list in a tuple

[vjktm]

On Dec 28, 1:34 am, "Gabriel Genellina" <gagsl-... at yahoo.com.ar>
wrote:
> En Thu, 27 Dec 2007 16:38:07 -0300, <montyphy... at gmail.com> escribió:
>
>
>
> > On Dec 27, 8:20 pm, Wildemar Wildenburger
> > <lasses_w... at klapptsowieso.net> wrote:
>
> >>  From that post:
> >>  > Ok, I do admit that doing
>
> >>  > a = ([1], 2)
> >>  > a[0].append(2)
>
> >>  > also doesn't throw an error, but this only confuses me more.
>
> >> Why? You mutate thelist, but thetupledoes not change. It is still
> >> atupleof alistand an int. At least that's how I think about it, and I
> >> seem to recall reading that beavior justified like this (don't ask me
> >> where though (might have been "Dive Into Python", but maybe not)).
>
> > That part is ok, I mean it doesn't confuse me I just wanted to say
> > that this is somewhat confusing behavior.
> > I agree that its not best put... But I was thinking about the last
> > part of the post, the part
> > that talks about trying to print a tuple and getting an error.
>
> Instead of trying to explain it myself, I'll refer you to this little
> essay [1] by Michael Hudson including some nice ASCII art, and a long
> reply from Alex Martelli from which I'll quote just a few memorable
> paragraphs. (Just replace "dictionary" with "tuple" in your example)
>
> """There is [...] a huge difference
> between changing an object, and changing (mutating) some
> OTHER object to which the first refers.
>
> In Bologna over 100 years ago we had a statue of a local hero
> depicted pointing forwards with his finger -- presumably to
> the future, but given where exactly it was placed, the locals
> soon identified it as "the statue that points to Hotel
> Belfiore".  The one day some enterprising developer bought
> the hotel's building and restructured it -- in particular,
> where the hotel used to be was now a restaurant, Da Carlo.
>
> So, "the statue that points to Hotel Belfiore" had suddenly
> become "the statue that points to Da Carlo"...!  Amazing
> isn't it?  Considering that marble isn't very fluid and the
> statue had not been moved or disturbed in any way...?
>
> This is a real anecdote, by the way (except that I'm not
> sure of the names of the hotel and restaurant involved --
> I could be wrong on those), but I think it can still help
> here.  The dictionary, or statue, has not changed at all,
> even though the objects it refers/points to may have been
> mutated beyond recognition, and the name people know it
> by (the dictionary's string-representation) may therefore
> change.  That name or representation was and is referring
> to a non-intrinsic, non-persistent, "happenstance"
> characteristic of the statue, or dictionary...
> """
>
> [1]http://python.net/crew/mwh/hacks/objectthink.html
>
> --
> Gabriel Genellina

Thank you very much for this discussion and reference [1] and other
pointers in [1].  This has definitely helped my understanding of such
matters.