[Tutor] Sort Output

[Alan Gauld]

"Wayne Watson" <sierra_mtnview at sbcglobal.net> wrote 
>  I was surprised to find the result below.

Well done,  you just found one of the standard beginners gotchas!

>>>> a =[4,2,5,8]
>>>> b = a

This makes b refer to the *same list* as a.

>>>> a.sort()

This sorts the list contents in-place

>>>> a
> [2, 4, 5, 8]

As shown.

>>>> b
> [2, 4, 5, 8]

And since b refers to the same luist as a it also reflects 
the rearrangement of the contents.

> b no longer has the same value as it began. 

Actually it does. It refers to exactly the same list as a 
before and after the sort. The list object has not changed 
only the contents of the list were sorrted.

> Apparently to prevent sort from making it the same 
> I have to resort to copying b into a first? 

No you need to take a copy of a when you assign it to b

b = a[:]

will result in b referring to a copy of a.
Now if you change a's list nothing will happen to b's copy

Remember that in Python variables are just names that 
refer to objects. Many names can refer to the same object.

HTH,

-- 
Alan Gauld
Author of the Learn to Program web site
http://www.freenetpages.co.uk/hp/alan.gauld