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Date: Thu, 7 May 2015 19:38:13 +1000
Subject: Re: Throw the cat among the pigeons
From: Chris Angelico <rosuav@gmail.com>
Cc: "python-list@python.org" <python-list@python.org>
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Xref: csiph.com comp.lang.python:90093

On Thu, May 7, 2015 at 7:14 PM, Alain Ketterlin
<alain@universite-de-strasbourg.fr.invalid> wrote:
> Dave Angel <davea@davea.name> writes:
>
>> On 05/06/2015 11:36 AM, Alain Ketterlin wrote:
>>> Yes, plus the time for memory allocation. Since the code uses "r *=
>>> ...", space is reallocated when the result doesn't fit. The new size is
>>> probably proportional to the current (insufficient) size. This means
>>> that overall, you'll need fewer reallocations, because allocations are
>>> made in bigger chunks.
>>
>> That sounds plausible, but  a=5; a*=4  does not update in place. It
>> calculates and creates a new object.  Updating lists can work as you
>> say, but an int is immutable.
>
> Ah, okay. Even for big ints? If that is the case, my suggestion doesn't
> explain anything. Anyway, with so many allocations for so little
> arithmetic, the difference is probably due to the behavior of the
> allocator (which maybe always finds blocks big enough, since one was
> released after the previous multiplication, or something like that). The
> only way to know would be to profile the VM.

Yes, all integers are immutable. This is true regardless of the size
of the integer, because:

x = some_big_long_calculation()
y = x
y += 1

should never change the value of x.

ChrisA