Adding key-value pair to a nested dictionary

Question

I'm having some trouble adding a key-value pair to a nested dictionary. The new pair seems to be getting added to ALL entries in the outer dictionary, instead of just the nested dictionary that I want.

Here's an example:

mykeys = ['key1', 'key2', 'key3']
mydictionary = dict.fromkeys(mykeys, {})
mydictionary['key1']['subkey1'] = 'value1'
mydictionary

I'm expecting to see the following, with 'subkey1' only being added to the 'key1' dictionary:

{'key1': {'subkey1': 'value1'},
 'key2': {},
 'key3': {}}

Instead, it adds the subkey1/value1 pair to every element of the outer dictionary:

{'key1': {'subkey1': 'value1'},
 'key2': {'subkey1': 'value1'},
 'key3': {'subkey1': 'value1'}}

Why is this the case, and how can I re-write my code to fix this mistake?

You provide the same object as subdictionary as value of all main dict keys. — Michael Butscher, Sep 10 '18 at 02:02
Instead of `fromkeys`, you can try `mydictionary = {k:{} for k in mykeys} ` — niraj, Sep 10 '18 at 02:09

score 4 · Answer 1 · answered Sep 10 '18 at 02:03

4

mydictionary['key1'] = {'subkey1': 'value1'}

answered Sep 10 '18 at 02:03

trandatnh

351
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2

Please add some explanation. – Mad Physicist Sep 10 '18 at 02:05
perfect , Thank you – karthik Mar 09 '21 at 21:30
2

by me it overwrites every time the dictionary:( – Qas Sep 14 '22 at 15:18

Adding key-value pair to a nested dictionary

1 Answers1