学习要点

• f.seek(0)意思是回到文件的第一个字节位置，也就是文件开始。
• f.readline()读取文件每一行，直到找到一个\n为止，然后它就停止读取文件，并且返回之前发现的文件内容。同时文件记录每次调用'readline()'后的读取位置，这样它就可以在下次被调用时读取接下来的一行。
• +=意思是 x += y 表示 x = x + y。

程序代码

from sys import argv

script, input_file = argv

def print_all(f):
    print(f.read())

def rewind(f):
    f.seek(0)

def print_a_line(line_count,f):
    print(line_count,f.readline(),end='')

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)

current_line = current_line + 1
print_a_line(current_line,current_file)

current_line = current_line + 1
print_a_line(current_line,current_file)

输出结果

PS D:\lpthw> python ex20.py test.txt
Frist let's print the whole file:

Mary had a little lamb
It's fleece was white as snow
It was also tasty

Now let's rewind, kind of like a tape.
Let's print three lines:
1 Mary had a little lamb

2 It's fleece was white as snow

3 It was also tasty