How to split a huge csv file based on content of first column?

Question

• I have a 250MB+ huge csv file to upload
• file format is group_id, application_id, reading and data could look like

1, a1, 0.1
1, a1, 0.2
1, a1, 0.4
1, a1, 0.3
1, a1, 0.0
1, a1, 0.9
2, b1, 0.1
2, b1, 0.2
2, b1, 0.4
2, b1, 0.3
2, b1, 0.0
2, b1, 0.9
.....
n, x, 0.3(lets say)  

• I want to divide the file based on group_id, so output should be n files where n=group_id

Output

File 1

1, a1, 0.1
1, a1, 0.2
1, a1, 0.4
1, a1, 0.3
1, a1, 0.0
1, a1, 0.9

and

File2
2, b1, 0.1
2, b1, 0.2
2, b1, 0.4
2, b1, 0.3
2, b1, 0.0
2, b1, 0.9
.....

and

File n
n, x, 0.3(lets say)  

How can I do this effectively?

Answer 1

If the file is already sorted by group_id, you can do something like:

import csv
from itertools import groupby

for key, rows in groupby(csv.reader(open("foo.csv")),
                         lambda row: row[0]):
    with open("%s.txt" % key, "w") as output:
        for row in rows:
            output.write(",".join(row) + "\n")

Answer 2

awk is capable:

 awk -F "," '{print $0 >> ("FILE" $1)}' HUGE.csv

Answer 3

Sed one-liner:

sed -e '/^1,/wFile1' -e '/^2,/wFile2' -e '/^3,/wFile3' ... OriginalFile 

The only down-side is that you need to put in n -e statements (represented by the ellipsis, which shouldn't appear in the final version). So this one-liner might be a pretty long line.

The upsides, though, are that it only makes one pass through the file, no sorting is assumed, and no python is needed. Plus, it's a one-freaking-liner!