# Author:Lee Sir

 import sys,os

 des_file = r'E:\StartPython\day3\test.txt'

 des_file1 = r'E:\StartPython\day3\test1.txt'

 parameter = ['','Somehow','',des_file]

 def usage(parameter):

     if len(parameter) == 4:

         if isinstance(parameter[1],str) and isinstance(parameter[2],str):

             old_str = parameter[1]

             new_str = parameter[2]

             if os.path.exists(parameter[3]):

                 des_file = parameter[3]

                 return True, old_str, new_str, des_file

     return False

 def check_string_exist(old,file):

     with open(file,encoding='utf-8') as fd:

         for line in fd:

             if old not in line:

                 return False

             else:

                 return True

 def replace(old,new,file):

     with open(file,'r+',encoding='utf-8') as fd,open(des_file1,'w+',encoding='utf-8') as fd1:

         for line in fd:

             if old in line:

                 new_line = line.replace(old,new)

             else:

                 new_line = line

             print(new_line)

             fd1.write(new_line)

 def main():

     result = usage(parameter)

     if result:

         if check_string_exist(result[1],result[3]):

             replace(result[1],result[2],result[3])

         else:

             print('the %s is not found in %s ' % (result[1],result[3]))

     else:

         exit('USAGE: %s  old_str  new_str  des_file' % sys.argv[0])

 if __name__ == '__main__':

     main()