Updating nested dictionaries when data has existin

Question

I am trying to update values in a nested dictionary, without over-writting previous entries when the key already exists. For example, I have a dictionary:

  myDict = {}
  myDict["myKey"] = { "nestedDictKey1" : aValue }

giving,

 print myDict
>> { "myKey" : { "nestedDictKey1" : aValue }}

Now, I want to add another entry , under "myKey"

myDict["myKey"] = { "nestedDictKey2" : anotherValue }}

This will return:

print myDict
>> { "myKey" : { "nestedDictKey2" : anotherValue }}

But I want:

print myDict
>> { "myKey" : { "nestedDictKey1" : aValue , 
                 "nestedDictKey2" : anotherValue }}

Is there a way to update or append "myKey" with new values, without overwriting the previous ones?

Answer 1

You could treat the nested dict as immutable:

myDict["myKey"] = dict(myDict["myKey"], **{ "nestedDictKey2" : anotherValue })

Answer 2

This is a very nice general solution to dealing with nested dicts:

import collections
def makehash():
    return collections.defaultdict(makehash)

That allows nested keys to be set at any level:

myDict = makehash()
myDict["myKey"]["nestedDictKey1"] = aValue
myDict["myKey"]["nestedDictKey2"] = anotherValue
myDict["myKey"]["nestedDictKey3"]["furtherNestedDictKey"] = aThirdValue

For a single level of nesting, defaultdict can be used directly:

from collections import defaultdict
myDict = defaultdict(dict)
myDict["myKey"]["nestedDictKey1"] = aValue
myDict["myKey"]["nestedDictKey2"] = anotherValue

And here's a way using only dict:

try:
  myDict["myKey"]["nestedDictKey2"] = anotherValue
except KeyError:
  myDict["myKey"] = {"nestedDictKey2": anotherValue}

Answer 3

myDict["myKey"]["nestedDictKey2"] = anotherValue

myDict["myKey"] returns the nested dictionary to which we can add another key like we do for any dictionary :)

Example:

>>> d = {'myKey' : {'k1' : 'v1'}}
>>> d['myKey']['k2'] = 'v2'
>>> d
{'myKey': {'k2': 'v2', 'k1': 'v1'}}

Answer 4

You can use collections.defaultdict for this, and just set the key-value pairs within the nested dictionary.

from collections import defaultdict
my_dict = defaultdict(dict)
my_dict['myKey']['nestedDictKey1'] = a_value
my_dict['myKey']['nestedDictKey2'] = another_value

Alternatively, you can also write those last 2 lines as

my_dict['myKey'].update({"nestedDictKey1" : a_value })
my_dict['myKey'].update({"nestedDictKey2" : another_value })