[Tutor] Variable Modification in a class

[Danny Yoo]

On Tue, 3 Jun 2003, Mehta, Anish wrote:

> I m sorry that i am repeating my last mail. Here also in c i m doing the
> same thing. Or is there any differnce? The point is clear to me that
> when i do ' c = b' it makes the memory locations same.
>
> C example which is creating the confusion:
>
> typedef struct ab
> {
>        int a;
>        int b;
> }AB;
>
> main()
> {
>   AB b;
>   AB c;
>
>   b.a = 5;
>   b.b = 10;
>
>   c = b;
>
>   c.a = 30;
>   c.b = 40;
>
>   printf("AB values %d %d\n", b.a, b.b);
>   printf("New values %d %d\n", c.a, c.b);
> }


Let me see if I can translate the situation in C code.

/******/
AB* b = malloc(sizeof(AB));
AB* c = malloc(sizeof(AB));
b->a = 5;
b->b = 10;

c = b;            /** This is the crucial line.  */
/******/



The equivalent Python code is:

###
b = AB()
c = AB
b.a = 5
b.b = 10

c = b            ## This is the crucial line.
###

Does this make sense?  Names in Python are things that point to objects.
Since you're a C programmer, I think you'll understand this: in Python,
everything's a pointer.  That is, all object access in Python goes through
a level of indirection.



The confusion that you're running into is related to the way C allows for
two different ways of working with structures: direct access, like

/***/
AB some_object;
/***/

vs indirect access through a pointer:

/***/
AB* some_object = malloc(sizeof(AB));
/***/


If you keep in your head that Python always uses the indirect method, the
problem you're running into should be immediately clear.


What you want to do, instead of reassigning c to b, is to make a copy of
b, and assign that copy to c.  We can do this with the copy module:

    http://www.python.org/doc/lib/module-copy.html