[Tutor] Tuple - Immutable ?

[col speed]

On 8 March 2012 19:18, John Jensen <jensenjohn59 at yahoo.com> wrote:
> ________________________________
> From: Steven D'Aprano <steve at pearwood.info>
> To: tutor at python.org
> Sent: Thursday, March 8, 2012 7:51:33 AM
> Subject: Re: [Tutor] Tuple - Immutable ?
>
> col speed wrote:
>
>> I was just thinking about the immutability of things and tried this
>> (which -at least I- find interesting:
>>
>>>>> id(1)
>> 154579120
>>>>> a = 1
>>>>> id(a)
>> 154579120
>>>>> a += 2
>>>>> id(a)
>> 154579096
>>>>> id(3)
>> 154579096
>>>>> a is 3
>> True
>> Although there is probably no other logical way of doing it - I learnt
>> something new again!
>
> Prepare to have your mind boggled:
>
> py> a = 99
> py> b = 99
> py> a is b
> True
> py> a = 9912345
> py> b = 9912345
> py> a is b
> False
>
>
> Well, okay, so it's not *much* of a boggle. Perhaps a bogglet.
>
> What happens is that Python caches the small integers, like -1, 0, 1, up to
> some limit, and re-uses them when and as needed. That limit various from
> version to version, so you can't rely on it. But larger integers are not
> cached, and so you get a fresh one each time.
>
> This makes sense, and is easy to understand. Now for the real boggle:
>
> py> a = 9912346; b = 9912346
> py> a is b
> True
>
> Can you guess what is going on here?
>
> (Answer will follow later.)
>
> I'll preface my answer by stating that I'm a noob and haven't checked the
> documentation for my answer, but I would guess that Python is saving the
> variables as integer values and that the larger values would require them
> being saved as doubles.
>
>
> -- Steven
>
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Thanks