Text21. Дан текстовый файл, содержащий более трех строк. Удалить из него последние три строки.

Решение на Python 3:

import random
from collections import deque

q = deque([])

f_input = "text21_in.txt"
f_output = "text21_out.txt"
#GenerateNumbers(f_input)
print("Read from:",f_input)
print("Write to:",f_output)

K = 3
N = random.randrange(K,15)
print("K = ",K)
print("N = ",N)
print("code of 'a' ", ord('a'))
try:
    f = open(f_input, "w")
    try:
        s = ''
        for i in range(N):
            s += chr(ord('a') + i)
            #print(s)
            f.write(s + "\n")
    finally:
        f.close()
except IOError:
    print('Файл не создан')

try:
    with open(f_input, 'r') as f_in, open(f_output, 'w') as f_out:
        for i in range(K):
            line = f_in.readline()
            q.append(line)
        #print(q)
        for line in f_in:
            q.append(line)
            line = q[0]
            f_out.write(line)
            q.popleft()
            #print(q)

except IOError:
    print('Open error: ',file1)