is it a bug in Module copy or i am wrong??
Chris Rebert
clp at rebertia.com
Fri Nov 7 03:21:37 EST 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!!
>
>
>
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