Try this. It only uses standard library and built in functions.
import csv # python has a csv module, see docs.
outfile = 'x.csv'
infile = 'x.data'
with open(outfile, 'w', newline = '') as fout: # need to add "newline = ''"" as csv does it own thing see docs
write = csv.writer(fout) # get a csv writer to output formatteded dats
with open(infile) as fin: # files closed at end with block
for line in fin: # looping over a text file gets you a new line each time.
line = line.strip() # get rid of excess space and end of line
lst = eval(line) # this works as the data in the input file looks like a list on each line
write.writerow(lst) # splits the list and adds the commas
Its not the shortest way to do it but I think its clear