[Tutor] Variable Modification in a class

[Mehta, Anish]

Thanks to everyone for giving consideration to my problem. It worked 
with copy module and it made me learn quite a few new things abt python. 
Thanks for your support.

Regards,

Anish

Danny Yoo wrote:

>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
>
>
>
>  
>