deepcopy problem

[Stephen C Phillips]

On Tue, 11 Mar 2003 18:44:53 +0000, Jp Calderone wrote:

>   I think the essense of this problem can be distilled thusly:
> 
>   import copy
>   class Foo(object): pass
>   f = Foo()
>   f.foo = f
>   g = copy.deepcopy(f)
> 
>   "f.foo is f" is True, while "g.foo is g" is False.

Yes, I get that too.  Thanks for making it a simpler example - I hope a
solution to this will solve my problem too!

>   I believe this to be a bug in the pickle module, one which is fixed in
> 2.3.  There seems to be a work-around in 2.2, though I haven't explored
> its behavior very thoroughly:
> 
>   import copy
>   class Foo(object): pass
>   f = Foo()
>   f.foo = f
>   memo = {id(f): f}
>   g = copy.deepcopy(f)
> 
>   Now, "g.foo is g" is True.

Did you mean "g = copy.deepcopy(f, memo)" ?
If I do that then "g.foo is g" is true but that's only because "g is f"
is also true.  From what I know about the copy module, the keys in the
memo are the id's of the objects being copied and the value of a key is
the copy of the object, so passing that memo to deepcopy tells it that the
copy of f is f and so g gets pointed at the original f.

So what we need is both "g is g.foo" and "g is not f" to be true.

I had a look at the SourceForge bug tracker for 2.2.2 and could not see a
bug listed for copy or pickle but I may be using it wrongly.


Stephen Phillips