一 操作文件
操作文件包括了文件的读、写和关闭，首先来谈谈打开方式：当我们执行 文件句柄 = open('文件路径', '模式') 操作的时候，要传递给open方法一个表示模式的参数：
打开文件的模式有：
- r，只读模式（默认）。
- w，只写模式。【不可读；不存在则创建；存在则删除内容；】
- a，追加模式。【可读； 不存在则创建；存在则只追加内容；】
"+" 表示可以同时读写某个文件
- r+，可读写文件。【可读；可写；可追加】
- w+，先写再读。【这个方法打开文件会清空原本文件中的所有内容，将新的内容写进去，之后也可读取已经写入的内容】
- a+，同a
"U"表示在读取时，可以将 自动转换成 （注意：只能与 r 或 r+ 模式同使用）
- rU
- r+U
- rbU
- rb+U
"b"表示处理二进制文件（如：FTP发送上传ISO镜像文件，linux可忽略，windows处理二进制文件时需标注）
- rb
- wb
- ab
以下是file操作的源码解析：
1 class file(object): 2 3 def close(self): # real signature unknown; restored from __doc__ 4 关闭文件 5 6 """close() -> None or (perhaps) an integer. Close the file. 7 8 Sets data attribute .closed to True. A closed file cannot be used for 9 further I/O operations. close() may be called more than once without 10 error. Some kinds of file objects (for example, opened by popen()) 11 may return an exit status upon closing. 12 """ 13 14 def fileno(self): # real signature unknown; restored from __doc__ 15 文件描述符 16 17 """fileno() -> integer "file descriptor". 18 19 This is needed for lower-level file interfaces, such os.read(). """ 20 21 return 0 22 23 def flush(self): # real signature unknown; restored from __doc__ 24 刷新文件内部缓冲区 25 26 """ flush() -> None. Flush the internal I/O buffer. """ 27 28 pass 29 30 def isatty(self): # real signature unknown; restored from __doc__ 31 判断文件是否是同意tty设备 32 33 """ isatty() -> true or false. True if the file is connected to a tty device. """ 34 35 return False 36 37 def next(self): # real signature unknown; restored from __doc__ 38 获取下一行数据，不存在，则报错 39 40 """ x.next() -> the next value, or raise StopIteration """ 41 42 pass 43 44 45 46 def read(self, size=None): # real signature unknown; restored from __doc__ 47 读取指定字节数据 48 49 """read([size]) -> read at most size bytes, returned as a string. 50 51 If the size argument is negative or omitted, read until EOF is reached. 52 Notice that when in non-blocking mode, less data than what was requested 53 may be returned, even if no size parameter was given.""" 54 55 pass 56 57 def readinto(self): # real signature unknown; restored from __doc__ 58 读取到缓冲区，不要用，将被遗弃 59 60 """ readinto() -> Undocumented. Don't use this; it may go away. """ 61 62 pass 63 64 65 def readline(self, size=None): # real signature unknown; restored from __doc__ 66 仅读取一行数据 67 """readline([size]) -> next line from the file, as a string. 68 69 Retain newline. A non-negative size argument limits the maximum 70 number of bytes to return (an incomplete line may be returned then). 71 Return an empty string at EOF. """ 72 73 pass 74 75 def readlines(self, size=None): # real signature unknown; restored from __doc__ 76 读取所有数据，并根据换行保存值列表 77 78 """readlines([size]) -> list of strings, each a line from the file. 79 80 Call readline() repeatedly and return a list of the lines so read. 81 The optional size argument, if given, is an approximate bound on the 82 total number of bytes in the lines returned. """ 83 84 return [] 85 86 87 88 def seek(self, offset, whence=None): # real signature unknown; restored from __doc__ 89 指定文件中指针位置 90 """seek(offset[, whence]) -> None. Move to new file position. 91 92 Argument offset is a byte count. Optional argument whence defaults to 93 0 (offset from start of file, offset should be >= 0); other values are 1 94 (move relative to current position, positive or negative), and 2 (move 95 relative to end of file, usually negative, although many platforms allow 96 seeking beyond the end of a file). If the file is opened in text mode, 97 only offsets returned by tell() are legal. Use of other offsets causes 98 undefined behavior. 