Python deepcopy changes the reference to an object

Question

Hi: This is a follow up to this question:

How to copy all python instances to a new class?

The problem is that deepcopy() is not copying the objects correctly, this can be seen through this working code:

from kivy.app import App
from kivy.uix.floatlayout import FloatLayout

class MyApp(App):
    def build(self):
        global Myroot    
        self.root = FloatLayout()
        Myroot = self.root
        Starthere(None)

        return

class VarStorage:
    pass

VS=VarStorage()
from kivy.uix.button import Button
import copy

def Starthere(instance):
    VS.MyButton=Button()
    print str(VS.MyButton)
    VSCopy=copy.deepcopy(VS)
    print str(VSCopy.MyButton)

MyApp().run()

From my understanding of copy, it should print twice the same button, but the result is:

<kivy.uix.button.Button object at 0x046C2CE0>
<kivy.uix.button.Button object at 0x04745500>

How to make deepcopy() copy the same object instead of a new (non existing) one? Thank you!

---------------------- EDIT -----------------------------

After trying copy() instead of deepcopy(), its not what Im intending to do:

What I get with deepcopy():

Copied class of VS, with copied items for those which are not objects (for instance, if VS.text="text", VSCopy.text will have the same contents without being linked whatsoever).
But, for objects, what I need is a copy of the reference to such object, which I dont get, since I get a new reference pointint a new object.

What I get with copy():

Copied class VSCopy with refferences pointing to original class VS. I dont want this since I want to change VS's contents (thats why Im trying to copy it) and still have the original ones available in VSCopy.

Is there such a function in copy module?

score 3 · Answer 1 · answered Jul 21 '13 at 13:56

3

You need to use shallowcopy instead of deepcopy. From copy module docs:

A shallow copy constructs a new compound object and then (to the extent possible) inserts references into it to the objects found in the original.

A deep copy constructs a new compound object and then, recursively, inserts copies into it of the objects found in the original.

You can use copy.copy() to create a shallow copy:

VSCopy = copy.copy(VS)

answered Jul 21 '13 at 13:56

Rohit Jain

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Very much appreciated, thank you! Will accept the answer as soon as stackoverflow allows me to ^_^ – I want badges Jul 21 '13 at 13:59
Updated the question, the answer doesnt actually solve what I intend to do, thanks – I want badges Jul 21 '13 at 14:20

score 2 · Accepted Answer · answered Jul 21 '13 at 15:50

Is there such a function in copy module?

No, I'm afraid there is not.

It sounds like you want some sort of hybrid deep/shallow behavior, and that's not what exists in the copy module. You either get a shallow copy, or a deep one.

Sorry for the bad news. :) You may just want to write your own copier, or see if you can accomplish your task without copying per se.

(I see you edited your question, so I've answered again to see if I can help. (Please see my first answer for a response to your original Q.))

Thank you Ron, this answer actually (iroically) answers the intended question of this post. — I want badges, Jul 21 '13 at 16:00

ron rothman · Answer 3 · 2013-07-21T14:03:34.960

deepcopy is working as expected (assuming VarStorages can be deepcopied). You might be getting confused by the Buttons' string representations.

When you deepcopy an object, it will (by definition) have a new memory address.

This:

<kivy.uix.button.Button object at 0x046C2CE0>

means that there's a Button object at memory location 0x046C2CE0.

And this:

<kivy.uix.button.Button object at 0x04745500>

means that there's a different Button object at memory location 0x04745500. Which is exactly what you want (right?)--a new copy of the first button.

Perhaps (though I don't think so) you actually want a shallow copy (copy.copy)?

Python deepcopy changes the reference to an object

3 Answers3