insert_dict_leaves

from copy import deepcopy

def insert_dict_leaves(source, additions):
    rv = deepcopy(source)
    for k, v in additions.items():
        if not (k in rv and isinstance(rv[k], dict)):
            rv[k] = v
        elif isinstance(v, dict):
            rv[k] = insert_dict_leaves(rv[k], v)
        else:
            rv[k] = v
    return rv

source_dict = {
    'untouched_scalar': 'untouched value',
    'untouched_dict': {
        'subkey': 'subkey value'
    },
    'touched_scalar': 'not yet touched value',
    'touched_dict': {
        'subkey 1': 'subkey 1 value'
    }
}

additions_dict = {
    'touched_scalar': 'rewritten value',
    'touched_dict': {
        'subkey 2': 'subkey 2 value'
    }
}
   
from pprint import pprint
pprint(insert_leaves(source_dict, additions_dict))

{'touched_dict': {'subkey 1': 'subkey 1 value', 'subkey 2': 'subkey 2 value'},
 'touched_scalar': 'rewritten value',
 'untouched_dict': {'subkey': 'subkey value'},
 'untouched_scalar': 'untouched value'}