Python文件处理 open()

男娘i 2021-09-22 06:04 389阅读 0赞

1. open()语法  
open(file\[, mode\[, buffering\[, encoding\[, errors\[, newline\[, closefd=True\]\]\]\]\]\])  
open函数有很多的参数，常用的是file，mode和encoding  
file文件位置，需要加引号  
mode文件打开模式，见下面3  
buffering的可取值有0，1，>1三个，0代表buffer关闭（只适用于二进制模式），1代表line buffer（只适用于文本模式），>1表示初始化的buffer大小；  
encoding表示的是返回的数据采用何种编码，一般采用utf8或者gbk；  
errors的取值一般有strict，ignore，当取strict的时候，字符编码出现问题的时候，会报错，当取ignore的时候，编码出现问题，程序会忽略而过，继续执行下面的程序。  
newline可以取的值有None, \\n,  \\r, '', '\\r\\n'，用于区分换行符，但是这个参数只对文本模式有效；  
closefd的取值，是与传入的文件参数有关，默认情况下为True，传入的file参数为文件的文件名，取值为False的时候，file只能是文件描述符，什么是文件描述符，就是一个非负整数，在Unix内核的系统中，打开一个文件，便会返回一个文件描述符。

2. Python中file()与open()区别  
两者都能够打开文件，对文件进行操作，也具有相似的用法和参数，但是，这两种文件打开方式有本质的区别，file为文件类，用file()来打开文件，相当于这是在构造文件类，而用open()打开文件，是用python的内建函数来操作，建议使用open

3. 参数mode的基本取值

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   <td style="border-width:0px 1px 1px 0px; border-style:solid; border-color:rgb(221,221,221); font-family:inherit; font-size:15px; font-style:inherit; margin:0px; outline:0px; padding:5px; vertical-align:baseline"> Character</td> 
   <td style="border-width:0px 1px 1px 0px; border-style:solid; border-color:rgb(221,221,221); font-family:inherit; font-size:15px; font-style:inherit; margin:0px; outline:0px; padding:5px; vertical-align:baseline"> Meaning</td> 
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   <td style="border-width:0px 1px 1px 0px; border-style:solid; border-color:rgb(221,221,221); font-family:inherit; font-size:15px; font-style:inherit; margin:0px; outline:0px; padding:5px; vertical-align:baseline"> open for reading (default)</td> 
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   <td style="border-width:0px 1px 1px 0px; border-style:solid; border-color:rgb(221,221,221); font-family:inherit; font-size:15px; font-style:inherit; margin:0px; outline:0px; padding:5px; vertical-align:baseline"> <span style="border:0px; font-family:inherit; font-style:inherit; margin:0px; outline:0px; padding:0px; vertical-align:baseline">'w'</span></td> 
   <td style="border-width:0px 1px 1px 0px; border-style:solid; border-color:rgb(221,221,221); font-family:inherit; font-size:15px; font-style:inherit; margin:0px; outline:0px; padding:5px; vertical-align:baseline"> open for writing, truncating the file first</td> 
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  <tr style="border:0px; font-family:inherit; font-style:inherit; margin:0px; outline:0px; padding:0px; vertical-align:baseline"> 
   <td style="border-width:0px 1px 1px 0px; border-style:solid; border-color:rgb(221,221,221); font-family:inherit; font-size:15px; font-style:inherit; margin:0px; outline:0px; padding:5px; vertical-align:baseline"> <span style="border:0px; font-family:inherit; font-style:inherit; margin:0px; outline:0px; padding:0px; vertical-align:baseline">'a'</span></td> 
   <td style="border-width:0px 1px 1px 0px; border-style:solid; border-color:rgb(221,221,221); font-family:inherit; font-size:15px; font-style:inherit; margin:0px; outline:0px; padding:5px; vertical-align:baseline"> open for writing, appending to the end of the file if it exists</td> 
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  <tr style="border:0px; font-family:inherit; font-style:inherit; margin:0px; outline:0px; padding:0px; vertical-align:baseline"> 
   <td style="border-width:0px 1px 1px 0px; border-style:solid; border-color:rgb(221,221,221); font-family:inherit; font-size:15px; font-style:inherit; margin:0px; outline:0px; padding:5px; vertical-align:baseline"> <span style="border:0px; font-family:inherit; font-style:inherit; margin:0px; outline:0px; padding:0px; vertical-align:baseline">'b'</span></td> 
   <td style="border-width:0px 1px 1px 0px; border-style:solid; border-color:rgb(221,221,221); font-family:inherit; font-size:15px; font-style:inherit; margin:0px; outline:0px; padding:5px; vertical-align:baseline"> binary mode</td> 
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   <td style="border-width:0px 1px 1px 0px; border-style:solid; border-color:rgb(221,221,221); font-family:inherit; font-size:15px; font-style:inherit; margin:0px; outline:0px; padding:5px; vertical-align:baseline"> <span style="border:0px; font-family:inherit; font-style:inherit; margin:0px; outline:0px; padding:0px; vertical-align:baseline">'t'</span></td> 
   <td style="border-width:0px 1px 1px 0px; border-style:solid; border-color:rgb(221,221,221); font-family:inherit; font-size:15px; font-style:inherit; margin:0px; outline:0px; padding:5px; vertical-align:baseline"> text mode (default)</td> 
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   <td style="border-width:0px 1px 1px 0px; border-style:solid; border-color:rgb(221,221,221); font-family:inherit; font-size:15px; font-style:inherit; margin:0px; outline:0px; padding:5px; vertical-align:baseline"> <span style="border:0px; font-family:inherit; font-style:inherit; margin:0px; outline:0px; padding:0px; vertical-align:baseline">'+'</span></td> 
   <td style="border-width:0px 1px 1px 0px; border-style:solid; border-color:rgb(221,221,221); font-family:inherit; font-size:15px; font-style:inherit; margin:0px; outline:0px; padding:5px; vertical-align:baseline"> open a disk file for updating (reading and writing)</td> 
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   <td style="border-width:0px 1px 1px 0px; border-style:solid; border-color:rgb(221,221,221); font-family:inherit; font-size:15px; font-style:inherit; margin:0px; outline:0px; padding:5px; vertical-align:baseline"> <span style="border:0px; font-family:inherit; font-style:inherit; margin:0px; outline:0px; padding:0px; vertical-align:baseline">'U'</span></td> 
   <td style="border-width:0px 1px 1px 0px; border-style:solid; border-color:rgb(221,221,221); font-family:inherit; font-size:15px; font-style:inherit; margin:0px; outline:0px; padding:5px; vertical-align:baseline"> universal newline mode (for backwards compatibility; should not be used in new code)</td> 
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r、w、a为打开文件的基本模式，对应着只读、只写、追加模式；  
b、t、+、U这四个字符，与以上的文件打开模式组合使用，二进制模式，文本模式，读写模式、通用换行符，根据实际情况组合使用、

