Appending (not replacing) items in a nested dictionary via a list of keys

Question

This is a follow up to following question on SO:

Access nested dictionary items via a list of keys?

All the solutions given in the above link allows to replace/create a value of the nested dict via a list of keys. But my requirement is to append (update) another dict to value.

If my dict is as follows:

dataDict = {
    "a":{
        "r": 1,
        "s": 2,
        "t": 3
        },
    "b":{
        "u": 1,
        "v": {
            "x": 1,
            "y": 2,
            "z": 3
        },
        "w": 3
        }
}

And my list of keys is as follows:

maplist = ["b", "v"]

How can I append another dict, say "somevalue": {} to the dataDict['b']['v'] using my maplist?

So, after appending new dataDict would look as follows:

dataDict = {
        "a":{
            "r": 1,
            "s": 2,
            "t": 3
            },
        "b":{
            "u": 1,
            "v": {
                "x": 1,
                "y": 2,
                "z": 3,
                "somevalue": {}
            },
            "w": 3
            }
    }

The solutions on the aforementioned SO link changes/creates the value like this:

dataDict[mapList[-1]] = value

But we can't call the dict's update function like dataDict[mapList[-1]].update().

Checked all of SO and Google, but no luck. Could someone please provide some help with this. Thanks!

`dataDict['a']['b']` is a KeyError. What is your expected output? — Austin, Dec 30 '19 at 16:09
Just updated my question with sample output and fixed the KeyError — Vivek Singh, Dec 30 '19 at 16:19
@ Ch3steR Hard luck bro! My reputation less than 125, so my vote doesn't show up publicly :) — Vivek Singh, Jan 01 '20 at 18:34

Ed Ward · Answer 1 · 2019-12-30T16:26:45.343

3

This should work for you:

def set_item(this_dict, maplist, key, value):
    for k in maplist:
        if k not in this_dict:
            this_dict[k] = {}
        this_dict = this_dict[k]

    this_dict[key] = value

dataDict = {
    "a":{
        "r": 1,
        "s": 2,
        "t": 3
        },
    "b":{
        "u": 1,
        "v": {
            "x": 1,
            "y": 2,
            "z": 3
        },
        "w": 3
        }
}

maplist = ["b", "v"]
new_key = "somevalue"
new_value = {}

set_item(dataDict, maplist, new_key, new_value)

print(dataDict)

Output:

{'a': {'r': 1, 's': 2, 't': 3}, 'b': {'u': 1, 'v': {'x': 1, 'y': 2, 'z': 3, 'somevalue': {}}, 'w': 3}}

edited Dec 30 '19 at 16:26

answered Dec 30 '19 at 16:22

Ed Ward

2,333
2
10
16

pylint would say that `not x in y` should be written as `x not in y` – avysk Dec 30 '19 at 16:24
also `k in some_dict.keys()` can be written simply as `k in some_dict`. – avysk Dec 30 '19 at 16:24
Thanks Ed. I was searching for something more concise, but something is better than nothing, so I'm happy with this :) – Vivek Singh Dec 30 '19 at 17:52

Ch3steR · Answer 2 · 2019-12-30T18:12:38.377

You can use setdefault() method.

>>>dataDict['b']['v'].setdefault('something',{})
>>>print(dataDict)
{'a': {'r': 1, 's': 2, 't': 3}, 
'b': {'u': 1, 'v': {'x': 1, 'y': 2, 'z': 3, 'something': {}}, 
'w': 3}}

The above is the idea of how it works. Make a function for the above.

def set_new_key(d:dict,x:list,y:str):
    for key in x:
        d=d[key]
    d.setdefault(y,{})

dataDict = {
    "a":{
        "r": 1,
        "s": 2,
        "t": 3
        },
    "b":{
        "u": 1,
        "v": {
            "x": 1,
            "y": 2,
            "z": 3
        },
        "w": 3
        }
}

mapList=['b','v']
set_new_key(dataDict,mapList,'something')
print(dataDict)
print()
set_new_key(dataDict,['a'],'another')
print(dataDict)
print()
set_new_key(dataDict,['b','v','something'],'inside something')
print(dataDict)

output:

{'a': {'r': 1, 's': 2, 't': 3}, 'b': {'u': 1, 'v': {'x': 1, 'y': 2, 'z': 3, 'something': {}}, 'w': 3}}

{'a': {'r': 1, 's': 2, 't': 3, 'another': {}}, 'b': {'u': 1, 'v': {'x': 1, 'y': 2, 'z': 3, 'something': {}}, 'w': 3}}

{'a': {'r': 1, 's': 2, 't': 3, 'another': {}}, 'b': {'u': 1, 'v': {'x': 1, 'y': 2, 'z': 3, 'something': {'inside something': {}}}, 'w': 3}}

Thanks Ch3steR! But this is not useful for my scenario because I need to update the dict dynamically using the list of keys in another function. — Vivek Singh, Dec 30 '19 at 17:55
@VivekSingh check the edited answer. btw it was dynamic from the start. — Ch3steR, Dec 30 '19 at 18:08

Appending (not replacing) items in a nested dictionary via a list of keys

2 Answers2