python文件读写操作

http://blog.csdn.net/xia7139/article/details/25544015

python中，可以通过open()函数打开一个文件创建一个file类的对象来操作文件，也可以在打开文件创建file对象时指定文件打开的模式（如果没有指定打开模式，默认为r），来决定能对文件进行的操作。这里说的文件读写操作就是利用file类中提供的read、readline、readlines和write等方法来操作文件。

1、read和write

 <strong>read([size]) -> read at most size bytes, returned as a string.</strong>
 If the size argument is negative or omitted, read until EOF is reached. Notice that when in non-blocking mode, less data than what was requested may be returned, even if no size parameter was given.
 <strong>write(str) -> None. </strong>
 Write string str to file. Note that due to buffering, flush() or close() may be needed before the file on disk reflects the data written.

read函数除非指定size，否则会默认读取文件的全部内容并返回。write函数会将一个str写入到文件当前写入内容的末尾（意思就是，f.write(str1), f.write(str2)，str2会出现在str1的后面）。

2、打开模式

和其它编程语言类似，python中的文件打开模式也有三种：读模式（'r'）、写模式（'w'）和追加模式（‘a’）。读模式下打开的文件只能用来进行读操作；写模式下打开的文件能进行写操作，但是文件中原来的内容会被清空掉；追加模式下打开的文件能进行写操作，而且所有被写入的内容都会追加在原来文件的末尾。

2.1 写模式和追加模式

写模式和追加模式下打开的文件如果不存在，该文件会默认被创建，其内容为空。

(1) r mode: write to a file

 $ cat file_w.py
 <span style="color:#663366;">filename = "temp.txt"
 f = open(filename,"w")
 f.write("hello, world.\n")
 f.write("hi, python!\n")
 f.close()</span>
 $ python file_w.py
 $ cat temp.txt
 <span style="color:#663366;">hello, world.
 hi, python!</span>

(2) r mode: write to the same file again

 $ cat file_w.py
 <span style="color:#663366;">filename = "temp.txt"
 f = open(filename,"w")
 f.write("Be serious!\n")
 f.write("Not funny at all!\n")
 f.close()</span>
 $ python file_w.py
 $ cat temp.txt
 <span style="color:#663366;">Be serious!
 Not funny at all!</span>

可以看出，之前文件中的内容都被清空了。

(3) a mode: write to the same file

 $ cat file_a.py
 filename = "temp.txt"
 f = open(filename,"a")
 f.write("hello, world.\n")
 f.write("hi, python!\n")
 f.close()
 $ python file_a.py
 $ cat temp.txt
 Be serious!
 Not funny at all!
 hello, world.
 hi, python!

2.2 读模式

对于读模式下打开的文件，可以进行读取操作。如果读取模式下打开一个不存在的文件，会报错(IOError: [Errno 2] No such file or directory)。

(1) read

像上文中说的read默认读取整个文件的内容并返回。

 $ cat file_r.py
 <span style="color:#663366;">filename = "temp.txt"
 f = open(filename,"r")
 print f.read()
 f.close()</span>
 $ python file_r.py
 <span style="color:#663366;">Be serious!
 Not funny at all!
 hello, world.
 hi, python!</span>

(2) readline

 readline([size]) -> next line from the file, as a string.
 Retain newline.  A non-negative size argument limits the maximum number of bytes to return (an incomplete line may be returned then).Return an empty string at EOF.

读取文件的一行，保留换行符，到文件末尾返回空串。

 $ cat file_readline.py
 <span style="color:#663366;">filename = "temp.txt"
 f = open(filename,"r")
 line =  f.readline()
 while line://最后的空行也会被打印出来
     #readline will retain an enter, so add ',' at the end to remove the enter of print
     print line,
     line = f.readline()
 f.close()</span>
 <span style="color:#663366;">$ cat temp.txt
 Be serious!
 Not funny at all!
 hello, world.
 hi, python!
 //这里有一个空行</span>
 $ python file_readline.py
 <span style="color:#663366;">Be serious!
 Not funny at all!
 hello, world.
 hi, python!
 //这里最后也会打出空行</span>

(3) readlines

 <strong>readlines([size]) -> list of strings, each a line from the file.</strong>
 Call readline() repeatedly and return a list of the lines so read. The optional size argument, if given, is an approximate bound on the total number of bytes in the lines returned.

重复调用readline读取文件中的所有行到一个list。

 $ cat file_readlines.py
 filename = "temp.txt"
 f = open(filename,"r")
 lines =  f.readlines()
 for line in lines:
     print line,
 f.close()
 $ cat temp.txt
 Be serious!
 Not funny at all!
 hello, world.
 hi, python!
 //这里有一个空行
 $ python file_readlines.py
 Be serious!
 Not funny at all!
 hello, world.
 hi, python!
 //这里最后也会打出空行

好了，到这里就可以用python进行文件的基本操作了，后面有复杂的需求会进一步补充。^_^http://blog.csdn.net/xia7139/article/details/25544015