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Python encoding processing: The difference and use of str and Unicode
Python encoding processing: The difference and use of str and Unicode
用python处理中文，读取文件或消息时，如果发现乱码(字符串处理，读写文件，print)，大多数人的做法是，调用encode/decode进行调试，并没有明确思考为何出现乱码， 今天我们来讨论一下如何处理编码问题。
注意: 以下讨论为Python2.x版本, Py3k下未测试
调试时最常出现的错误
错误1
Traceback (most recent call last): File "<stdin>", line 1, in <module> UnicodeDecodeError: ‘ascii‘ codec can‘t decode byte 0xe6 in position 0: ordinal
错误2
Traceback (most recent call last): File "<stdin>", line 1, in <module> File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/encodings/utf_8.py", line 16, in decode return codecs.utf_8_decode(input, errors, True) UnicodeEncodeError: ‘ascii‘ codec can‘t encode characters in position 0-1: ordinal not in range(128)
首先
必须有大体概念，了解下字符集，字符编码
ASCII | Unicode | UTF-8 | 等等
字符编码笔记：ASCII，Unicode和UTF-8
str 和 unicode
str和unicode都是basestring的子类
所以有判断是否是字符串的方法
def is_str(s): return isinstance(s, basestring)
str和unicode 转换
str -> decode(‘the_coding_of_str‘) -> unicode unicode -> encode(‘the_coding_you_want‘) -> str
区别
str是字节串，由unicode经过编码(encode)后的字节组成的
声明方式
>>> s = ‘中文‘ s = u‘中文‘.encode(‘utf-8‘) >>> type(‘中文‘) <type ‘str‘>
求长度(返回字节数)
>>> u‘中文‘.encode(‘utf-8‘) ‘\xe4\xb8\xad\xe6\x96\x87‘ >>> len(u‘中文‘.encode(‘utf-8‘)) 6
unicode才是真正意义上的字符串，由字符组成
声明方式
>>> s = u‘中文‘ >>> s = ‘中文‘.decode(‘utf-8‘) >>> s = unicode(‘中文‘, ‘utf-8‘) >>> type(u‘中文‘) <type ‘unicode‘>
求长度(返回字符数),在逻辑中真正想要用的
>>> u‘中文‘ u‘\u4e2d\u6587‘ >>> len(u‘中文‘) 2
结论
搞明白要处理的是str还是unicode, 使用对的处理方法(str.decode/unicode.encode)
下面是判断是否为unicode/str的方法
>>> isinstance(u‘中文‘, unicode) True >>> isinstance(‘中文‘, unicode) False >>> isinstance(‘中文‘, str) True >>> isinstance(u‘中文‘, str) False
简单原则：不要对str使用encode，不要对unicode使用decode (事实上可以对str进行encode的，具体见最后，为了保证简单，不建议)
>>> ‘中文‘.encode(‘utf-8‘) Traceback (most recent call last): File "", line 1, in UnicodeDecodeError: ‘ascii‘ codec can‘t decode byte 0xe4 in position 0: ordinal not in range(128) >>> u‘中文‘.decode(‘utf-8‘) Traceback (most recent call last): File "<stdin>", line 1, in <module> File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/encodings/utf_8.py", line 16, in decode return codecs.utf_8_decode(input, errors, True) UnicodeEncodeError: ‘ascii‘ codec can‘t encode characters in position 0-1: ordinal not in range(128)
不同编码转换,使用unicode作为中间编码
#s是code_A的str s.decode(‘code_A‘).encode(‘code_B‘)
文件处理,IDE和控制台
处理流程，可以这么使用，把python看做一个水池，一个入口，一个出口
入口处，全部转成unicode, 池里全部使用unicode处理，出口处，再转成目标编码(当然，有例外，处理逻辑中要用到具体编码的情况)
读文件 外部输入编码，decode转成unicode 处理(内部编码，统一unicode) encode转成需要的目标编码 写到目标输出(文件或控制台)
IDE和控制台报错，原因是print时，编码和IDE自身编码不一致导致
输出时将编码转换成一致的就可以正常输出
>>> print u‘中文‘.encode(‘gbk‘) ???? >>> print u‘中文‘.encode(‘utf-8‘) 中文
建议
规范编码
统一编码，防止由于某个环节产生的乱码
环境编码，IDE/文本编辑器, 文件编码，数据库数据表编码
保证代码源文件编码
这个很重要
py文件默认编码是ASCII, 在源代码文件中，如果用到非ASCII字符，需要在文件头部进行编码声明 文档
不声明的话，输入非ASCII会遇到的错误,必须放在文件第一行或第二行
File "XXX.py", line 3 SyntaxError: Non-ASCII character ‘\xd6‘ in file c.py on line 3, but no encoding declared; see http://www.python.org/peps/pep-0263.html for details
# -*- coding: utf-8 -*- 或者 #coding=utf-8
若头部声明coding=utf-8, a = ‘中文‘ 其编码为utf-8
若头部声明coding=gb2312, a = ‘中文‘ 其编码为gbk
so, 同一项目中所有源文件头部统一一个编码,并且声明的编码要和源文件保存的编码一致(编辑器相关)
在源代码用作处理的硬编码字符串，统一用unicode
将其类型和源文件本身的编码隔离开, 独立无依赖方便流程中各个位置处理
if s == u‘中文‘: #而不是 s == ‘中文‘ pass #注意这里 s到这里时，确保转为unicode
以上几步搞定后，你只需要关注两个 unicode和 你设定的编码(一般使用utf-8)
处理顺序
1. Decode early 2. Unicode everywhere 3. Encode later
相关模块及一些方法
获得和设置系统默认编码
>>> import sys >>> sys.getdefaultencoding() ‘ascii‘ >>> reload(sys) <module ‘sys‘ (built-in)> >>> sys.setdefaultencoding(‘utf-8‘) >>> sys.getdefaultencoding() ‘utf-8‘ >>> str.encode(‘other_coding‘)
在python中，直接将某种编码的str进行encode成另一种编码str
#str_A为utf-8 str_A.encode(‘gbk‘) 执行的操作是 str_A.decode(‘sys_codec‘).encode(‘gbk‘) 这里sys_codec即为上一步 sys.getdefaultencoding() 的编码
‘获得和设置系统默认编码‘和这里的str.encode是相关的，但我一般很少这么用，主要是觉得复杂不可控,还是输入明确decode，输出明确encode来得简单些
chardet
文件编码检测，下载
>>> import chardet
>>> f = open(‘test.txt‘,‘r‘)
>>> result = chardet.detect(f.read())
>>> result {‘confidence‘: 0.99, ‘encoding‘: ‘utf-8‘}\u字符串转对应unicode字符串
>>> u‘中‘ u‘\u4e2d‘ >>> s = ‘\u4e2d‘ >>> print s.decode(‘unicode_escape‘) 中 >>> a = ‘\\u4fee\\u6539\\u8282\\u70b9\\u72b6\\u6001\\u6210\\u529f‘ >>> a.decode(‘unicode_escape‘) u‘\u4fee\u6539\u8282\u70b9\u72b6\u6001\u6210\u529f‘
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