How to Change a Value in a Dict in another Dict?

Question

I created a dict of dicts in python.

dict1 = {}
dict2 = {'a': 0,
         'b': 0,
         'c': 0}
for i in range(1, 4):
    dict1.update({i : dict2})

print(dict1)
output: {1: {'a': 0, 'c': 0, 'b': 0}, 
         2: {'a': 0, 'c': 0, 'b': 0}, 
         3: {'a': 0, 'c': 0, 'b': 0}}

I now want to change the value of 'a' in the first dictionary.

dict1[1]['a'] += 1

The output looks like this.

print(dict1)
output: {1: {'a': 1, 'c': 0, 'b': 0}, 
         2: {'a': 1, 'c': 0, 'b': 0}, 
         3: {'a': 1, 'c': 0, 'b': 0}}

All 'a' values changed and not just the one in the first dictionary. Why is that? And how do I change only the value in the first dict?

https://stackoverflow.com/questions/17246693/what-exactly-is-the-difference-between-shallow-copy-deepcopy-and-normal-assignm — Rakesh, Jan 21 '19 at 10:51
all three keys in the new dictionary you create, refer to `dict2` so when you update one, you update them all. Re-built the inner one instead (`dict1.update({i : {k: v for k, v in dict2.items()}})` or similar) — Ma0, Jan 21 '19 at 10:53
I believe you need the deep copy instead of a shallow copy so that the changes you make in the 1st Dict does not impact the 2nd Dict — gireesh4manu, Jan 21 '19 at 10:56

score 3 · Answer 1 · answered Jan 21 '19 at 10:53

3

You are referencing the same dict (dict2) in all the new dict entries. Just copy the dict2 when updating the values in dict1:

>>> dict1 = {}
>>> dict2 = {'a': 0,
...          'b': 0,
...          'c': 0}
>>> dict1.update({i:dict2.copy() for i in range(1,4)})
>>> dict1
{1: {'a': 0, 'b': 0, 'c': 0}, 2: {'a': 0, 'b': 0, 'c': 0}, 3: {'a': 0, 'b': 0, 'c': 0}}
>>> dict1[1]["a"] = 1
>>> dict1
{1: {'a': 1, 'b': 0, 'c': 0}, 2: {'a': 0, 'b': 0, 'c': 0}, 3: {'a': 0, 'b': 0, 'c': 0}}

answered Jan 21 '19 at 10:53

Netwave

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1

A Deep copy is what is required right? is this not a shallow copy? – gireesh4manu Jan 21 '19 at 10:57
2

@gireesh4manu, it depens if `dict2` has other references on it or not. In this case **strings** and **integers** are inmutable so there is no need. – Netwave Jan 21 '19 at 10:59
hmm i'll try out this example as well just to be doubly sure. Thanks. – gireesh4manu Jan 21 '19 at 11:01
I agree. A shallow copy will suffice in this case :-) I looked at your example and i tried out myself as well. Thank you again. – gireesh4manu Jan 21 '19 at 11:28

Jay · Accepted Answer · 2019-01-21T11:38:20.427

In your code, you are updating the reference of the same dictionary in all the places. Since, all those references point to the same object, when you modify one, it is reflected everywhere.

You need a deep copy and not a shallow copy i.e. not the reference of the same dictionary in all the places.

Try the below code:

import copy

dict1 = {}
dict2 = {'a': 0,
         'b': 0,
         'c': 0}
for i in range(1, 4):
    dict1.update({i : copy.deepcopy(dict2)})

EDIT:

As correctly pointed out by @Netwave and @gireesh4manu, I am adding an update here.

For the example in this particular question, a copy.deepcopy is not needed as contents are dictionary are all immutable. In this case, the below answer would suffice.

dict1 = {}
dict2 = {'a': 0,
         'b': 0,
         'c': 0}
for i in range(1, 4):
    dict1.update({i : dict2.copy()})

But, in case if dict2 were to contain some mutable objects, then copy.deepcopy will be needed. One such example of dict2 is below:

dict2: {'a': [1,2,3],
        'b': [4,5,6],
        'c':[]
       }

In this case, since dict2 contains lists, any changes to those lists will reflected everywhere as they are linked by reference. To avoid this, copy.deepcopy will be needed.

There is no need of a deepcopy here. Since dict2 do not contain references to other mutable objects. — Netwave, Jan 21 '19 at 11:02
@Jay - A shallow copy works just fine. you can try modifying the values of the dicts and see to be doubly sure — gireesh4manu, Jan 21 '19 at 11:29
Thanks @gireesh4manu for pointing out. I have updated my answer. — Jay, Jan 21 '19 at 11:39

score 1 · Answer 3 · answered Jan 21 '19 at 10:55

Dictionnaries are mutable objects, which means all of the values of dict2 are pointing to the same object in memory.

One way to force the instanciation of a new dict object at each iteration is using the dict() function:

for i in range(1, 4):
    dict1.update({i : dict(dict2)})

How to Change a Value in a Dict in another Dict?

3 Answers3