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']

gog · Accepted Answer · 2024-01-11 13:24:25Z

43

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}

With an arbitrary terminal value:

result = ... # terminal value 
for k in reversed(keys):
    result = {k:  result}
# result is the nested dict

edited Jan 11, 2024 at 13:24

gog

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answered Nov 3, 2016 at 12:49

Sven Marnach

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1 Comment

MisterMiyagi Over a year ago

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.

Guillaume · Accepted Answer · 2016-11-03 12:50:59Z

13

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 3, 2016 at 12:50

Guillaume

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Comments

mkrieger1 · Accepted Answer · 2021-07-01 20:13:47Z

10

This is a short solution:

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

edited Jul 1, 2021 at 20:13

mkrieger1

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answered Nov 3, 2016 at 12:53

Kh40tiK

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3 Comments

j-i-l Over a year ago

@Chris_Rands ah I totally agree with you. But readability is just one criterion.

Sven Marnach Over a year ago

You can save two characters by passing {} as the third argument to reduce() instead of prepending it to the list.

Kh40tiK Over a year ago

@Sven Marnach Good one!

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