Detailed explanation of Unicode and utf-8 in Python

In the Python language, Uincode string processing has always been a confusing problem. Many Python enthusiasts often have trouble figuring out the differences between Unicode, UTF-8, and many other encodings. This article will introduce the relevant knowledge of Unicode and Python's Chinese processing. Let’s take a look with the editor below

In the Python language, Uincode string processing has always been a confusing problem. Many Python enthusiasts often have trouble figuring out the differences between Unicode, UTF-8, and many other encodings. The author was once a member of this "troublesome group", but after more than half a year of hard work, I have finally figured out some of the relationships. It is now organized as follows and shared with all colleagues. At the same time, I also hope that this short article can attract more real experts to join in and jointly improve our Python Chinese environment.

The various opinions mentioned in this article are partly obtained by consulting the data, and partly obtained by the author using various existing coded data using the "guessing and verification" method. The author thinks that he has little talent and knowledge, and I am afraid that there are many mistakes hidden in it. There are many experts among the readers. If any of you find any mistakes in it, I hope that the experts will give you some advice. It is a small matter for the author to be embarrassed himself, but it is a big matter for others to have wrong opinions, so you don’t have to worry about the author’s face.

Section 1 Text Encoding and Unicode Standard

To explain Unicode strings, we must first start with what Unicode encoding is. As we all know, text display has always been a basic problem that computer display functions must solve. The computer is not literate. It actually regards the text as a string of "Pictures", and each "picture" corresponds to a character. When each computer program displays text, it must use a collection of "pictures" that records how the text "picture" is displayed, find the data corresponding to the "picture" for each character, and then "draw" the word in the same way. to the screen. This "picture" is called a "font", and the collection of recorded font display data is called "Character Set". In order to facilitate program search, the font data of each character must be arranged in an orderly manner in the character set, and each character will be assigned a unique ID. This ID is the character's encoding. When computers process character data, this encoding is always used to represent the character it represents. Therefore, a character set specifies a set of character data that a computer can process. Obviously, different countries specify different character set sizes, and the corresponding character encodings are also different.

In the history of computers, the most widely used standardized character set is the ASCII character set. It is actually a standard formulated in the United States and developed for North American users. It uses 7 binary bit encoding and can represent 128 characters. This character set was eventually officially adopted by the ISO organization as an international standard and is widely used in various computer systems. Nowadays, the BIOS of all PCs contains the font model of the ASCII character set, which is evident from its popularity.

However, when computers became popular in various countries, the limitations of ASCII encoding were exposed: its character space is really limited and cannot accommodate more characters, but most languages ​​need to use The number of characters is far more than 128. In order to correctly process their own characters, officials or private individuals in various countries have begun the work of designing their own character encoding sets, and eventually many character encodings for various national characters emerged, such as the ISO-8859-1 encoding for Western European characters. There are GB series codes for Simplified Chinese, and SHIFT-JIS codes for Japanese, etc. At the same time, in order to ensure that each new character set is compatible with the original ASCII text, most character sets invariably use ASCII characters as their first 128 characters, and make their encodings correspond to ASCII encodings one-to-one.

In this way, the problem of displaying characters in various countries is solved, but it also brings a new problem: garbled characters. Character sets used in different countries and regions usually do not have unified specifications for constraints, so the encodings of various character sets are often incompatible with each other. The encoding of the same word in two different character sets is generally different; and the corresponding characters of the same encoding in different character sets are also different. A piece of text written in encoding A will often be displayed as a mess of characters on a system that only supports encoding B. To make matters worse, the encoding lengths used by different character sets are often different. Programs that can only handle single-byte encoding often fail to handle text correctly when encountering double-byte or even multi-byte encoding. The infamous "half-word" problem. This made the already chaotic situation even more confusing.

