Reference

[Grant Edwards]

On 2014-03-03, ast <nomail at invalid.com> wrote:
> hello
>
> Consider following code:
>
>>>> A=7
>>>> B=7
>>>> A is B
> True
>
> I understand that there is a single object 7 somewhere in memory

Maybe, maybe not.  Integer are immutable, so that's allowed but not
required. In CPython, that's true for small integers, but that is an
implementation detail, and you shouldn't depend on it.

> and both variables A and B point toward this object 7

They might.  They might not.

> now do the same with a list:
>
>>>> l1 = [1, 2]
>>>> l2 = [1, 2]
>>>> l1 is l2
> False
>
> It seems this time that there are 2 distincts objects [1, 2] in
> memory.

Yep.  Lists are mutable, therefore each literal produces a distinct
object.

> l1 points toward the first one and l2 points toward the
> second one.

Yep.

> I dont really understand why the behavior is different. 
> Both integer 7 and list [1, 2] are objects. Why is it
> different ?

Integer objects are immutable (they can't change value), therefore you
can reuse them without causing problems.  Lists are mutable (you can
change the values in them), so you can't reuse them.

-- 
Grant Edwards               grant.b.edwards        Yow! Maybe I should have
                                  at               asked for my Neutron Bomb
                              gmail.com            in PAISLEY --