Python-сообщество

#!/usr/bin/env python3

import sys

if __name__ == '__main__':
    if len(sys.argv) != 4:
        print('usage: field infile outfile', file=sys.stderr)
        sys.exit(1)

    n = int(sys.argv[1])
    ifname = sys.argv[2]
    ofname = sys.argv[3]

    with open(ifname, encoding='utf-8') as fin, \
         open(ofname, "w", encoding='utf-8') as fout:
        state = 0
        for line in fin:
            if state == 0:
                if line == '\n':
                    fout.write(line)
                else:
                    assert n > 0
                    n -= 1
                    if n > 0:
                        fout.write(line)
                        state = 1
                    else:
                        state = 2
            elif state == 1:
                fout.write(line)
                if line == '\n':
                    state = 0
            elif state == 2:
                if line == '\n':
                    fout.write(line)
                    state = 3
            elif state == 3:
                fout.write(line)
    print('ok')