[Edu-sig] looking for explanations... globals dynamic dict

[Carl Cerecke]

Yes. Laura, you're correct. I'll try again:

Massimo was trying to say that after:
a = 1
b = a

b and a refer to *different* 'one's because changing one of them (b += 2)
doesn't change the other.

This isn't true. They refer to the same 'one' object (regardless of python
implementation). b += 2 doesn't change the 'one' object, but simply points b
to a 'three' object.

There. How's that?

Carl.

On 29 March 2011 10:25, Laura Creighton <lac at openend.se> wrote:

> In a message of Tue, 29 Mar 2011 09:59:40 +1300, Carl Cerecke writes:
> >Well, not really. Although I see what you are trying to say.
>
> >numbers (like strings) are immutable. There can ever be only one number 1
> >You can't change a number to something else. If add 5 to 3, the number 3
> >doesn't change, I get a new number, 8.
>
> This is merely an implementation detail.
>
> Python 2.6.6
> Type "help", "copyright", "credits" or "license" for more information.
> >>> w = 1
> >>> y = 1
> >>> w is y  # this might surprise you
> True
>
> [PyPy 1.4.1]
> Type "help", "copyright", "credits" or "license" for more information.
>
> >>>> w = 1
> >>>> y = 1
> >>>> w is y # or are you so used to CPython that this surprises you?
> False
>
> You can have as many number ones as you like.  In CPython they are
> cached to be the same object, but you shouldn't rely on that behaviour.
> It's actually pretty dodgy to test the object identity on immutable
> objects -- unless you want to surprise people like I tried to.
>
> Incidentally
>
> Python 2.6.6
> Type "help", "copyright", "credits" or "license" for more information.
> >>> w = 5234
> >>> y = 5324
> >>> w is y
> False
>
> so after a certain point, the CPython developers have decided that the
> performance gains of cacheing all number xs as the same x isn't worth
> it.
>
> Laura
>
>
>
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