Nested dictionaries from file in Python

Question

So I'm trying to create a nested dictionary but I can't seem to wrap my head around the logic.

So say I have input coming in from a csv:

1,2,3
2,3,4
1,4,5

Now'd like to create a dictionary as follows:

d ={1:{2:3,4:5}, 2:{3:4}}

Such that for the first being some ID column that we create keys in the sub dictionary corresponding to second value.

The way I tried it was to go:

d[row[0]] = {row[1]:row[2]}

But that overwrites the first instead of appending/pushing to it, how would I go about this problem? I can't seem to wrap my mind around what keys to use.

Any guidance is appreciated.

dict overwrites a builtin function. Pick a different name for your dictionary. dict[row[0]] = ... just overwrites the existing object with a new one. If it exists, add a new property on it. — ggorlen
– ggorlen, Commented Aug 7, 2019 at 22:18
Should've been more clear, I just wrote that example. I'll change the name. — fynmnx
– fynmnx, Commented Aug 7, 2019 at 22:19

snamef · Accepted Answer · 2019-08-07 22:15:24Z

2

Yes, cause dict[row[0]] = is dict[1] = what overwrites previous dict[1] value You should use :

dict.setdefault(row[0],{})[row[1]] = row[2]

remember there must be no duplicates for row[1] then

or

dict.setdefault(row[0],{}).update({row[1]:row[2]})

answered Aug 7, 2019 at 22:15

snamef

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2 Comments

fynmnx Over a year ago

I didn't know about set default that was what I was looking for. Thank you!

snamef Over a year ago

oh yes and you should not call the valiable dict cause it overwrites the dict type

Guilherme Lopes · Accepted Answer · 2019-08-07 22:19:12Z

0

if dict[row[0]] == None:
    dict[row[0]] = {row[1]:row[2]}
else:
    dict[row[0]][row[1]] = row[2]

answered Aug 7, 2019 at 22:19

Guilherme Lopes

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1 Comment

ggorlen Over a year ago

No need for == None (switch the statements in the blocks), and it's best practice to use is None if you do need to compare against None. Correct answer though, thanks!

Djaballah Mohammed DJEDID · Accepted Answer · 2019-08-07 22:25:35Z

0

you can use defaultdict, which is similaire to the built-in dictionaries, but will create dictionaries with a default values, in your case a default value will be a dictionary

from collections import  defaultdict
res = defaultdict(dict)

with this code we are creating a defaultdict, with it's values default value being of type dict, so next we would do

for row in l:
    res[row[0]][row[1]] = row[2]

answered Aug 7, 2019 at 22:25

Djaballah Mohammed DJEDID

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