"gettext" --- 多语种国际化服务
******************************

**源代码：** Lib/gettext.py

======================================================================

"gettext" 模块为 Python 模块和应用程序提供国际化
(Internationalization, I18N) 和本地化 (Localization, L10N) 服务。它同
时支持 GNU **gettext** 消息编目 API 和更高级的、基于类的 API，后者可能
更适合于 Python 文件。下方描述的接口允许用户使用一种自然语言编写模块和
应用程序消息，并提供翻译后的消息编目，以便在不同的自然语言下运行。

同时还给出一些本地化 Python 模块及应用程序的小技巧。


GNU **gettext** API
===================

模块 "gettext" 定义了下列  API，这与 **gettext** API 类似。如果你使用
该 API，将会对整个应用程序产生全局的影响。如果你的应用程序支持多语种，
而语言选择取决于用户的区域设置，这通常正是你所想要的。而如果你正在本地
化某个 Python 模块，或者你的应用程序需要在运行时切换语言，相反你或许想
用基于类的API。

gettext.bindtextdomain(domain, localedir=None)

   将 *domain* 绑定到本地目录 *localedir*。 更具体地来说，模块
   "gettext" 将使用路径 (在 Unix 系统中):
   "*localedir*/*language*/LC_MESSAGES/*domain*.mo" 查找二进制 ".mo"
   文件，此处对应地查找 *language* 的位置是环境变量 "LANGUAGE",
   "LC_ALL", "LC_MESSAGES" 和 "LANG" 中。

   如果遗漏了 *localedir* 或者设置为 "None"，那么将返回当前 *domain*
   所绑定的值 [1]

gettext.bind_textdomain_codeset(domain, codeset=None)

   将 *domain* 绑定到 *codeset*，修改 "lgettext()", "ldgettext()",
   "lngettext()" 和 "ldngettext()" 函数返回的字节串的字符编码。如果省
   略了 *codeset*，则返回当前绑定的编码集。

   Deprecated since version 3.8, will be removed in version 3.10.

gettext.textdomain(domain=None)

   修改或查询当前的全局域。如果 *domain* 为 "None"，则返回当前的全局域
   ，不为 "None" 则将全局域设置为 *domain*，并返回它。

gettext.gettext(message)

   返回 *message* 的本地化翻译，依据包括当前的全局域、语言和区域目录。
   本函数在本地命名空间中通常有别名 "_()" （参考下面的示例）。

gettext.dgettext(domain, message)

   与 "gettext()" 类似，但在指定的 *domain* 中查找 message。

gettext.ngettext(singular, plural, n)

   与 "gettext()" 类似，但考虑了复数形式。如果找到了翻译，则将 *n* 代
   入复数公式，然后返回得出的消息（某些语言具有两种以上的复数形式）。
   如果未找到翻译，则 *n* 为 1 时返回 *singular*，为其他数时返回
   *plural*。

   复数公式取自编目头文件。它是 C 或 Python 表达式，有一个自变量 *n*，
   该表达式计算的是所需复数形式在编目中的索引号。关于在 ".po" 文件中使
   用的确切语法和各种语言的公式，请参阅 GNU gettext 文档 。

gettext.dngettext(domain, singular, plural, n)

   与 "ngettext()" 类似，但在指定的 *domain* 中查找 message。

gettext.pgettext(context, message)

gettext.dpgettext(domain, context, message)

gettext.npgettext(context, singular, plural, n)

gettext.dnpgettext(domain, context, singular, plural, n)

   与前缀中没有 "p" 的相应函数类似（即 "gettext()", "dgettext()",
   "ngettext()", "dngettext()" ），但是仅翻译给定的 message *context*
   。

   3.8 新版功能.

gettext.lgettext(message)

gettext.ldgettext(domain, message)

gettext.lngettext(singular, plural, n)

gettext.ldngettext(domain, singular, plural, n)

   与前缀中没有 "l" 的相应函数等效（ "gettext()", "dgettext()",
   "ngettext()" 和 "dngettext()" ），但是如果没有用
   "bind_textdomain_codeset()" 显式设置其他编码，则返回的翻译将以首选
   系统编码来编码字节串。

   警告:

     在 Python 3 中应避免使用这些函数，因为它们返回的是编码后的字节串
     。最好使用返回 Unicode 字符串的其他方法，因为大多数 Python 应用程
     序都希望将人类可读的文本作为字符串而不是字节来处理。此外，如果翻
     译后的字符串存在编码问题，则可能会意外出现与 Unicode 相关的异常。

   Deprecated since version 3.8, will be removed in version 3.10.

注意，GNU **gettext** 还定义了 "dcgettext()" 方法，但它被认为不实用，
因此目前没有实现它。

这是该 API 的典型用法示例:

   import gettext
   gettext.bindtextdomain('myapplication', '/path/to/my/language/directory')
   gettext.textdomain('myapplication')
   _ = gettext.gettext
   # ...
   print(_('This is a translatable string.'))


