[python拾遗]文件操作
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文件操作
1.open()函数
open()函数主要用于文件处理，一般分为下面3个过程：
1.打开文件
2.操作文件
3.关闭文件
常见的格式示例：
f = open(‘note.txt‘,‘r‘) f.read() f.close()
1.打开文件
文件句柄 = open(‘文件路径‘,‘模式‘)
常见的模式有：
1.‘r’，只读
2.‘w’，只写（当对打开执行只写操作后，文件原内容将会被清空，注意备份）
3.‘a’,追加
"+" 表示可以同时读写某个文件
1.‘r+’
2.‘w+’
3.‘a+’
“b”表示处理二进制文件
1.‘rb’，‘rb+’
2.‘wb’,‘wb+’
3.‘ab’,‘ab+’
"U"表示在读取时，可以将 \r \n \r\n自动转换成 \n （与 r 或 r+ 模式同使用）
1.‘rU’
2.‘r+U’
2.操作文件
1 class file(object) 2 def close(self): # real signature unknown; restored from __doc__ 3 关闭文件 4 """ 5 close() -> None or (perhaps) an integer. Close the file. 6 7 Sets data attribute .closed to True. A closed file cannot be used for 8 further I/O operations. close() may be called more than once without 9 error. Some kinds of file objects (for example, opened by popen()) 10 may return an exit status upon closing. 11 """ 12 13 def fileno(self): # real signature unknown; restored from __doc__ 14 文件描述符 15 """ 16 fileno() -> integer "file descriptor". 17 18 This is needed for lower-level file interfaces, such os.read(). 19 """ 20 return 0 21 22 def flush(self): # real signature unknown; restored from __doc__ 23 刷新文件内部缓冲区 24 """ flush() -> None. Flush the internal I/O buffer. """ 25 pass 26 27 28 def isatty(self): # real signature unknown; restored from __doc__ 29 判断文件是否是同意tty设备 30 """ isatty() -> true or false. True if the file is connected to a tty device. """ 31 return False 32 33 34 def next(self): # real signature unknown; restored from __doc__ 35 获取下一行数据，不存在，则报错 36 """ x.next() -> the next value, or raise StopIteration """ 37 pass 38 39 def read(self, size=None): # real signature unknown; restored from __doc__ 40 读取指定字节数据 41 """ 42 read([size]) -> read at most size bytes, returned as a string. 43 44 If the size argument is negative or omitted, read until EOF is reached. 45 Notice that when in non-blocking mode, less data than what was requested 46 may be returned, even if no size parameter was given. 47 """ 48 pass 49 50 def readinto(self): # real signature unknown; restored from __doc__ 51 读取到缓冲区，不要用，将被遗弃 52 """ readinto() -> Undocumented. Don‘t use this; it may go away. """ 53 pass 54 55 def readline(self, size=None): # real signature unknown; restored from __doc__ 56 仅读取一行数据 57 """ 58 readline([size]) -> next line from the file, as a string. 59 60 Retain newline. A non-negative size argument limits the maximum 61 number of bytes to return (an incomplete line may be returned then). 62 Return an empty string at EOF. 63 """ 64 pass 65 66 def readlines(self, size=None): # real signature unknown; restored from __doc__ 67 读取所有数据，并根据换行保存值列表 68 """ 69 readlines([size]) -> list of strings, each a line from the file. 70 71 Call readline() repeatedly and return a list of the lines so read. 72 The optional size argument, if given, is an approximate bound on the 73 total number of bytes in the lines returned. 74 """ 75 return [] 76 77 def seek(self, offset, whence=None): # real signature unknown; restored from __doc__ 78 指定文件中指针位置 79 """ 80 seek(offset[, whence]) -> None. Move to new file position. 81 82 Argument offset is a byte count. Optional argument whence defaults to 83 (offset from start of file, offset should be >= 0); other values are 1 84 (move relative to current position, positive or negative), and 2 (move 85 relative to end of file, usually negative, although many platforms allow 86 seeking beyond the end of a file). If the file is opened in text mode, 87 only offsets returned by tell() are legal. Use of other offsets causes 88 undefined behavior. 