Creating a nested dictionary from a CSV file with Python

Question

I have a a csv file "input.csv" which has the following data.

UID,BID,R
U1,B1,4
U1,B2,3
U2,B1,2

I want the above to look like the following dictionary; group by the UID as they key and BID and R as a nested dictionary value.

{"U1":{"B1":4, "B2": 3}, "U2":{"B1":2}}

I have the below code:

new_data_dict = defaultdict(str)
with open("input.csv", 'r') as data_file:
    data = csv.DictReader(data_file, delimiter=",")
    headers = next(data)
    for row in data:
        new_data_dict[row["UID"]] += {row["BID"]:int(row["R"])}

The above throws an obvious error of

TypeError: cannot concatenate 'str' and 'dict' objects

Is there a way to do this?

Nils Werner · Accepted Answer · 2014-08-25 12:16:11Z

3

Using the regular dict() you can use get() to initialize a new sub-dict and fill it afterwards.

import csv

new_data_dict = {}
with open("data.csv", 'r') as data_file:
    data = csv.DictReader(data_file, delimiter=",")
    for row in data:
        item = new_data_dict.get(row["UID"], dict())
        item[row["BID"]] = int(row["R"])

        new_data_dict[row["UID"]] = item

print new_data_dict

Also, your call to next(data) was superfluous as the headers were automatically detected and stripped from the result.

answered Aug 25, 2014 at 12:16

Nils Werner

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Padraic Cunningham · Accepted Answer · 2014-08-25 12:45:39Z

2

This is a more efficient version using defaultdict:

from collections import defaultdict

new_data_dict = {}
with open("input.csv", 'r') as data_file:
    data_file.next()
    for row in data_file:
        row = row.strip().split(",")
        new_data_dict.setdefault(row[0],{})[row[1]] = int(row[2])

edited Aug 25, 2014 at 12:45

answered Aug 25, 2014 at 12:35

Padraic Cunningham

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