python copy class object

1"""
2I've read that there are problems with this approach, but it never let me down
3"""
4
5import copy
6
7class Dummy:
8    def __init__(self, signature = 1, some_list = []):
9        self.signature = signature
10        self.list = some_list
11
12    def change_signature(self):
13        self.signature += 1
14
15    def change_list(self):
16        self.list[0][0] += 1
17
18
19def naive_test():
20    print('---> Naive Copy Test')
21    dummy1 = Dummy(1)
22    #dummy2 points to dummy1
23    dummy2 = dummy1
24    if dummy1.signature == dummy2.signature:
25        print ('\tDummy instances are the same')
26    dummy1.change_signature()
27    if dummy1.signature == dummy2.signature:
28        print ('\tDummy \'copy\' changed after changing the original. They are still the same.')
29
30def shallow_copy_test():
31    print('\n---> Shallow Copy Test')
32    dummy1 = Dummy(1, [[1,2],[3,4]])
33    #Create shallow copy
34    dummy2 = copy.copy(dummy1)
35    if dummy1.signature == dummy2.signature:
36        print ('\tDummy instances are the same')
37    dummy1.change_signature()
38    if dummy1.signature != dummy2.signature:
39        print ('\tDummy copy didn\'t change after changing the original. They are now different.')
40    dummy1.change_list()
41    if dummy1.list == dummy2.list:
42        print('\tBut their lists change together! This is because the copy only made copies of the original instance\'s higher level list, but it is a list of lists, so each instance points to two different lists which, in turn, point to the SAME lists')
43
44def deep_copy_test():
45    print('\n---> Deep Copy Test')
46    dummy1 = Dummy(1, [[1,2],[3,4]])
47    #Create deep copy
48    dummy2 = copy.deepcopy(dummy1)
49    if dummy1.signature == dummy2.signature and dummy1.list == dummy2.list:
50        print ('\tDummy instances are the same')
51    dummy1.change_list()
52    if dummy1.list != dummy2.list:
53        print('\tNow each instance is 100% independent. Deep copy goes through all nested references!')
54
55
56naive_test()
57shallow_copy_test()
58deep_copy_test()