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 is:

['person', 'address', 'city'] 

and the value is

'New York' 

I want as a result a dictionary object like:

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

Basically, the list represents a 'path' into a nested dictionary.

I think I can construct the dictionary itself, but where I'm stumbling is how to set the value. Obviously if I was just writing code for this manually it would be:

dict['Person']['address']['city'] = 'New York' 

But how do I index into the dictionary and set the value like that programmatically if I just have a list of the indices and the value?

Python

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

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}}