In order to solve these problems once and for all, many large companies and organizations in the industry jointly proposed a standard, which is Unicode. Unicode is actually a new character encoding system. It encodes each character in the character set with a two-byte long ID number, thereby defining a coding space that can accommodate up to 65536 characters, and including all commonly used words in encodings from various countries in the world. . Due to careful consideration in designing the encoding, Unicode has well solved the problems of garbled characters and "half-words" caused by other character sets in data exchange. At the same time, the designers of Unicode fully considered the fact that a large amount of font data today still uses various encodings formulated by various countries, and put forward the design concept of "using Unicode as an internal encoding". In other words, the character display program still uses the original encoding and code, and the internal logic of the application will use Unicode. When displaying text, the program always converts the Unicode-encoded string into the original encoding for display. In this way, everyone does not have to redesign the font data system in order to use Unicode. At the same time, in order to distinguish it from the encodings that have been formulated by various countries, the designers of Unicode call Unicode "wide characters encodings", while the encodings formulated by various countries are customarily called "multi-byte encodings". encodings). Today, the Unicode system has introduced a four-byte extended encoding, and is gradually converging with UCS-4, which is the ISO10646 encoding specification. It is hoped that one day the ISO10646 system can be used to unify all text encodings around the world.

The Unicode system received high hopes as soon as it was born, and was quickly accepted as an international standard recognized by ISO. However, during the promotion process of Unicode, it encountered opposition from European and American users. The reason for their opposition is very simple: the original encodings used by European and American users are single-byte long, and the double-byte Unicode processing engine cannot process the original single-byte data; and if all existing single-byte texts need to be converted To convert it into Unicode, the workload will be too much. Furthermore, if all single-byte encoded text were converted to double-byte Unicode encoding, all their text data would take up twice as much space, and all handlers would have to be rewritten. They cannot accept this expense.

Although Unicode is an internationally recognized standard, it is impossible for the standardization organization to ignore the requirements of European and American users, the largest computer user group. So after consultations between all parties, a variant version of Unicode was produced, which is UTF-8. UTF-8 is a multi-byte encoding system. Its encoding rules are as follows:

1. UTF-8 encoding is divided into four areas:

The first area is single-byte encoding,

The encoding format is: 0xxxxxxx;
corresponds to Unicode: 0x0000 - 0x007f

The second area is double-byte encoding,

The encoding format is: 110xxxxx 10xxxxxx;

corresponds to Unicode: 0x0080 - 0x07ff

The three areas are three-byte encoding,

The encoding format is: 1110xxxx 10xxxxxxx 10xxxxxx

corresponds to Unicode: 0x0800 - 0xffff

The four areas are four-byte encoding,

The encoding format is: 11110xxx 10xxxxxxx 10xxxxxx 10xxxxxx

corresponds to Unicode: 0x0 0010000 - 0x0001ffff

The five areas are five-byte encoding,

The encoding format is: 111110xx 10xxxxxxx 10xxxxxxx 10xxxxxxx 10xxxxxxx

corresponds to Unicode: 0x00200000 - 0x03ffffff

## The six areas are six-byte encoding,

The encoding format is: 111110x 10xxxxxxx 10xxxxxxx 10xxxxxxx 10xxxxxxx 10xxxxxxx

corresponds to Unicode: 0x04000000 - 0x7ffffffff

Among them, the first, second and third areas correspond to the double-byte encoding area of ​​Unicode, and the fourth The area is for the four-byte extended part of Unicode (according to this definition, UTF-8 also has five and six areas, but the author did not find it in the GNU glibc library, I don’t know why);

2. Each area is arranged in the order of one, two, three, four, five, and six, and the characters in the corresponding positions remain the same as Unicode;

3. Unicode characters that cannot be displayed are encoded as 0 bytes. In other words, they are not included in UTF-8 (this is the statement I got from the GNU C library comment , which may not be consistent with the actual situation);

According to UTF-8 encoding rules, it is not difficult to find that the 128 codes in the first area are actually ASCII codes. So the UTF-8 processing engine can directly process ASCII text. However, UTF-8's compatibility with ASCII encoding comes at the expense of other encodings. For example, originally Chinese, Japanese, and Korean characters were basically double-byte encodings, but their positions in Unicode encoding correspond to the three areas in UTF-8, and each character encoding is three bytes long. In other words, if we convert all existing non-ASCII character text data encoded in China, Japan, and Korea into UTF-8 encoding, its size will become 1.5 times the original size.

