is it a bug in Module copy or i am wrong??

Chris Rebert clp at rebertia.com
Fri Nov 7 09:21:37 CET 2008


On Thu, Nov 6, 2008 at 11:59 PM, yoma <yomatatata at gmail.com> wrote:
> python version 2.5 in module copy
>
> we all know that copy have two method: copy() and deepcopy().
> and the explain is
> - A shallow copy constructs a new compound object and then (to the
>  extent possible) inserts *the same objects* into it that the
>  original contains.
>
> - A deep copy constructs a new compound object and then, recursively,
>  inserts *copies* into it of the objects found in the original.
>
> so i try a example:
> import copy
>
> class A:
>    i = 1
>
> class B:
>    a = A()

Note that `a` is a class variable, not an instance variable. This ends
up being important.

>
>
> b = B()
>
> x=copy.copy(b)
>
> y=copy.deepcopy(b)

I believe these only copy the instance variables of `b`. They do NOT
copy the class `B` (IMHO, copying B would be weird and unexpected
behavior here anyway) or its constituent variables, such as `a`.

>
> print id(x.a), id(b.a)
>
> print id(y.a), id(y.a)
>
> the result:
> 14505264 14505264
> 14505264 14505264

Thus this makes sense. These all refer to B's variable `a`, which is a
class variable and therefore not copied by copy() or deepcopy()-ing
`b`, an *instance* of class B.
The fact that you can access `a` through B instances does not mean
that `a` "belongs" to any instance of B and is merely a result of how
Python's object system works.

Disclaimer: I am not a CPython dev and did not look at the `copy`
module's sources.

Cheers,
Chris
-- 
Follow the path of the Iguana...
http://rebertia.com

>
> So maybe i have a wrong understand to deep copy and shallow copy or
> it is  a bug ?
>
> please help me!!
>
>
>
> --
> http://mail.python.org/mailman/listinfo/python-list
>



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