99 Note that not all file objects are seekable. """ 100 101 pass 102 103 104 105 def tell(self): # real signature unknown; restored from __doc__ 106 获取当前指针位置 107 108 """ tell() -> current file position, an integer (may be a long integer). """ 109 pass 110 111 112 def truncate(self, size=None): # real signature unknown; restored from __doc__ 113 截断数据，仅保留指定之前数据 114 115 """ truncate([size]) -> None. Truncate the file to at most size bytes. 116 117 Size defaults to the current file position, as returned by tell().“"" 118 119 pass 120 121 122 123 def write(self, p_str): # real signature unknown; restored from __doc__ 124 写内容 125 126 """write(str) -> None. Write string str to file. 127 128 Note that due to buffering, flush() or close() may be needed before 129 the file on disk reflects the data written.""" 130 131 pass 132 133 def writelines(self, sequence_of_strings): # real signature unknown; restored from __doc__ 134 将一个字符串列表写入文件 135 """writelines(sequence_of_strings) -> None. Write the strings to the file. 136 137 Note that newlines are not added. The sequence can be any iterable object 138 producing strings. This is equivalent to calling write() for each string. """ 139 140 pass 141 142 143 144 def xreadlines(self): # real signature unknown; restored from __doc__ 145 可用于逐行读取文件，非全部 146 147 """xreadlines() -> returns self. 148 149 For backward compatibility. File objects now include the performance 150 optimizations previously implemented in the xreadlines module. """ 151 152 pass
obj1 = open('filetest.txt','w+')
obj1.write('I heard the echo, from the valleys and the heart
')
obj1.writelines(['Open to the lonely soul of sickle harvesting
',
'Repeat outrightly, but also repeat the well-being of
',
'Eventually swaying in the desert oasis'])
obj1.seek(0)
print obj1.readline()
print obj1.tell()
print obj1.readlines()
obj1.close()
以‘w+’的打开方式为例，write是向文件中写入一个字符串，而writelines是想文件中写入一个字符串数组。
写文件操作
write,writelines，相比于那些五花八门的读方法，写方法就单纯的多了，只有wite和writelines两种。看下面的例子和写入的结果，其实write方法和writelines方法都差不多，只不过一个接受的参数是list格式，一个接受的参数是字符串格式而已。这里使用的时候要注意换行符。
1 obj1 = open('E:PythonL\11-8\filetest.txt','r') 2 obj1 = open('filetest.txt','w+') 3 obj1.write('I heard the echo, from the valleys and the heart Open to the lonely soul of sickle harvesting ') 4 obj1.writelines([ 5 'Repeat outrightly, but also repeat the well-being of ', 6 'Eventually swaying in the desert oasis' 7 ])
读文件操作
我们以上面这个文件为例，来说说读文件：
首先来看一下直接读取文件中所有内容的方法read和readlines，从下面的结果来看就知道这两种方法一个返回列表，一个是返回字符串，和上面的write方法相对应：
1 #readline方法
2 obj1 = open('E:PythonL\11-8\filetest.txt','r')
3 print 'readlines:',obj1.readlines()
5 #readline方法
6 print "read:",obj1.read()
1 readlines: ['I heard the echo, from the valleys and the heart ', 'Open to the lonely soul of sickle harvesting ', 'Repeat outrightly, but also repeat the well-being of ', 'Eventually swaying in the desert oasis']
1 read: I heard the echo, from the valleys and the heart 2 Open to the lonely soul of sickle harvesting 3 Repeat outrightly, but also repeat the well-being of 4 Eventually swaying in the desert oasis
readlines和read方法虽然简便好用，但是如果这个文件很庞大，那么一次性读入内存就降低了程序的性能，这个时候我们就需要一行一行的读取文件来降低内存的使用率了。
readline,next,xreadlines:用来按行读取文件，其中需要仔细看xreadlines的用法，因为xreadlines返回的是一个迭代器，并不会直接返回某一行的内容
需要注意的是，尽管我把这一大坨代码放在一起展示，但是要是真的把这一大堆东西放在一起执行，就会报错（ValueError: Mixing iteration and read methods would lose data），具体的原因下面会进行解释
1 obj1 = open('E:PythonL\11-8\filetest.txt','r') 2 #readline方法 3 print "readline:",obj1.readline() 5 #readline方法 6 print "next:",obj1.next() 8 #readline方法 9 r = obj1.xreadlines() 10 print 'xreadlines:',r.next() 12 #readline方法 13 print 'readlines:',obj1.readlines() 15 #readline方法 16 print "read:",obj1.read(）
左侧是代码，右侧是执行结果。