#函数和文件

 #readline：只读取文本文件的一行

 #seek(0):将读写位置移动到文件开头

 from sys import argv

 script , input_file = argv      # input_file = ex20.txt

                                 #ex20.txt 至少有三行数据

 def print_all(f):               # f 为文件名

     print(f.read())             #读取完文件后，指针指向文件末尾

 def rewind(f):

     f.seek(0)                   #seek(0):将读写位置移动到文件开头

 #readline：只读取文本文件的一行 ，下次读取自动接着下一行开始读取

 def print_a_line(line_count, f):

     print(line_count, f.readline())

 current_file = open(input_file)

 print("First let's print the whole file: \n")

 print_all(current_file)

 print("Now let's rewind, kind of like a tape.")

 rewind(current_file)

 print("Let's print three lines:")

 current_line = 1

 print_a_line(current_line, current_file)

 current_line = current_line + 1

 print_a_line(current_line, current_file)

 current_line = current_line + 1

 print_a_line(current_line, current_file)