Python set dictionary nested key with dot delineated string

Question

If I have a dictionary that is nested, and I pass in a string like "key1.key2.key3" which would translate to:

myDict["key1"]["key2"]["key3"]

What would be an elegant way to be able to have a method where I could pass on that string and it would translate to that key assignment? Something like myDict.set_nested('key1.key2.key3', someValue)

I believe the following question can help you: https://stackoverflow.com/questions/1957780/how-to-override-the-operator-in-python — Itay, Aug 19 '19 at 16:05
This seems like what you're trying to do: https://stackoverflow.com/questions/14692690/access-nested-dictionary-items-via-a-list-of-keys — anderw, Aug 19 '19 at 16:16

score 4 · Answer 1 · answered Aug 19 '19 at 17:53

Using only builtin stuff:

def set(my_dict, key_string, value):
    """Given `foo`, 'key1.key2.key3', 'something', set foo['key1']['key2']['key3'] = 'something'"""

    # Start off pointing at the original dictionary that was passed in.
    here = my_dict

    # Turn the string of key names into a list of strings.
    keys = key_string.split(".")

    # For every key *before* the last one, we concentrate on navigating through the dictionary.
    for key in keys[:-1]:
        # Try to find here[key]. If it doesn't exist, create it with an empty dictionary. Then,
        # update our `here` pointer to refer to the thing we just found (or created).
        here = here.setdefault(key, {})

    # Finally, set the final key to the given value
    here[keys[-1]] = value


myDict = {}
set(myDict, "key1.key2.key3", "some_value")
assert myDict == {"key1": {"key2": {"key3": "some_value"}}}

This traverses myDict one key at a time, ensuring that each sub-key refers to a nested dictionary.

You could also solve this recursively, but then you risk RecursionError exceptions without any real benefit.

score 1 · Answer 2 · edited Jun 20 '20 at 09:12

1

There are a number of existing modules that will already do this, or something very much like it. For example, the jmespath module will resolve jmespath expressions, so given:

>>> mydict={'key1': {'key2': {'key3': 'value'}}}

You can run:

>>> import jmespath
>>> jmespath.search('key1.key2.key3', mydict)
'value'

The jsonpointer module does something similar, although it likes / for a separator instead of ..

Given the number of pre-existing modules I would avoid trying to write your own code to do this.

edited Jun 20 '20 at 09:12

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answered Aug 19 '19 at 16:13

larsks

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How can I do the opposite of search, and set nested dictionary path to a value? – Jasonca1 Aug 19 '19 at 17:46
is there no way to do this with dot notation? – Jasonca1 Aug 19 '19 at 18:04

David Culbreth · Answer 3 · 2019-08-19T16:47:54.237

EDIT: OP's clarification makes it clear that this answer isn't what he's looking for. I'm leaving it up here for people who find it by title.

I implemented a class that did this a while back... it should serve your purposes. I achieved this by overriding the default getattr/setattr functions for an object.

Check it out! AndroxxTraxxon/cfgutils

This lets you do some code like the following...

from cfgutils import obj

a = obj({
    "b": 123,
    "c": "apple",
    "d": {
        "e": "nested dictionary value"
    }
})

print(a.d.e)
>>> nested dictionary value

Python set dictionary nested key with dot delineated string

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