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")