常见的mode取值组合

r或rt  默认模式，文本模式读
    rb     二进制文件
     
    w或wt 文本模式写，打开前文件存储被清空
    wb    二进制写，文件存储同样被清空
     
    a   追加模式，只能写在文件末尾
    a+  可读写模式，写只能写在文件末尾
     
    w+ 可读写，与a+的区别是要清空文件内容
    r+ 可读写，与a+的区别是可以写到文件任何位置

4. 测试  
测试文件test.txt，内容如下：

Hello,Python
    www.linuxeye.com
    This is a test file

用一小段代码来测试写入文件直观的显示它们的不同

test = [ "test1\n", "test2\n", "test3\n" ]
    f = open("test.txt", "a+")
    try:
      #f.seek(0)
      for l in test:
        f.write(l)
    finally:
      f.close()

a+、w+和r+模式的区别(测试后还原test.txt)  
a+模式

# cat test.txt
    Hello, Python
    www.linuxeye.com
    This is a test file
    test1
    test2
    test3

w+模式

# cat test.txt
    test1
    test2
    test3

r+模式  
在写入文件前，我们在上面那段代码中加上一句f.seek(0)，用来定位写入文件写入位置(文件开头)，直接覆盖字符数(注意\\n也是一个字符)

# cat test.txt
    test1
    test2
    test3
    inuxeye.com
    This is a test file

注意：r+模式打开文件时，此文件必须存在，否则就会报错，‘r’模式也如此  
其他测试

>>> f = open('test.txt')
    >>> f.read() #读取整个文件，字符串显示
    'Hello,Python\nwww.linuxeye.com\nThis is a test file\n'
    >>> f.read() #指针在文件末尾，不能再读取内容
    ''

>>> f = open('test.txt')
    >>> f.readline() #一次读一行，指针在该行末尾
    'Hello,Python\n'
    >>> f.tell() #改行的字符长度
    13
    >>> f.readline()
    'www.linuxeye.com\n'
    >>> f.tell()
    30
    >>> f.readline()
    'This is a test file\n'
    >>> f.tell()
    50
    >>> f.readline()
    ''
    >>> f.tell() #指针停在最后一行
    50

>>> f = open('test.txt')
    >>> f.readlines() #读取整个文件，以列表显示
    ['Hello,Python\n', 'www.linuxeye.com\n', 'This is a test file\n']
    >>> f.tell() #指针在最后一行
    50

>>> f = open('test.txt','w') #覆盖创建新文件
    >>> f.write('Hello,Python!')  #如果写入内容小于1024，会存在内存，否则需要刷新
    >>> f.flush() #写入到硬盘
    >>> f.close() #关闭文件会自动刷新
    >>> f.write('Hello,Linuxeye') #关闭后，写失败，提示文件已经关闭
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    ValueError: I/O operation on closed file