Changing value from a nested dictionary in Python

Question

I'm just trying to change the value from a nested dictionary.

The first dictionary is empty and the second one is populated. When I'm trying to change the value of one of the item in the nested dictionary, also the value with the same key of the others items has been changed.

name = dict()
work = {"hour" : 4, "age":21}

name['bob'] = work
name['harry'] = work

name['bob']['hour'] = 7

Now I obtain this if i print the new name dictionary:

{'bob': {'hour': 7, 'age': 21}, 'harry': {'hour': 7, 'age': 21}}

I'd like to have this situation:

{'bob': {'hour': 7, 'age': 21}, 'harry': {'hour': 4, 'age': 21}}

Why does it change the hour from the dictionary of harry?

Use name['bob'] = work.copy() and name['harry'] = work.copy().It will pass the reference not the value. — jizhihaoSAMA
– jizhihaoSAMA, Commented May 5, 2020 at 14:56
They are both references to the same value. You're going to want to copy it. — Kenny Ostrom
– Kenny Ostrom, Commented May 5, 2020 at 14:58

David · Accepted Answer · 2020-05-05 15:09:55Z

1

This is because both names refer to the same element, i.e. references to the same value. You should copy the dict instead of simple assignment like the following:

name = dict()
work = {"hour" : 4, "age":21}

name['bob'] = work.copy()
name['harry'] = work.copy()

name['bob']['hour'] = 7

edited May 5, 2020 at 15:09

answered May 5, 2020 at 14:58

David

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1 Comment

Gabio Over a year ago

you don't need to import copy for shallow copy

Gabio · Accepted Answer · 2020-05-05 15:11:56Z

1

The problem is that you assign the same object that you defined before.

Try to use dict copy:

name = dict()
work = {"hour" : 4, "age":21}

name['bob'] = work.copy()
name['harry'] = work.copy()

name['bob']['hour'] = 7

If work was nested dictionary, you would have to use deepcopy (copy creates shallow copy which doesn't copy nested structures):

from copy import deepcopy
name['bob'] = deepcopy(work)
name['harry'] = deepcopy(work)

edited May 5, 2020 at 15:11

answered May 5, 2020 at 15:01

Gabio

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