lines_per_file = 300
smallfile = None
with open('really_big_file.txt') as bigfile:
    for lineno, line in enumerate(bigfile):
        if lineno % lines_per_file == 0:
            if smallfile:
                smallfile.close()
small_filename = 'small_file_{}.txt'.format(lineno + lines_per_file)
smallfile = open(small_filename, 'w')
smallfile.write(line)
if smallfile:
    smallfile.close()
Splitting large text file into smaller text files by line numbers using Python