ファイルをM行を分割して処理

a.py

import itertools

file_line_num = 3

if __name__ == '__main__':
    for i in range(1, 10):
        with open("test.txt") as f:
            start = file_line_num * (i - 1)
            stop = file_line_num * i
            for l in itertools.islice(f, start, stop):
                print(i, start, stop, l.strip())


def func_improve(input_file_name):
    with open(input_file_name) as f:
        group_key_func = lambda x: x[0] // file_line_num
        for key, grouper in itertools.groupby(enumerate(f), group_key_func):
            writefile = str(key) + '.txt'
            with open(writefile, "w") as out:
                out.write('header\n')

                for line_num, line in grouper:
                    out.write(str(line_num) + ':deco_' + line.strip() + '__lated\n')

                out.write('footer\n')