Although the author personally thinks that the encoding method of UTF-8 seems a bit unfair, it has solved the transition problem from ASCII text to the Unicode world, so it has won wide recognition. Typical examples are XML and Java: the default encoding of XML text is UTF-8, and Java source code can actually be written in UTF-8 characters (JBuilder users should be impressed). There is also the well-known GTK 2.0 in the open source software world, which uses UTF-8 characters as internal encoding.

Having said so much, it seems that the topic is a bit far away. Many Python enthusiasts may have begun to worry: "What does this have to do with Python?" Okay, now we turn our attention to the world of Python Come.

Section 2 Python’s Unicode encoding system

In order to correctly handle multi-language text, Python introduced Unicode strings after version 2.0. Since then, strings in the Python language have been divided into two types: traditional Python strings that have been used for a long time before version 2.0, and new Unicode strings. In the Python language, we use the unicode() built-in function to "decode" a traditional Python string to get a Unicode string, and then use the encode() method of the Unicode string to decode this Unicode string. The string is "encoded" and "encoded" into a traditional Python string. The above content must be familiar to every Python user. But did you know that Python's Unicode string is not a true "Unicode-encoded string", but follows its own unique rules. The content of this rule is very simple:

1. The Python Unicode encoding of ASCII characters is the same as their ASCII encoding. In other words, the ASCII text in Python's Unicode string is still a single-byte length encoding;

2. The encoding of characters other than ASCII characters is Unicode A two-byte (or four-byte) encoding of the standard encoding. (The author guesses that the reason why the Python community wants to formulate such a weird standard may be to ensure the versatility of ASCII strings)

Usually in Python applications, Unicode strings are It is used for internal processing, and the terminal display work is completed by traditional Python strings (in fact, Python's print statement cannot print out double-byte Unicode encoded characters at all). In the Python language, traditional Python strings are so-called "multi-byte encoded" strings, which are used to represent various strings that are "encoded" into specific character set encodings (such as GB, BIG5, KOI8-R, JIS, ISO-8859-1, and of course UTF-8); and Python Unicode strings are "wide character encoding" strings, which represent Unicode data "decoded" from a specific character set encoding. So usually, a Python application that requires Unicode encoding will often process string data in the following way:

def foo(string, encoding = "gb2312"):
# 1. convert multi-byte string to wide character string
u_string = unicode(string, encoding)

# 2. do something
...

# 3. convert wide character string to printable multi-byte string
return u_string.encode(encoding)

We can give an example: often in the Red Hat Linux environment Python colleagues who use PyGTK2 for XWindowprogramming may have discovered this situation long ago: if we directly write the following statement:

import pygtk
pygtk.require('2.0')
import gtk

main = gtk.Window() # create a window
main.set_title("你好") # NOTICE!

Such a statement will appear on the terminal when executed Such a warning:

Error converting from UTF-8 to 'GB18030': An invalid character sequence

appears in the conversion input and the program window title will not be set to " Hello"; but if the user installs the Chinese codec and changes the last sentence above to:

u_string = unicode('你好','gb2312')
main.set_title(u_string)

, the program window title will be correctly set to "Hello ". Why is this?

the reason is simple. The gtk.Window.set_title() method always treats the title string it receives as a Unicode string. When the PyGTK system receives the user's main.set_title() request, it processes the string obtained as follows somewhere:

class Window(gtk.Widget):
...
def set_title(self, title_unicode_string):
...
# NOTICE! unicode -> multi-byte utf-8
real_title_string = title_unicode_string.encode('utf-8')
...
# pass read_title_string to GTK2 C API to draw the title
...

我们看到，字符串title_unicode_string在程序内部被“编码”成了一个新的字符串：real_title_string。显然，这个real_title_string是一个传统Python字符串，而它的编码用的是UTF-8。在上一节中笔者曾经提到过，GTK2的内部使用的字符串都是按UTF-8编码的，所以，GTK2核心系统在接收到real_title_string后可以正确显示出标题来。

那么，如果用户输入的标题是ASCII字符串（比如：“hello world”），又当如何？我们回想一下Python Unicode字符串的定义规则就不难发现，如果用户的输入是ASCII字符串，则对其进行重编码得到的就是其自身。也就是说，如果title_unicode_string的值是ASCII字符串，则real_title_string与title_unicode_string的值将完全一致。而一个ASCII字符串也就是一个UTF-8字符串，把它传递给GTK2系统不会有任何问题。

