How do I create a nested dictionary under a key that is yet to exist?

Question

I have an empty array.

I want to assign a value like this: array[key][subkey] = 'value'

This produces a KeyError as array[key] does not exist yet.

What do I do? I tried the following...

array['key'] = None
array['key']['subkey'] = 'value'

TypeError: 'NoneType' object does not support item assignment

I tried:

array['key'] = []
array['key']['subkey'] = 'value'

TypeError: list indices must be integers, not str

I tried:

array['key'] = ['subkey']
array['key']['subkey'] = 'value'

TypeError: list indices must be integers, not str

So what do I do?

Nested dictionaries you meant? – Moses Koledoye Jul 09 '17 at 01:17 — Moses Koledoye, Jul 09 '17 at 01:17

Moses Koledoye · Accepted Answer · 2017-07-09T01:27:36.960

29

You could use collections.defaultdict, passing the default factory as dict:

>>> from collections import defaultdict
>>> d = defaultdict(dict)
>>> d['key']['subkey'] = 'value'
>>> d
defaultdict(<type 'dict'>, {'key': {'subkey': 'value'}})

To apply further levels of nesting, you can create a defaultdict that returns defaultdicts to a n-th depth of nesting, using a function, preferably anonymous, to return the nested default dict(s):

>>> d = defaultdict(lambda: defaultdict(dict))
>>> d['key']['subkey']['subsubkey'] = 'value'
>>> d
defaultdict(<function <lambda> at 0x104082398>, {'key': defaultdict(<type 'dict'>, {'subkey': {'subsubkey': 'value'}})})

_{Example shows nesting up to depth n=1}

edited Jul 09 '17 at 01:27

answered Jul 09 '17 at 01:19

Moses Koledoye

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The issue with using it is that the last value can't be of type list for example, can you find a solution for that ? d['key']['subkey'].append(msg) will raise AttributeError which need to be handled d['key']['subkey'] = [msg]. can we have dynamic nested with last one as list ? – Sion C May 09 '23 at 13:52
@SionC https://godbolt.org/z/K54GMTz4n – Moses Koledoye Jun 04 '23 at 10:33
it will only be 2d .. we are looking for nested unlimited. – Sion C Jun 05 '23 at 11:28
1

@SionC It should be possible to implement a factory class which defines `__call__` that can do that sort of thing. – Moses Koledoye Jun 06 '23 at 23:57
can you give an example ? – Sion C Jul 12 '23 at 10:10

score 7 · Answer 2 · edited Jan 09 '19 at 09:44

You are using a [] list literal not a {} dict literal:

array['key'] = {}
array['key']['subkey'] = 'value'

But this isn't very useful in a loop.
In a loop you could test if 'key' is not in array - which is a cheap operation (O(1) lookup):

if 'key' not in array:
    array['key'] = {}
array['key']['subkey'] = 'value'

But you can use setdefault() to do the same thing and give key a default value if it doesn't already have a value, e.g.:

array.setdefault('key', {})['subkey'] = 'value'

And if this looks ugly, then you can always use collection.defaultdict.

Stefan Pochmann · Answer 3 · 2017-07-09T02:01:30.060

4

You could use a nested defaultdict like this, goes arbitrarily deep automatically:

>>> from collections import defaultdict
>>> def ndd():
        return defaultdict(ndd)

>>> array = ndd()
>>> array['key']['subkey']['subsubkey'] = 'value'
>>> array
defaultdict(<function ndd at 0x041AC270>,
            {'key': defaultdict(<function ndd at 0x041AC270>,
                                {'subkey': defaultdict(<function ndd at 0x041AC270>,
                                                       {'subsubkey': 'value'})})})

(I indented the output for readability.)

Originally saw that from severb.

edited Jul 09 '17 at 02:01

answered Jul 09 '17 at 01:52

Stefan Pochmann

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The issue with using it is that the last value can't be of type list for example, can you find a solution for that ? array['key']['subkey']['subsubkey'].append(msg) will raise AttributeError which need to be handled array['key']['subkey']['subsubkey'] = [msg] – Sion C May 09 '23 at 13:50

Anton vBR · Answer 4 · 2017-07-09T01:29:02.017

3

I have used this alternative way before (edit: although I preferred Moses Koledoye answer):

array = {}

try:
    array[key][subkey] = 'value'
except KeyError:
    array[key] = {}
    array[key][subkey] = 'value'

edited Jul 09 '17 at 01:29

answered Jul 09 '17 at 01:22

Anton vBR

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score 0 · Answer 5 · answered Jul 09 '17 at 01:30

0

Can the below codes be used as a different method?

>>> array = {}
>>> array["key"] = {"subkey": "value"}
>>> array
{'key': {'subkey': 'value'}}
>>>

answered Jul 09 '17 at 01:30

dildeolupbiten

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1

Where I have used it (creating dictionaries inside loops) this is not an option as it does not "fill" the dictionary but rather creates a new one. – Anton vBR Jul 09 '17 at 01:33

How do I create a nested dictionary under a key that is yet to exist?

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