Text10. Дано целое число K и текстовый файл. Вставить пустую строку после строки файла с номером K. Если строки с таким номером нет, то оставить файл без изменений.

Решение на Python 3:

import random
import string
import os
import sys

def CountF(fname):
#count number of rows in given text-file
    N = 0
    try:
        with open(fname,'r') as f:
            for line in f:
                N += 1
                #print(N,":",line)
    except IOError:
        print("Open error:",fname)
        return -1
    finally:
        return N
    
#file for reading and adding empty row
file1 = "text10.txt"

K = random.randrange(1,20)
print("Adding empty row before row #",K)

N = CountF(file1)
if N < K:
    print("File {0} has less then {1} rows".format(file1,K))
    sys.exit()

#file for temporary data
N = random.randrange(5,8)
S = ''.join(random.choice(string.ascii_uppercase + string.digits) for _ in range(N))
temp_file = "temp_" + S + ".txt"
print("Temp file:",temp_file)
print("Write to:",temp_file)
try:
    f_temp = open(temp_file, "w")
    try:
        N = 0
        try:
            with open(file1,'r') as f_open:
                for line in f_open:
                    if N == K:
                        f_temp.write("\n")
                    f_temp.write(line)
                    N += 1
        except IOError:
            print("Read error:",file1)
    finally:
        f_temp.close()
except IOError:
    print('Write error: ',file1)

print("Rename {0} to {1}".format(temp_file,file1))
try:
    os.rename(temp_file, file1)
except WindowsError:
    os.remove(file1)
    os.rename(temp_file, file1)
except OSError as e:
    print("Error:", e)