exercise 20 函数和文件

from sys import argv

script, input_file = argv

def print_all(f):

    print f.read()

def rewind(f):

    f.seek(0)       ##seek参数为0时 表示将文件指针移到起始位置---- 同倒带rewind

def print_a_line(line_count, f):

    print line_count, f.readline()  ## 此处加 ，能避免每一行下面空一行？？？

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)

print_a_line(current_line+=1, current_file)

print_a_line(current_line+=1, current_file)

/or

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)

##########################what you see
$ python ex20.py test.txt
First let's print the whole file:

This is line 1
This is line 2
This is line 3

Now let's rewind, kind of like a tape.
Let's print three lines:
1 This is line 1

2 This is line 2

3 This is line 3   ##

file.seek(off, whence=0)：从文件中移动off个操作标记（文件指针），正往结束方向移动，负往开始方向移动。如果设定了whence参数，就以whence设定的起始位为准，0代表从头开始，1代表当前位置，2代表文件最末尾位置。

seek(offset,where):  where=0从起始位置移动（默认），1从当前位置移动，2从结束位置移动。当有换行时，会被换行截断。seek（）无返回值，故值为None。

Python文件操作seek()方法：此方法设置文件的当前位置偏移。

Why are there empty lines between the lines in the file?

The readline() function returns the \n that's in the file at the end of that line. Add a , at the end of your print function calls to avoid adding double\n to every line.