以上我们举的例子是关于Linux下的PyGTK2的，但类似的问题不仅出现在PyGTK中。除了PyGTK之外，现今各种Python绑定的图形包，如PyQT、Tkinter等，多多少少都会遇到与Unicode处理有关的问题。

现在我们弄清了Python的Unicode字符串编码机制，但是我们最想知道的问题还是没有解决：我们如何才能让Python支持用Unicode处理中文呢？这个问题我们将在下一节说明。

第三节　如何让Python的Unicode字符串支持中文

看完这一节的标题，有一些Python同道们可能会有些不以为然：“为什么一定要用Unicode处理中文呢？我们平时用传统Python字符串处理得不是也不错吗？”的确，其实在一般情况下像字符串连接、子串匹配等操作用传统Python字符串也就足够了。但是，如果涉及到一些高级的字符串操作，比如包含多国文字的正则表达式匹配、文本编辑、表达式分析等等，这些大量混杂了单字节和多字节文本的操作如果用传统字符串处理就非常麻烦了。再说，传统字符串始终无法解决那该死的“半个字”问题。而如果我们可以使用Unicode，则这些问题都可以迎刃而解。所以，我们必须正视并设法解决中文Unicode的处理问题。

由上一节的介绍我们知道，如果要想利用Python的Unicode机制处理字符串，只要能够拥有一个能够把多字节的中文编码（包括GB编码系列和BIG5系列）和Unicode编码进行双向转换的编码／解码模块就可以了。按照Python的术语，这样的编码／解码模块被称为codec。于是接下来的问题就变成了：我们该如何编写这样一个codec？

如果Python的Unicode机制是硬编码在Python核心中的话，那么给Python添加一个新的codec就将是一项艰苦卓绝的工作了。幸亏Python的设计者们没有那么傻，他们提供了一个扩充性极佳的机制，可以非常方便地为Python添加新的codecs。

Python的Unicode处理模块有三个最重要的组成部分：一是codecs.py文件，二是encodings目录，三是aliases.py文件。前两者都位于Python系统库的安装目录之中（如果是Win32发行版，就在$PYTHON_HOME/lib/目录下；如果是Red Hat Linux，就在/usr/lib/python-version/目录下，其它系统可以照此寻找），而最后一个则位于encodings目录下。接下来，我们分别对这三者加以说明。

先来看看codecs.py文件。这个文件定义了一个标准的Codec模块应有的接口。其具体内容大家可以在自己的Python发行版中找到，在此不再赘述。按照codecs.py文件的定义，一个完整的codec应该至少拥有三个类和一个标准函数：

1、Codec类

用途：

用于将用户传入的缓冲区数据（一个buffer）作为一个传统Python字符串，并将

其“解码”为对应的Unicode字符串。一个完整的Codec类定义必须提供Codec.decode()和

Codec.encode()两个方法：

Codec.decode(input, <a href="http://www.php.cn/wiki/222.html" target="_blank">errors</a> = "strict")

用于将输入的数据看做是传统Python字符串，并将其“解码”，转换成对应的Unicode字符串。

参数：

input：输入的buffer（可以是字符串，也可以是任何可以转换成字符串表示的对象）

errors：发生转换错误时的处理选择。可选择如下三种取值：

strict（默认值）：如果发生错误，则抛出UnicodeError异常；

replace：如果发生错误，则选取一个默认的Unicode编码代替之；

ignore：如果发生错误，则忽略这个字符，并继续分析余下的字符。

返回值：

一个常数列表（tuple）：首元素为转换后的Unicode字符串，尾元素为输入数据的长度。

Codec.encode(input, errors = "strict")

用于将输入的数据看做是Unicode字符串，并将其“编码”，转换成对应的传统Python字符串。

参数：

input：输入的buffer（通常就是Unicode字符串）

errors：发生转换错误时的处理选择。取值规则与Codec.decode()方法相同。

返回值：

一个常数列表（tuple）：首元素为转换后的传统Python字符串，尾元素为输入数据的长度。

2、StreamReader类（通常应该继承自Codec类）

用于分析文件输入流。提供所有对文件对象的读取操作，如readline()方法等。

3、StreamWriter类（通常应该继承自Codec类）

用于分析文件输出流。提供所有对文件对象的写入操作，如writeline()方法等。

5、getregentry()函数

即“GET REGistry ENTRY”之意，用于获取各个Codec文件中定义的四个关键函数。其函数体统一为：

def getregentry():
return tuple(Codec().encode,Codec().decode,StreamReader,StreamWriter)

