Python string formatting too slow

Question

I use the following code to log a map, it is fast when it only contains zeroes, but as soon as there is actual data in the map it becomes unbearably slow... Is there any way to do this faster?

log_file = open('testfile', 'w')
for i, x in ((i, start + i * interval) for i in range(length)):
    log_file.write('%-5d %8.3f %13g %13g %13g %13g %13g %13g\n' % (i, x,
        map[0][i], map[1][i], map[2][i], map[3][i], map[4][i], map[5][i]))

Answer 1

I suggest you run your code using the cProfile module and postprocess the results as described on http://docs.python.org/library/profile.html . This will let you know exactly how much time is spent in the call to str.__mod__ for the string formatting and how much is spent doing other things, like writing the file and doing the __getitem__ lookups for map[0][i] and such.