Reemplazar linea de un archivo

#!/usr/bin/python
import shutil
out = open('file2.txt', 'w')

for line in open('file.txt'):
    if line.strip() == '123':
        line = "AAAAAAAAAAA123\n"
    out.write(line)
out.close()
shutil.move('file2.txt', 'file.txt')