Python dictionary pointers and pointer to a reference?-CodePudding

I have the following class that simply create a dictionary of objects for me. Then copy the created dictionary, main_dict, to a new dictionary second_dic.

import numpy as np

class MainClass:
    def __init__(self):
        self.main_dict = dict()
    def add(self, k, val):
        self.main_dict[k]=val

main_obj = MainClass()
for i in range(1,5):
    main_obj.add(k=i, val=np.random.randint(1,10))

print(main_obj.main_dict)
{1: 1, 2: 2, 3: 7, 4: 7}

second_dic = main_obj.main_dict.copy()
print(second_dic)
{1: 1, 2: 2, 3: 7, 4: 7}

It seems Python doesn't support pointers like C . So when I change a value in my second_dic, the changes won't reflect in my main_dict. I am wondering what are my options in order to have this happens.

second_dic[1]=1000
print(second_dic)
{1: 1000, 2: 2, 3: 7, 4: 7}
print(main_obj.main_dict)
{1: 1, 2: 2, 3: 7, 4: 7}

CodePudding user response：

In your above code, the changes you make to second_dic won't be reflected in main_dict because why would they? Those two are two completely different dictionary objects.

If you want aliasing (though it's really not recommended anyway because it leads to hard-to-detect bugs) you have to assign the reference (kinda like the C pointers):

second_dic = main_obj.main_dict # Now second_dic is a pointer to main_dict, in a way

second_dic[1] = 1000

print(main_obj.main_dict)
{1: 1000, ...}

CodePudding user response：

The address of second_dic and main_dict are different. So the changes made to second_dic will not reflect to main_dict. I used the same above code and displayed the address of them and as expected they are different.

>>> id(second_dic) # To print the address
1911354287192
>>> id(main_obj.main_dict) # To print the address
1911354286952

If you require the second_dic to point to the main_dict. we can do it as below.

>>> main_obj.main_dict
{1: 6, 2: 10, 3: 10, 4: 3}
>>> second_dic = main_obj.main_dict # pointing to the same location
>>> second_dic
{1: 6, 2: 10, 3: 10, 4: 3}
>>> second_dic[1] = 111 #Changes the value
>>> second_dic
{1: 111, 2: 10, 3: 10, 4: 3}
>>> main_obj.main_dict  # Value gets reflected.
{1: 111, 2: 10, 3: 10, 4: 3}