Writing and Reading Integer List of Lists to / from CSV file

July 29, 2024

I want to write a list of lists to csv file with one script and simply read it again into a list of lists with the same format with another script. Problem is when I read the csv file back, there is apostrophes around each list when printing the entire line, like this:

a = [  [ [1,2,3], [4,5,6], [7,8,9] ],
       [ [1,2,3], [4,5,6], [7,8,9] ],
       [ [1,2,3], [4,5,6], [7,8,9] ],
       [ [1,2,3], [4,5,6], [7,8,9] ],
       [ [1,2,3], [4,5,6], [7,8,9] ] ]

path = "enter path here"

print("")
print(a)
print("")

with open(path, 'w', newline='') as f:
    writer = csv.writer(f)
    writer.writerows(a)

with open(path, 'r') as csv_file:
    csv_reader = csv.reader(csv_file)
    rows = list(csv_reader)

    print(rows)

Printing list a looks like this as before writing expected:

[[[1, 2, 3], [4, 5, 6], [7, 8, 9]], [[1, 2, 3], [4, 5, 6], [7, 8, 9]], [[1, 2, 3], [4, 5, 6], [7, 8, 9]], [[1, 2, 3], [4, 5, 6], [7, 8, 9]], [[1, 2, 3], [4, 5, 6], [7, 8, 9]]]

But reading list rows back prints this:

[['[1, 2, 3]', '[4, 5, 6]', '[7, 8, 9]'], ['[1, 2, 3]', '[4, 5, 6]', '[7, 8, 9]'], ['[1, 2, 3]', '[4, 5, 6]', '[7, 8, 9]'], ['[1, 2, 3]', '[4, 5, 6]', '[7, 8, 9]'], ['[1, 2, 3]', '[4, 5, 6]', '[7, 8, 9]']]

But if I print this rows[0], there are no apostrophes:

[1, 2, 3]

I think this is caused by the way the list is written to csv as the apostrophes indicate that each inner list element is written to the csv file as a string. How can I get around this so I don’t read back the apostrophes? Or is there a simple way I can remove them?

>Solution :

A CSV is only two dimensions, and the columns are read in as strings, so post-processing of the column data could make it work. ast.literal_eval will convert a string representation of a list into an actual Python list:

import csv
import ast
from pprint import pprint

a = [[[1,2,3], [4,5,6], [7,8,9]],
     [[1,2,3], [4,5,6], [7,8,9]],
     [[1,2,3], [4,5,6], [7,8,9]],
     [[1,2,3], [4,5,6], [7,8,9]],
     [[1,2,3], [4,5,6], [7,8,9]]]

path = 'output.csv'

with open(path, 'w', newline='') as f:
    writer = csv.writer(f)
    writer.writerows(a)

with open(path, 'r', newline='') as csv_file:
    csv_reader = csv.reader(csv_file)
    rows = [[ast.literal_eval(col) for col in row] for row in csv_reader]

pprint(rows)

output.csv:

"[1, 2, 3]","[4, 5, 6]","[7, 8, 9]"
"[1, 2, 3]","[4, 5, 6]","[7, 8, 9]"
"[1, 2, 3]","[4, 5, 6]","[7, 8, 9]"
"[1, 2, 3]","[4, 5, 6]","[7, 8, 9]"
"[1, 2, 3]","[4, 5, 6]","[7, 8, 9]"

Terminal output:

[[[1, 2, 3], [4, 5, 6], [7, 8, 9]],
 [[1, 2, 3], [4, 5, 6], [7, 8, 9]],
 [[1, 2, 3], [4, 5, 6], [7, 8, 9]],
 [[1, 2, 3], [4, 5, 6], [7, 8, 9]],
 [[1, 2, 3], [4, 5, 6], [7, 8, 9]]]