在以上提到的所有四个类中，实际上只有Codec类和getregentry()函数是必须提供的。必须提供前者是因为它是实际提供转换操作的模块；而后者则是Python系统获得Codec定义的标准接口，所以必须存在。至于StreamReader和StreamWriter，理论上应该可以通过继承codecs.py中的StreamReader和StreamWriter类，并使用它们的默认实现。当然，也有许多codec中将这两个类进行了改写，以实现一些特殊的定制功能。

接下来我们再说说encodings目录。顾名思义，encodings目录就是Python系统默认的存放所有已经安装的codec的地方。我们可以在这里找到所有Python发行版自带的codecs。习惯上，每一个新的codec都会将自己安装在这里。需要注意的是，Python系统其实并不要求所有的codec都必须安装于此。用户可以将新的codec放在任何自己喜欢的位置，只要Python系统的搜索路径可以找得到就行。

仅仅将自己写的codec安装在Python能够找到的路径中还不够。要想让Python系统能找到对应的codec，还必须在Python中对其进行注册。要想注册一个新的codec，就必须用到encodings目录下的aliases.py文件。这个文件中只定义了一个哈希表aliases，它的每个键对应着每一个codec在使用时的名称，也就是unicode()内建函数的第二个参数值；而每个键对应的值则是一个字符串，它是这个codec对应的那个处理文件的模块名。比如，Python默认的解析UTF-8的codec是utf_8.py，它存放在encodings子目录下，则aliases哈希表中就有一项表示其对应关系：

'utf-8' : 'utf_8', # the <a href="http://www.php.cn/code/8212.html" target="_blank">module</a> `utf_8' is the codec <a href="http://www.php.cn/wiki/125.html" target="_blank">for</a> UTF-8

同理，如果我们新写了一个解析‘mycharset'字符集的codec，假设其编码文件为mycodec.py，存放在$PYTHON_HOME/lib/site-packages/mycharset/目录下，则我们就必须在aliases哈希表中加入这么一行：

'mycharset' : 'mycharset.mycodec',

这里不必写出mycodec.py的全路径名，因为site-packages目录通常都在Python系统的搜索路径之中。

Python解释器在需要分析Unicode字符串时，会自动加载encodings目录下的这个aliases.py文件。如果mycharset已经在系统中注册过，则我们就可以像使用其它内建的编码那样使用我们自己定义的codec了。比如，如果按照上面的方式注册了mycodec.py，则我们就可以这样写：

my_unicode_string = unicode(a_multi_byte_string, 'mycharset')

print my_unicode_string.encode('mycharset')

现在我们可以总结一下要编写一个新的codec一共需要那些步骤：

首先，我们需要编写一个自己的codec编码／解码模块；

其次，我们要把这个模块文件放在一个Python解释器可以找到的地方；

最后，我们要在encodings/aliases.py文件中对其进行注册。

从理论上说，有了这三步，我们就可以将自己的codec安装到系统中去了。不过这样还不算完，还有一个小问题。有时候，我们出于种种原因，不希望随便修改自己的系统文件（比如，一个用户工作在一个集中式的系统中，系统管理员不允许别人对系统文件进行修改）。在以上介绍的步骤中，我们需要修改aliases.py文件的内容，这是一个系统文件。可如果我们不能修改它，难道我们就不能添加新的codec吗？不，我们当然有办法。

这个办法就是：在运行时修改encodings.aliases.aliases哈希表的内容。

还是使用上面那个假设，如果用户工作系统的管理员不允许用户把mycodec.py的注册信息写入aliases.py，那么我们就可以如此处理：

1、将mycodec.py放在一个目录下，比如/home/myname/mycharset/目录；

2、这样编写/home/myname/mycharset/init.py文件：

import encodings.aliases
# update aliases hash map
encodings.aliases.aliases.update({/
'mycodec' : 'mycharset.mycodec',/
}}

