to the name of Napoleon so we carried on calling
it this name.

This story ends quite sadly.  One day Napoleon escaped out the front
door and got run over by a passing van.  Our son kept asking for Polion
so we decided to get a new cat and called him Napoleon as well (we
decided it would be easier for him!).

Now some questions about the story:

1. Is Napoleon a copy of Dobby or are they the same cat?

2. Is Polion a copy of Napoleon or are they the same cat?

3. When we got rid of Napoleon's fleas, was Nelson deflea-ed as well?

4. When Napoleon died, did Nelson die as well?

5. When we got a new Napoleon, does this mean that our neighbours got a
   new Nelson?

Now a question about the questions about the story:

   To be able to understand the story and answer questions 1-5, do we
   need to think of Napoleon, Nelson and Polion as variables containing
   references to cat objects, or is it enough to think of them as three
   names for cats?

-- 
Arnaud