文件倒讀

In [1]: cat /tmp/1.txt
a  
b
c
d
In [2]: ll /tmp/1.txt  應該是有8個字符，每個字符後邊都有一個換行符號\n,加上換行符共8個字節。
-rw-r--r--  1 admin  wheel  8  7  3 15:18 /tmp/1.txt
In [3]: f = open(‘/tmp/1.txt‘)
In [4]: f.tell()   指針位置：0 在文件的開頭
Out[4]: 0
In [5]: f.read(1) 讀一個字符
Out[5]: ‘a‘
In [6]: f.tell() 指針位置：1
Out[6]: 1
In [13]: f.read(1) 讀一個字符
Out[13]: ‘\n‘

In [14]: f.tell() 指針位置：2
Out[14]: 2
In [15]: f.read(2)  讀2個字節，b和\n
Out[15]: ‘b\n‘

In [16]: f.tell() 指針位置：4
Out[16]: 4

In [17]: f.read(100)
Out[17]: ‘c\nd\n‘

In [18]: f.tell()
Out[18]: 8

In [30]: help(f.seek)
Help on built-in function seek:

seek(...)
    seek(offset[, whence]) -> None.  Move to new file position.
    
    Argument offset is a byte count.  Optional argument whence defaults to
    0 (offset from start of file, offset should be >= 0); other values are 1
    (move relative to current position, positive or negative), and 2 (move
    relative to end of file, usually negative, although many platforms allow
    seeking beyond the end of a file).  If the file is opened in text mode,
    only offsets returned by tell() are legal.  Use of other offsets causes
    undefined behavior.
    Note that not all file objects are seekable.
    
    
0  從開頭指針位置移動
In [45]: f.seek(2)
In [46]: f.tell()
Out[46]: 2
In [10]: f.seek(0,0) 移動指針到開頭，從開頭向後移動0位
In [11]: f.tell()
Out[11]: 0
1  從當前指針位置移動
f.tell()
Out[42]: 5
In [43]: f.seek(-2,1)
In [44]: f.tell()    
Out[44]: 3

2  從結尾的指針位置開始移動move relative to end of file
In [8]: f.seek(3,2)   8+3=11從結尾8向後移動三位，
In [9]: f.tell()
Out[9]: 11
In [12]: f.seek(0,2) 把指針移動到最後
In [13]: f.tell()
Out[13]: 8

admindeMacBook-Air-62:~ admin$ cat /tmp/1.txt 
a
b
c
d
#!/usr/bin/env python

f = open(‘/tmp/1.txt‘)
f.seek(0,2)
while True:
    if f.tell() == 1:
        break
    else:
        f.seek(-2,1)
        c = f.read(1)
        print c,
        
python seek.py
       
d 
c 
b 
a



#!/usr/bin/env python
#encoding:utf8
import sys
f = open(‘/etc/hosts‘)
f.seek(0,2)
line = ‘‘

while True:
    if f.tell() == 1:
        print line  #打印1a的條件
        break
    else:
        f.seek(-2,1)
        c = f.read(1)
        if c != ‘\n‘:
            line = c + line #空＋d＝d，4+d＝4d
        else:
            print line
            line = ‘‘   #重置line＝空

python seek3.py
221.228.208.76 dmp.chinadep.com admin.chinadep.com

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文件倒讀