Setting a value in a nested python dictionary given a list of indices and value

Question

I\'m trying to programmatically set a value in a dictionary, potentially nested, given a list of indices and a value.

So for example, let\'s say my list of indices i

Answer 1

Something like this could help:

def nested_set(dic, keys, value):
    for key in keys[:-1]:
        dic = dic.setdefault(key, {})
    dic[keys[-1]] = value

And you can use it like this:

>>> d = {}
>>> nested_set(d, ['person', 'address', 'city'], 'New York')
>>> d
{'person': {'address': {'city': 'New York'}}}

Answer 2

First off, you probably want to look at setdefault

As a function I'd write it as

def get_leaf_dict( dict, key_list):
    res=dict
    for key in key_list:
        res=dict.setdefault( key, {} )
    return res

This would be used as:

get_leaf_dict( dict, ['Person', 'address', 'city']) = 'New York'

This could be cleaned up with error handling and such, also using *args rather than a single key-list argument might be nice; but the idea is that you can iterate over the keys, pulling up the appropriate dictionary at each level.

Answer 3

Here's a variant of Bakuriu's answer that doesn't rely on a separate function:

keys = ['Person', 'address', 'city']
value = 'New York'

nested_dict = {}

# Build nested dictionary up until 2nd to last key
# (Effectively nested_dict['Person']['address'] = {})
sub_dict = nested_dict
for key_ind, key in enumerate(keys[:-1]):
    if not key_ind:
        # Point to newly added piece of dictionary
        sub_dict = nested_dict.setdefault(key, {})
    else:
        # Point to newly added piece of sub-dictionary
        # that is also added to original dictionary
        sub_dict = sub_dict.setdefault(key, {})
# Add value to last key of nested structure of keys 
# (Effectively nested_dict['Person']['address']['city'] = value)
sub_dict[keys[-1]] = value

print(nested_dict)

>>> {'Person': {'address': {'city': 'New York'}}}

Answer 4

Here is my simple solution: just write

terms = ['person', 'address', 'city'] 
result = nested_dict(3, str)
result[terms] = 'New York'  # as easy as it can be

You can even do:

terms = ['John', 'Tinkoff', '1094535332']  # account in Tinkoff Bank
result = nested_dict(3, float)
result[terms] += 2375.30

Now the backstage:

from collections import defaultdict


class nesteddict(defaultdict):
    def __getitem__(self, key):
        if isinstance(key, list):
            d = self
            for i in key:
                d = defaultdict.__getitem__(d, i)
            return d
        else:
            return defaultdict.__getitem__(self, key)
    def __setitem__(self, key, value):
        if isinstance(key, list):
            d = self[key[:-1]]
            defaultdict.__setitem__(d, key[-1], value)
        else:
            defaultdict.__setitem__(self, key, value)


def nested_dict(n, type):
    if n == 1:
        return nesteddict(type)
    else:
        return nesteddict(lambda: nested_dict(n-1, type))

Answer 5

I took the freedom to extend the code from the answer of Bakuriu. Therefore upvotes on this are optional, as his code is in and of itself a witty solution, which I wouldn't have thought of.

def nested_set(dic, keys, value, create_missing=True):
    d = dic
    for key in keys[:-1]:
        if key in d:
            d = d[key]
        elif create_missing:
            d = d.setdefault(key, {})
        else:
            return dic
    if keys[-1] in d or create_missing:
        d[keys[-1]] = value
    return dic

When setting create_missing to True, you're making sure to only set already existing values:

# Trying to set a value of a nonexistent key DOES NOT create a new value
print(nested_set({"A": {"B": 1}}, ["A", "8"], 2, False))
>>> {'A': {'B': 1}}

# Trying to set a value of an existent key DOES create a new value
print(nested_set({"A": {"B": 1}}, ["A", "8"], 2, True))
>>> {'A': {'B': 1, '8': 2}}

# Set the value of an existing key
print(nested_set({"A": {"B": 1}}, ["A", "B"], 2))
>>> {'A': {'B': 2}}

Answer 6

Here's another option:

from collections import defaultdict
recursivedict = lambda: defaultdict(recursivedict)
mydict = recursivedict()

I originally got this from here: https://stackoverflow.com/a/10218517/1530754.

Quite clever and elegant if you ask me.