Nested dictonary in python

Question

I am trying to update the values in the internal dictonary but it is updating in the both, I tried a lot but still facing the same problem. Thanks in advance.

arr = [1,2]
d_int = {'a':False, 'e':False, 'i':False, 'o':False, 'u':False}
d_main ={i : d_int for i in range(len(arr))}
print(d_main)
d_main[0]['a'] = True
d_main[1]['i'] = True
print(d_main)

Result:

{0: {'a': True, 'e': False, 'i': True, 
     'o': False, 'u': False}, 
 1: {'a': True, 'e': False, 'i': True, 
     'o': False, 'u': False}
}

Need something like this:

{0: {'a': True, 'e': False, 'i': False,
     'o': False, 'u': False},
 1: {'a': False, 'e': False, 'i': True,
     'o': False, 'u': False}
}

Answer 1

This is because all values of the d_main refer to the same dict object d_int . Changes to any one of them will reflect across all references. Further read

To avoid the issue, ensure you create a new copy of the dict object during your list comprehension, using copying methods such as .copy .

arr = [1,2]
d_int = {'a':False, 'e':False, 'i':False, 'o':False, 'u':False}
d_main ={i : d_int.copy() for i in range(len(arr))} #modified.
print(d_main)
d_main[0]['a'] = True
d_main[1]['i'] = True
print(d_main)
#Output:
{0: {'a': True, 'e': False, 'i': False, 'o': False, 'u': False}, 1: {'a': False, 'e': False, 'i': True, 'o': False, 'u': False}}

Answer 2

Short workaround: Change d_int to d_int.copy() and you'll have your output. You are referencing the same object. Look at the difference in the id in these two cases:

Case 1 (your attempt)

arr = [1,2]
d_int = {'a':False, 'e':False, 'i':False, 'o':False, 'u':False}
d_main ={i : d_int for i in range(len(arr))}
print(list(map(id,d_main.values())))

Returns:

[140545244867248, 140545244867248] # <-- they are the same

Case 2 (possible work-around)

arr = [1,2]
d_int = {'a':False, 'e':False, 'i':False, 'o':False, 'u':False}
d_main ={i : d_int.copy() for i in range(len(arr))} # <-- .copy() added
print(list(map(id,d_main.values())))

Returns:

[140545244796464, 140545244798480]

Answer 3

If you are new without any programming experience in any programming language, For understanding why the problem happen, you'd better to know what is pointer in C programming language firstly.

---

copy.deepcopy is the best. dict.copy have same trouble in deep nested dict. as follows.

a = {"1": {"2": 2, "3": 3}, "4":4}

c = {1:a.copy()}
b = {1:a.copy()}

c[1]['1']['2'] = 5
c[1]['4'] = 40

print(c)
# {1: {'1': {'3': 3, '2': 5}, '4': 40}}
print(b)
# {1: {'1': {'3': 3, '2': 5}, '4': 4}}

But copy.deepcopy is work well with deep nested dict.

from copy import deepcopy

a = {"1": {"2": 2, "3": 3}, "4":4}

c = {1: deepcopy(a)}
b = {1: deepcopy(a)}

c[1]['1']['2'] = 5
c[1]['4'] = 40

print(c)
# {1: {'1': {'2': 5, '3': 3}, '4': 40}}

print(b)
# {1: {'1': {'2': 2, '3': 3}, '4': 4}}

Actually, this problem is not only happen to nested dict, other types as well, likes nested list, class with dict property and so on, and copy.deepcopy is work well.