89 Note that not all file objects are seekable. 90 """ 91 pass 92 93 def tell(self): # real signature unknown; restored from __doc__ 94 获取当前指针位置 95 """ tell() -> current file position, an integer (may be a long integer). """ 96 pass 97 98 def truncate(self, size=None): # real signature unknown; restored from __doc__ 99 截断数据，仅保留指定之前数据 100 """ 101 truncate([size]) -> None. Truncate the file to at most size bytes. 102 103 Size defaults to the current file position, as returned by tell(). 104 """ 105 pass 106 107 def write(self, p_str): # real signature unknown; restored from __doc__ 108 写内容 109 """ 110 write(str) -> None. Write string str to file. 111 112 Note that due to buffering, flush() or close() may be needed before 113 the file on disk reflects the data written. 114 """ 115 pass 116 117 def writelines(self, sequence_of_strings): # real signature unknown; restored from __doc__ 118 将一个字符串列表写入文件 119 """ 120 writelines(sequence_of_strings) -> None. Write the strings to the file. 121 122 Note that newlines are not added. The sequence can be any iterable object 123 producing strings. This is equivalent to calling write() for each string. 124 """ 125 pass 126 127 def xreadlines(self): # real signature unknown; restored from __doc__ 128 可用于逐行读取文件，非全部 129 """ 130 xreadlines() -> returns self. 131 132 For backward compatibility. File objects now include the performance 133 optimizations previously implemented in the xreadlines module. 134 """ 135 pass 136 137 Python 2.x
1 class TextIOWrapper(_TextIOBase): 2 """ 3 def close(self, *args, **kwargs): # real signature unknown 4 关闭文件 5 pass 6 7 def fileno(self, *args, **kwargs): # real signature unknown 8 文件描述符 9 pass 10 11 def flush(self, *args, **kwargs): # real signature unknown 12 刷新文件内部缓冲区 13 pass 14 15 def isatty(self, *args, **kwargs): # real signature unknown 16 判断文件是否是同意tty设备 17 pass 18 19 def read(self, *args, **kwargs): # real signature unknown 20 读取指定字节数据 21 pass 22 23 def readable(self, *args, **kwargs): # real signature unknown 24 是否可读 25 pass 26 27 def readline(self, *args, **kwargs): # real signature unknown 28 仅读取一行数据 29 pass 30 31 def seek(self, *args, **kwargs): # real signature unknown 32 指定文件中指针位置 33 pass 34 35 def seekable(self, *args, **kwargs): # real signature unknown 36 指针是否可操作 37 pass 38 39 def tell(self, *args, **kwargs): # real signature unknown 40 获取指针位置 41 pass 42 43 def truncate(self, *args, **kwargs): # real signature unknown 44 截断数据，仅保留指定之前数据 45 pass 46 47 def writable(self, *args, **kwargs): # real signature unknown 48 是否可写 49 pass 50 51 def write(self, *args, **kwargs): # real signature unknown 52 写内容 53 pass 54 55 Python 3.x
但其实常用的操作也就那几个：
f.read(3) # python2中表示指定读取3个字节，python3中表示指定读取3个字符！ f.readline() # 读取文件内容中的一行 f.readlines() # 自动将文件内容解析为一个< 行的列表 >，可以用 for line in f.readlines(): 处理 f.write(‘helloPython‘) f.seek(9) # 按照字节来执行，用来指定当前文件指针位置,seek(0)表示文件指针移到文件头，seek(0,2)指向文件尾，便于追加内容 f.tell() # 是按照字节来执行的，用来查看当前指针位置
还有一个truncate()函数，用于截断文件内容且仅保留文件内容截断处之前的内容，不容易理解可以看示例：
f = open(‘test.log‘,‘r+‘,encoding=‘utf-8‘) # encoding=‘utf-8‘，有处理汉字的时候这样用 f.seek(9) # 原文件内容是‘小苹果helloPython’ f.truncate() #执行truncate()后，仅保留原文件截断之前的内容，这里即为‘小苹果’ f.close()
2.with语句
上面利用open()函数进行文件处理时，必须在文件打开进行操作后执行f.close()关闭文件，十分的麻烦。而使用with()语句则可以避免这一步繁琐的操作，自动在文件操作后关闭文件。并且，在python中引入with语句的目的是在异常处理中把try,except和finally关键字，以及与资源分配释放相关的代码全部去掉，从而减少代码的编写量，使代码更简洁！
如：
with open(‘name.txt‘, ‘w‘) as f:
f.write(‘Somebody^Fancy1‘)
等价于：
try:
f = open(‘name.txt‘,‘w‘)
f.write(‘Somebody^Fancy1‘)
finally:
if f:
f.close()