上节中的代码没有关闭文件，这样并不好。
一、代码
from sys import argv
script, filename = argv
print "We're going to erase %r." % filename
print "If you don't want that, hit CTRL+C."
print "If you do want that, hit RETURN."
raw_input("?")
print "Opening the file..."
target = open(filename, 'w')
print "Truncating the file. Goodbye!"
target.truncate()
print "Now I'm going to ask you for three lines."
line1 = raw_input("line 1: ")
line2 = raw_input("line 2: ")
line3 = raw_input("line 3: ")
print "I'm going to write these to the file."
target.write(line1)
target.write("\n")
target.write(line2)
target.write("\n")
target.write(line3)
target.write("\n")
print "And finally, we close it."
target.close()
这段代码的文件操作流程如下：
打开-清空-写入-关闭
文件打开模式见上一节。
文件写入后记得要close()。
二、更多的文件操作函数
close：关闭文件。跟你的编辑器的文件--》保存。。一个意思。
readline：读取文本文件中一行。
truncate：清空文件，请小心使用。
write：将字符串写入文件。
老办法，使用帮助命令查看各个函数：python -m pydoc file
1.close
close(...)
close() -> None or (perhaps) an integer. Close the file.
Sets data attribute .closed to True. A closed file cannot be used for
further I/O operations. close() may be called more than once without
error. Some kinds of file objects (for example, opened by popen())
may return an exit status upon closing.
一个已经被关闭的文件不能用于进一步的I/O操作
2.readline
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.
可选参数限定最大返回字节数
3.truncate
truncate(...)
truncate([size]) -> None. Truncate the file to at most size bytes.
Size defaults to the current file position, as returned by tell().
截断size大小的文件，size取决于当前文件的位置，这个位置可用tell()获取。
4.write
write(...)
write(str) -> None. 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.
返回值None.注意由于缓冲的作用，在数据保存到磁盘之前需要flush()或者close()
