[Tutor] Tuple - Immutable ?

[Steven D'Aprano]

Sudip Bhattacharya wrote:
>>>> s=(1,2,3)
>>>> s=s+(4,5,6)
>>>> s
> (1,2,3,4,5,6)
> 
> The tuple has changed.

No it hasn't. You have created a *new* tuple, and assigned it to the same 
name. Consider:

py> s = (1, 2, 3)
py> id(s)
3083421332
py> t = s
py> id(t)
3083421332


This shows that both s and t are names for the same tuple, with ID 3083421332. 
Now watch when we use += on s:

py> s += (4, 5)
py> id(s)
3083534812
py> id(t)
3083421332
py> t
(1, 2, 3)

The ID of s has changed, but t stays the same. So the original tuple remains 
untouched, and a new tuple is created.

If you do the same thing with lists, you will see that because lists are 
mutable, it does *not* create a new list, but changes the original in place:

py> a = [1, 2, 3]
py> b = a
py> a += [4, 5]
py> b
[1, 2, 3, 4, 5]


-- 
Steven