Python編程中的文件讀寫(xiě)及相關(guān)的文件對(duì)象方法講解

python文件讀寫(xiě)
python 進(jìn)行文件讀寫(xiě)的內(nèi)建函數(shù)是open或file
file_hander(文件句柄或者叫做對(duì)象)= open(filename,mode)
mode：
模式    說(shuō)明
r        只讀
r+      讀寫(xiě)
w       寫(xiě)入,先刪除源文件,在重新寫(xiě)入,如果文件沒(méi)有則創(chuàng)建
w+     讀寫(xiě),先刪除源文件,在重新寫(xiě)入,如果文件沒(méi)有則創(chuàng)建(可以寫(xiě)入寫(xiě)出)
讀文件:    
>>> fo = open("/root/a.txt")
>>> fo    
<open file '/root/a.txt', mode 'r' at 0x7f5095dec4e0>    
>>> fo.read()    
'hello davehe\ni am emily\nemily emily\n'    
>>> fo.close()
>>> fo.read()                     #對(duì)象已關(guān)閉，在讀取就讀不到    
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
ValueError: I/O operation on closed file    
>>> f1 = file("/root/a.txt")         
>>> f1.read()    
'hello davehe\ni am emily\nemily emily\n'
    
>>> f1.close()

寫(xiě)文件:    
root@10.1.6.200:~# ls -l new.txt    
ls: cannot access new.txt: No such file or directory    
>>> fnew = open("/root/new.txt",'w')  w參數(shù)文件沒(méi)有則創(chuàng)建
>>> fnew.write('hello \n i am dave')

這時(shí)查看文件數(shù)據(jù)其實(shí)還只是在緩存區(qū)中,沒(méi)有真正落到文件上.    
root@10.1.6.200:~# cat new.txt
root@10.1.6.200:~#

只要我把文件關(guān)閉,數(shù)據(jù)會(huì)從緩存區(qū)寫(xiě)到文件里    
>>> fnew.close()
root@10.1.6.200:~# cat new.txt     
hello
i am dave

再次使用w參數(shù),文件會(huì)被清空,所以用該參數(shù)需要謹(jǐn)慎.    
>>> fnew = open("/root/new.txt","w")    
root@10.1.6.200:~# cat new.txt
root@10.1.6.200:~#

mode使用r+參數(shù):    
>>> fnew = open("/root/new.txt",'r+')
>>> fnew.read()    
'hello dave'    
>>> fnew.write('i am dave')
>>> fnew.close()    
root@10.1.6.200:~# cat new.txt     
hello davei am dave

這次打開(kāi)文件,直接寫(xiě)入,會(huì)發(fā)現(xiàn)ooo替換開(kāi)頭字母,因?yàn)樯厦孀x取操作使用了指針在寫(xiě)就寫(xiě)在后面.而這次是直接從頭寫(xiě)入.    
>>> fnew = open("/root/new.txt",'r+')
>>> fnew.write('ooo')
>>> fnew.close()    
root@10.1.6.200:~# cat new.txt     
ooolo davei am dave

文件對(duì)象方法
下面文件對(duì)象方法

    FileObject.close()
    String=FileObject.readline([size])
    List = FileObject.readlines([size])
    String = FileObject.read([size])   read:讀取所有數(shù)據(jù)
    FileObject.next()          
    FileObject.write(string)
    FileObject.writelines(List)
    FlieObject.seek(偏移量,選項(xiàng))
    FlieObject.flush() 提交更新    
>>> for i in open("/root/a.txt"):  用open可以返回迭代類型的變量,可以逐行讀取數(shù)據(jù)
...   print i
...     
hello davehe
i am emily
emily emily

FileObject.readline: 每次讀取文件的一行,size是指每行每次讀取size個(gè)字節(jié)，直到行的末尾，超出范圍會(huì)讀取空字符串    
>>> f1 = open("/root/a.txt")
>>> f1.readline()    
'hello davehe\n'    
>>> f1.readline()    
'i am emily\n'    
>>> f1.readline()    
'emily emily\n'    
>>> f1.readline()
''
>>> f1.readline()
''
>>>f1.close()

 FileObject.readlines:返回一個(gè)列表    
>>> f1 = open("/root/a.txt")
>>> f1.readlines()    
['hello davehe\n', 'i am emily\n', 'emily emily\n']''

FileObject.next:返回當(dāng)前行,并將文件指針到下一行,超出范圍會(huì)給予警示,停止迭代.    
>>> f1 = open("/root/a.txt")
>>> f1.next()    
'hello davehe\n'    
>>> f1.next()    
'i am emily\n'    
>>> f1.next()    
'emily emily\n'    
>>> f1.next()    
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
StopIteration

FileObject.write:write和后面writelines在寫(xiě)入前會(huì)是否清除文件中原來(lái)所有的數(shù)據(jù),在重新寫(xiě)入新的內(nèi)容,取決于打開(kāi)文件的模式.

FileObject.writelines(List):多行寫(xiě),效率比write高,速度更快,少量寫(xiě)入可以使用write    
>>> l = ["python\n","python\n","python\n"]
>>> f1 = open('/root/a.txt','a')
>>> f1.writelines(l)
>>> f1.close()    
root@10.1.6.200:~# cat a.txt     
hello davehe
i am emily
emily emily
python
python
python

FlieObject.seek(偏移量,選項(xiàng)):可以在文件中移動(dòng)文件指針到不同的位置.

位置的默認(rèn)值為0,代表從文件開(kāi)頭算起(即絕對(duì)偏移量),1代表從當(dāng)前位置算起,2代表從文件末尾算起.    
>>> f1 = open('/root/a.txt','r+')
>>> f1.read()    
'hello davehe\ni am emily\nemily emily\npython\npython\npython\n'    
>>> f1.seek(0,0)   指針指到開(kāi)頭,在讀
>>> f1.read()    
'hello davehe\ni am emily\nemily emily\npython\npython\npython\n'    
>>> f1.read()
''
>>> f1.seek(0,0)
>>> f1.seek(0,2)   指針指到末尾,在讀
>>> f1.read()
''
下面看個(gè)小實(shí)例,查找a.txt中emily出現(xiàn)幾次    
root@10.1.6.200:~# vim file.py     
#!/usr/bin/env python
import re
f1 = open('/root/a.txt')
count = 0
for s in f1.readlines():
  li = re.findall("emily",s)
  if len(li) > 0:
    count = count + len(li)
print "this is have %d emily" % count
f1.close()    
root@10.1.6.200:~# cat a.txt     
hello davehe
i am emily
emily emily    
root@10.1.6.200:~# python file.py     
this is have 3 emily