ex20.py

# -*- coding: utf-8 -*-
from sys import argv

script, input_file = argv

def print_all(f):
    print (f.read())
    
def rewind(f):
    f.seek(0)  
#seek那个函数不返回值，你print淡然显示为None了，file.seek(0)是重新定位在文件的第0位及开始位置
#file.seek(3) 定位到第3个

def print_a_line(line_count, f):
    print (line_count, f.readline())  #如果readline 中加入数值，看看什么含义？
    
current_file = open(input_file)

print ("Frist 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),   #X += Y 和X = X + Y 意思相同

current_file.seek(20)   #注意到这个作用，它是将readline()中的读取后光标的位置重新定位到seek()中字符的位置

current_line = current_line + 1
print_a_line(current_line,current_file),

current_line = current_line + 1
print_a_line(current_line, current_file)