以后每次要使用Python时，我们可以将/home/myname/加入搜索路径，并且在使用自己的codec时预先执行：

import mycharset # execute the script in mycharset/init.py

这样我们就可以在不改动原有系统文件的情况下使用新的codecs了。另外，如果借助Python的site机制，我们还可以让这个import工作自动化。如果大家不知道什么是site，就请在自己的Python交互环境中运行：

import site
print site.doc

浏览一下site模块的文档，即可明白个中技巧。如果大家手头有Red Hat Linux v8，v9，还可以参考一下Red Hat的Python发行版中附带的日文codec，看看它是如何实现自动加载的。也许不少同道可能找不到这个日文的codec在哪里，这里列出如下：

　　Red Hat Linux v8：在/usr/lib/python2.2/site-package/japanese/目录下；
　　Red Hat Linux v9：在/usr/lib/python2.2/lib-dynload/japanese/目录下；

提示：请Red Hat用户注意site-packages目录下的japanese.pth文件，结合site模块的文档，相信马上就能豁然开朗。

结束语

记得当初笔者在Dohao论坛上夸下海口：“如果可以的话，我可以为大家编写一个（中文模块）”，现在回想起来，不禁为自己当初的不知天高地厚而汗颜。一个把自己所有的的时间都花在学习上，一个学期只学七门课程，还落得个两门课不及格的傻瓜研究生，哪里有什么资格在大家面前如此嚣张。现如今，第二个学期由于这两门课的缘故负担陡增（十门课呀！），家中老父老母还眼巴巴地等着自己的儿子能给他们挣脸。要想在有限的时间之内，既保证学习，又保证工作（我要承担导师的课程辅导工作，同时还有一个学校的教学改革方案需要我在其中挑大梁），已经是疲于应付，再加上一个中文模块……唉，请恕笔者分身乏术，不得不食言。

因此，笔者斗胆，在此和盘托出自己这半年以来的心得，只希望能够找到一批，不，哪怕是一个也好，只要是对这个项目感兴趣的同道中人，能够接下笔者已经整理出来的知识，把一个完整的（至少应该包含GB、BIG5、笔者个人认为甚至还应包括HZ码）中文模块编写出来，贡献给大家（不论是有偿的还是无偿的），那就是我们广大Python爱好者之福了。另外，Python的发行版至今尚未包括任何中文支持模块。既然我等平日深爱Python，如果我们的工作能因此为Python的发展做出一点贡献，何乐而不为呢？

附录　几个小小提示

1、LUO Jian兄已经编写了一个非常不错的中文模块（Dohao上有链接，文件名是showfile.zip，这个模块比我已经写完的草稿版本要快得多），同时支持GB2312和GB18030编码，可惜不支持BIG5。如果大家有兴趣，可以下载这个模块研究一下；

2、和其它字符集编码相比，中文模块有其特殊性，那就是其海量的字符数目。一些相对较小的字符集还好说，比如GB2312，可以利用哈希表查找。而对于巨大的GB18030编码，如果简单地将所有数据制成一个特大的编码对照表，则查询速度会慢得让人无法容忍（笔者在编写模块时最头疼的就是这一点）。如果要编写一个速度上能让人满意的codec，就必须考虑设计某种公式，能够通过简单地运算从一种编码推算出另一种来，或者至少能推算出它的大概范围。这就要求程序员要能对整个编码方案做统计，设法找到规律。笔者认为，这应该是编写中文模块时的最大难点。或许是数学功底实在太差的缘故，笔者费尽心机也未能找出一个规律来。希望能有数学高手不吝赐教；

3. Chinese coding is divided into two major factions: GB and BIG5. Among them, GB is divided into three codes: GB2312, GBK and GB18030, and BIG5 is also divided into two types: BIG5 and BIG5-HKSCS (corresponding to the original BIG5 and Hong Kong extended versions respectively). Although the encodings of the same faction can be backward compatible, considering the large number of characters and in order to speed up the search, the author personally thinks it is more reasonable to encode them separately. Of course, if the conversion formula for the corresponding character set can be found, this separation is not necessary;

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