how to copy class object in an array in Python

Question

I have defined a Color class as below. Since I need to store multiple colors and their respective node ID (which has the color), I made a list colors to store them. However, every time a node color is changed, I do not want to update list colors directly (another function will decide whether to update or not), so I need to store a copy of colors to *tmp_colors* before calling the decision func, and update colors with *tmp_colors* if result is Yes.

I managed to make a copy of new list *tmp_colors*, but *tmp_colors[0]* is still pointing to colors[0] , resulting in the update of both list.

1. How can I make a copy of class object in colors[0] to *tmp_colors[0]*?
2. If I were to update colors[0] later on, what's the best way?
3. Is there any better design instead of the example below (define class, and list of class object)?

---

class Color:
    __elems__ = "num", "nodelist",

    def __init__(self):
        self.num = 0
        self.num_bad_edge = 0

    def items(self):
        return [
                (field_name, getattr(self, field_name)) 
                 for field_name in self.__elems__]

def funcA():

    nodeCount = 2
    colors = []
    for i in range(0, nodeCount):
        colors.append(Color())

    colors[0].num = 2
    colors[0].nodelist = [10,20]
    colors[1].num = 3
    colors[1].nodelist = [23,33, 43]

    print "colors"
    for i in range(0, nodeCount):
        print colors[i].items()

    tmp_colors = list(colors)
    print "addr of colors:" 
    print id(colors)
    print "addr of tmp_colors:" 
    print id(tmp_colors)    
    print "addr of colors[0]:" 
    print id(colors[0])
    print "addr of tmp_colors[0]:" 
    print id(tmp_colors[0])

    tmp_colors[0].num = 2
    tmp_colors[0].nodelist = [10,21]

    print "\ntmp_colors"
    for i in range(0, nodeCount):
        print tmp_colors[i].items()

    print "\ncolors <<< have been changed"
    for i in range(0, nodeCount):
        print colors[i].items()

---

Result:

colors
[('num', 2), ('nodelist', [10, 20])]
[('num', 3), ('nodelist', [23, 33, 43])]
addr of colors:
32480840
addr of tmp_colors:
31921032
addr of colors[0]:
32582728
addr of tmp_colors[0]:
32582728                           <<<<<< --- expecting a new addr

tmp_colors
[('num', 2), ('nodelist', [10, 21])]
[('num', 3), ('nodelist', [23, 33, 43])]

colors <<< have been changed
[('num', 2), ('nodelist', [10, 21])]   <<<<<< --- expecting no change [10, 20]
[('num', 3), ('nodelist', [23, 33, 43])]

Answer 1

You copied the list, but not the contents, which you then change. Your Color instances are mutable too, and tmp_colors[0] refers to the same instance as colors[0] does. Sure, tmp_colors is a copy, but the Color instance is not.

Use copy.deepcopy() to recursively create copies of your objects:

from copy import deepcopy

tmp_colors = deepcopy(colors)

Answer 2

You can use the copy module

import copy
tmp_colors = copy.deepcopy(colors)