You can use map to transform a list, then join them up.

",".join( map( str, list_of_things ) )

BTW, this works for any objects (not just longs).

If you are dealing with a csv file that is something other than the trivial example here, please do remember you are better off using the csv module to read and write csv, since CSV can be surprisingly complex.

Here is a write example:

import csv

x = [['Header\n1', 'H 2', 'H 3', 'H, 4'],[1,2,3,4],[5,6,7,8],[9,10,11,12]]

with open('/tmp/test.csv', 'w') as fout:
    writer=csv.writer(fout)
    for l in x:
        writer.writerow(l)

Note the embedded comma in 'H, 4' and the embedded new line in 'Header\n1' that may trip up efforts to decode if not properly encoded.

The file 'test.csv' that the csv module produces:

"Header
1",H 2,H 3,"H, 4"
1,2,3,4
5,6,7,8
9,10,11,12

Note the quoting around "Header\n1" and "H, 4"` in the file so that the csv is correctly decoded in the future.

Now check you can read that back:

with open('/tmp/test.csv') as f:
    csv_in=[[int(e) if e.isdigit() else e for e in row] 
               for row in csv.reader(f)]

>>> csv_in==x
True

Just sayin' You are usually better off using the tools in the library for csv.