How to create a nested dictionary from a list in Python?

Question

I have a list of strings: tree_list = ['Parents', 'Children', 'GrandChildren']

How can i take that list and convert it to a nested dictionary like this?

tree_dict = {
    'Parents': {
        'Children': {
            'GrandChildren' : {}
        }
    }
}

print tree_dict['Parents']['Children']['GrandChildren']

score 42 · Accepted Answer · answered Nov 03 '16 at 12:49

42

This easiest way is to build the dictionary starting from the inside out:

tree_dict = {}
for key in reversed(tree_list):
    tree_dict = {key: tree_dict}

answered Nov 03 '16 at 12:49

Sven Marnach

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1

Probably worth pointing out that if the leave has to be some other value than a `dict`, one can just initialise `tree_dict` accordingly. For example, `tree_dict = 42` leads to a nested dict where the innermost value is `42`. – MisterMiyagi Jun 10 '22 at 18:55

score 12 · Answer 2 · answered Nov 03 '16 at 12:50

12

Using a recursive function:

tree_list = ['Parents', 'Children', 'GrandChildren']

def build_tree(tree_list):
    if tree_list:
        return {tree_list[0]: build_tree(tree_list[1:])}
    return {}

build_tree(tree_list)

answered Nov 03 '16 at 12:50

Guillaume

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score 10 · Answer 3 · edited Jul 01 '21 at 20:13

10

This is a short solution:

lambda l:reduce(lambda x,y:{y:x},l[::-1],{})

edited Jul 01 '21 at 20:13

mkrieger1

19,194
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answered Nov 03 '16 at 12:53

Kh40tiK

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@Chris_Rands ah I totally agree with you. But readability is just one criterion. – j-i-l Nov 03 '16 at 13:17
1

You can save two characters by passing `{}` as the third argument to `reduce()` instead of prepending it to the list. – Sven Marnach Nov 03 '16 at 14:23
@Sven Marnach Good one! – Kh40tiK Nov 03 '16 at 14:27

How to create a nested dictionary from a list in Python?

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