Double Nesting defaultdict

Poked around but couldn't figure it out, probably a very simple solution but please help me understand.

Source (sample.txt):

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

This:

import csv
from collections import defaultdict

input = "sample.txt"

with open(input) as f:
    r = csv.reader(f)
    d = defaultdict(list)
    rlabel = 1

    for row in r:
        d[rlabel].append(row)
        rlabel += 1

print(d)

Gets This:

defaultdict(<class 'list'>, {1: [['1', '1', '2', '3']], 2: [['2', '3', '2', '4', '4']]})

Why are there double brackets around my lists?

Solution

Why are there double brackets around my lists?

Your code works exactly as expected. The key point is the usage of extend and append.

append adds the parameter you passed as a single element. Due to a list is an object and your defaultdict class is list, so the list is appended as a list of lists.
extend method iterate in the input and extend the original list by adding all elements from an iterable.

So, in this case, if you want to add a single list to your defaultdict you should use list.extend method. And your output will be:

defaultdict(<class 'list'>, {1: ['1', '1', '2', '3'], 2: ['2', '3', '2', '4', '4']})