使用Python3中的gettext模块翻译Python源码以支持多语言
你写了一个Python 3程序，还想要它适用于其他语言。你能复制全部代码库，然后刻意地检查每个.py文件，替换掉所有找到的文本字符串。但这意味着你有两份你代码的独立副本，每当你要做出个改动或修复个bug，你的工作量会加倍。而且如果你想要程序还适用于其他语言，就更糟了。
幸运的是，Python给了一个解决办法，就是用gettext模块。
一个Hack解法
你应该把你自己的解决办法统一改变。例如，你可以把你程序中的每个字符串替换为一个函数调用（函数名简单些，比如像_()一样），这会返回被翻译为该正确语言的字符串。举个例子，如果你的程序原本是：
print('Hello world!')
……你可以将它改为：
print(_('Hello world!'))
……函数_()会返回'Hello world!'的翻译，它基于程序设置有的语言。比如，如果这个语言设置之前被存在一个叫LANGUAGE的全局变量中，函数_()看起来像这样：
def _(s):
spanishStrings = {'Hello world!': 'Hola Mundo!'}
frenchStrings = {'Hello world!': 'Bonjour le monde!'}
germanStrings = {'Hello world!': 'Hallo Welt!'}
if LANGUAGE == 'English':
return s
if LANGUAGE == 'Spanish':
return spanishStrings[s]
if LANGUAGE == 'French':
return frenchStrings[s]
if LANGUAGE == 'German':
return germanStrings[s]
这可以，但是你这是在重复造轮子。Python的gettext模块可以做更多。gettext是一系列工具，文件格式在20世纪90年代被发明出来，来规范软件国际化（也叫I18N）。gettext是个作为对于所有编程语言的系统化的设计，但是我们会在本篇文章中只专注于Python。
程序例子
设想你有个想要翻译的用Python3写的简单“猜数字”游戏。程序的源代码在这里。有四步来使这个程序国际化：
调整这个.py文件的源代码，这样使字符串输入进一个名为_()的函数。
用和Python一起安装的pygettext.py文本，从源代码创建一个”pot”文件。
用这个免费的跨平台Poedit软件，从pot文件创建.po和.mo文件。
再次调整你的.py文件源代码导入gettext模块的代码，设置语言。
第一步：添加 _() 函数
首先，检查你程序中的所有需要被翻译和用_()的调用来替代的字符串。针对Python使用的gettext系统用_()作为得到翻译了的字符串的通用名，因为它是个短名。
注意：用格式型字符串而不是连接型字符串会是你的程序翻译起来更简单。例如，用连接型字符串你的程序会像这样：
print('Good job, ' + myName + '! You guessed my number in ' + guessesTaken + ' guesses!')
print(_('Good job, ') + myName + _('! You guessed my number in ') + guessesTaken + _(' guesses!'))
This results in three separate strings that need to be translated, as opposed to the single string needed in the string formatting approach:
这会导致三个独立的字符串都需要翻译，然而相反的是在格式型的字符串中，只需翻译一个字符串：
print('Good job, %s! You guessed my number in %s guesses!' % (myName, guessesTaken))
print(_('Good job, %s! You guessed my number in %s guesses!') % (myName, guessesTaken))
当你改完“猜数字”源代码后，它会像这样。你并不能运行它，因为_()函数还没定义。这个变化只是让pygettext.py文本可以找到所有需要翻译的字符串。
第二步：用pygettext.py提取字符串
在你Python安装（Windows上的C:Python34Toolsi18n）中的Tools/i18n就是pygettext.py文本。对于可译字符串普通 gettext unix 命令解析 C/C++ 源码并且 xgettext unix 命令可以解析其他语言，而pygettext.py则知道怎样去解析Python源码。它会找到所有字符串并产生个”pot”文件。
在Windows上我已经运行了这个文本像这样：
C:>py -3.4 C:Python34Toolsi18npygettext.py -d guess guess.py
这创建了一个pot文件，叫guess.pot。这只是个普通纯文本文件，它列出来了全部的在源码中寻找_()的调用的要翻译的字符串。你可以在这儿看guess.pot文件.
第三步：用Poedit翻译字符串
你可以用文本编辑器填写翻译但是免费的Poedit软件会更容易从这儿下载http://poedit.net. 选择 > New from POT/PO file… 然后选择你的guess.po文件。
Poedit会问你想要翻译成什么语言。我们举例用西班牙语：
填写翻译吧。（我用 http://translate.google.com，所以对于真的使用西班牙语的人会感觉有点奇怪。)
现在储存文件在它的gettext形式的文件夹里。保存会创建.po文件（一个人类可读的文本文件不同于原始.pot文件，除了是有西语翻译的）和一个.mo文件（一个gettext会读取的机器可读版本。这些文件会存在一个特定的文件夹内，为的是让gettext能够找到他们。他们看起来像这样（比如西语文件中的”es”和德语文件中”de”）：
./guess.py ./guess.pot ./locale/es/LC_MESSAGES/guess.mo ./locale/es/LC_MESSAGES/guess.po ./locale/de/LC_MESSAGES/guess.mo ./locale/de/LC_MESSAGES/guess.po
这些两种性质的语言像西语中的”es”和德语中的 ”de” 被称作ISO 639-1 codes 是语言的标准缩写。你不一定要用他们，但是遵循标准是有道理的。
第四步：给你程序加上gettext代码
现在你有包含翻译的.mo文件，调整你的Python代码去用它。在你的程序中加上下面的：
import gettext
es = gettext.translation('guess', localedir='locale', languages=['es'])
es.install()
第一个 'guess' 是”定义域”，这其实是意味着guess.mo文件名中“猜”的部分。 localedir是你创建的locale文件夹的目录地址。这会是相对或绝对的路径。'es'描述在locale文件夹下面的文件。LC_MESSAGES文件夹是个标准名
install()方法会导致调用_()返回翻译为西语的字符串。如果你想回到原始的英语只需要分配一个lambda函数值给_，这会返回当时输入的字符串：
import gettext
es = gettext.translation('guess', localedir='locale', languages=['es'])
print(_('Hello! What is your name?')) # prints Spanish
_ = lambda s: s
你可以检查准备翻译的”Guess the Number”源码。如果你想要运行此程序，下载并解压这个压缩文件和它的locale文件夹和.mo安装文件。
延伸阅读
我怎样都称不上是 I18N or gettext的专家，如果我的教程讲解不够好，请一定要留言。大多数情况下，你的软件运行时不会转换语言，而是会去读LANGUAGE，LC_ALL，LC_MESSAGES，和LANG这些环境变量中的一个来确定计算机的工作地点。我会边学习边更新本教程的。
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