Python nested dictionary and nested update

Question

I would like to update or create a nested dictionary without altering the existing content.

In the example below I will try to add the couple {'key2.2': 'value2.2'} nested as a child to the entry key2.

Case 1: the parent key (key2) exists :

    mydict = {'key1': 'value1', 'key2': {'key2.1': 'value2.1'}}
    mydict['key2'].update({'key2.2': 'value2.2'})
    pprint(mydict)

{'key1': 'value1', 'key2': {'key2.1': 'value2.1', 'key2.2': 'value2.2'}}

This is what I expected, no problem here.

Case 2: the parent key (key2) does not exists :*

    mydict = {'key1': 'value1'}
    mydict['key2'].update({'key2.2': 'value2.2'})

KeyError: 'key2'

Seems logic, so let's try something else..

    mydict = {'key1': 'value1'}
    mydict.update({'key2': {'key2.2': 'value2.2'}})
    pprint(mydict)

{'key1': 'value1', 'key2': {'key2.2': 'value2.2'}}

Perfect ! Let's check if this works with case 1 too.

Case 1B: the parent key (key2) exists :

    mydict = {'key1': 'value1', 'key2': {'key2.1': 'value2.1'}}
    mydict.update({'key2': {'key2.2': 'value2.2'}})
    pprint(mydict)

{'key1': 'value1', 'key2': {'key2.2': 'value2.2'}}

The existing entry {'key2.1': 'value2.1'} got deleted, this is not what I want.

What would be the best way to achieve what I want ? Like a nested update ? I know this would be possible with a couple of try:/except KeyError:, but it doesn't seems very clean.

I don't know in advance if I will be in the case 1 or case 2 situation, so I need a way to achieve the expected outcome no matter the case ! — Flof
– Flof, Commented May 12, 2020 at 12:28

Gábor Fekete · Accepted Answer · 2020-05-12 12:27:25Z

3

You could use defaultdict:

from collections import defaultdict

mydict = defaultdict(dict)
mydict.update({'key1': 'value1', 'key2': {'key2.1': 'value2.1'}})
mydict['key2'].update({'key2.2': 'value2.2'})
print(mydict)

mydict = defaultdict(dict)
mydict.update({'key1': 'value1'})
mydict['key2'].update({'key2.2': 'value2.2'})
print(mydict)

Outputs:

defaultdict(<class 'dict'>, {'key1': 'value1', 'key2': {'key2.1': 'value2.1', 'key2.2': 'value2.2'}})
defaultdict(<class 'dict'>, {'key1': 'value1', 'key2': {'key2.2': 'value2.2'}})

answered May 12, 2020 at 12:27

Gábor Fekete

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Comments

Gabio · Accepted Answer · 2020-05-12 12:27:57Z

0

Try:

mydict = {'key1': 'value1', 'key2': {'key2.1': 'value2.1'}}
if "key2" in mydict:
    mydict["key2"].update({'key2.2': 'value2.2'})
else:
    mydict["key2"] = {'key2.2': 'value2.2'}

answered May 12, 2020 at 12:27

Gabio

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Comments

Sowjanya R Bhat · Accepted Answer · 2020-05-12 12:44:44Z

0

Code:

mydict = {'key1': 'value1', 'key2': {'key2.1': 'value2.1'}}
mydict['key2.2'.split('.')[0]].update({'key2.2' : 'value2.2'})
print(mydict)

Output:

{'key2': {'key2.2': 'value2.2', 'key2.1': 'value2.1'}, 'key1': 'value1'}

answered May 12, 2020 at 12:44

Sowjanya R Bhat

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