基于类的 API
============

与 GNU **gettext** API 相比，"gettext" 模块的基于类的 API 提供了更多的
灵活性和更强的便利性。这是本地化 Python 应用程序和模块的推荐方法。
"gettext" 定义了一个 "GNUTranslations" 类，该类实现了 GNU ".mo" 格式文
件的解析，并且具有用于返回字符串的方法。本类的实例也可以将自身作为函数
"_()" 安装到内建命名空间中。

gettext.find(domain, localedir=None, languages=None, all=False)

   本函数实现了标准的 ".mo" 文件搜索算法。它接受一个 *domain*，它与
   "textdomain()" 接受的域相同。可选参数 *localedir* 与
   "bindtextdomain()" 中的相同。可选参数 *languages* 是多条字符串的列
   表，其中每条字符串都是一种语言代码。

   如果没有传入 *localedir*，则使用默认的系统区域设置目录。 [2] 如果没
   有传入 *languages*，则搜索以下环境变量："LANGUAGE"、"LC_ALL"、
   "LC_MESSAGES" 和 "LANG"。从这些变量返回的第一个非空值将用作
   *languages* 变量。环境变量应包含一个语言列表，由冒号分隔，该列表会
   被按冒号拆分，以产生所需的语言代码字符串列表。

   "find()" 将扩展并规范化 language，然后遍历它们，搜索由这些组件构建
   的现有文件:

   "*localedir*/*language*/LC_MESSAGES/*domain*.mo"

   "find()" 返回找到类似的第一个文件名。如果找不到这样的文件，则返回
   "None"。如果传入了 *all*，它将返回一个列表，包含所有文件名，并按它
   们在语言列表或环境变量中出现的顺序排列。

gettext.translation(domain, localedir=None, languages=None, class_=None, fallback=False, codeset=None)

   根据 *domain*、*localedir* 和 *languages*，返回 "*Translations" 实
   例，首先应将前述参数传入 "find()" 以获取关联的 ".mo" 文件路径的列表
   。名字与 ".mo" 文件名相同的实例将被缓存。如果传入 *class_*，它将是
   实际被实例化的类，否则实例化 "GNUTranslations"。类的构造函数必须只
   接受一个 *文件对象* 参数。如果传入 *codeset*，那么在 "lgettext()"
   和 "lngettext()" 方法中，对翻译后的字符串进行编码的字符集将被改变。

   如果找到多个文件，后找到的文件将用作先前文件的替补。为了设置替补，
   将使用 "copy.copy()" 从缓存中克隆每个 translation 对象。实际的实例
   数据仍在缓存中共享。

   如果 ".mo" 文件未找到，且 *fallback* 为 false（默认值），则本函数引
   发 "OSError" 异常，如果 *fallback* 为 true，则返回一个
   "NullTranslations" 实例。

   在 3.3 版更改: "IOError" 代替 "OSError" 被引发。

   Deprecated since version 3.8, will be removed in version 3.10:
   *codeset* 参数。

gettext.install(domain, localedir=None, codeset=None, names=None)

   根据传入 "translation()" 函数的 *domain*、*localedir* 和 *codeset*
   ，在 Python 内建命名空间中安装 "_()" 函数。

   *names* 参数的信息请参阅 translation 对象的 "install()" 方法的描述
   。

   如下所示，通常将字符串包括在 "_()" 函数的调用中，以标记应用程序中待
   翻译的字符串，就像这样:

      print(_('This string will be translated.'))

   为了方便，一般将 "_()" 函数安装在 Python 内建命名空间中，以便在应用
   程序的所有模块中轻松访问它。

   Deprecated since version 3.8, will be removed in version 3.10:
   *codeset* 参数。


"NullTranslations" 类
---------------------

translation 类实际实现的是，将原始源文件消息字符串转换为已翻译的消息字
符串。所有 translation 类使用的基类为 "NullTranslations"，它提供了基本
的接口，可用于编写自己定制的 translation 类。以下是 "NullTranslations"
的方法：

class gettext.NullTranslations(fp=None)

   接受一个可选参数 *文件对象* *fp*，该参数会被基类忽略。初始化由派生
   类设置的 "protected" （受保护的）实例变量 *_info* 和 *_charset*，与
   *_fallback* 类似，但它是通过 "add_fallback()" 来设置的。如果 *fp*
   不为 "None"，就会调用 "self._parse(fp)"。

   _parse(fp)

      在基类中没有操作，本方法接受文件对象 *fp*，从该文件读取数据，用
      来初始化消息编目。如果你手头的消息编目文件的格式不受支持，则应重
      写本方法来解析你的格式。

   add_fallback(fallback)

      添加 *fallback* 为当前 translation 对象的替补对象。如果
      translation 对象无法为指定消息提供翻译，则应向替补查询。

   gettext(message)

      如果设置了替补，则转发 "gettext()" 给替补。否则返回 *message*。
      在派生类中被重写。

   ngettext(singular, plural, n)

      如果设置了替补，则转发 "ngettext()" 给替补。否则，*n* 为 1 时返
      回 *singular*，为其他时返回 *plural*。在派生类中被重写。

   pgettext(context, message)

      如果设置了替补，则转发 "pgettext()" 给替补。否则返回已翻译的消息
      。在派生类中被重写。

      3.8 新版功能.

   npgettext(context, singular, plural, n)

      如果设置了替补，则转发 "npgettext()" 给替补。否则返回已翻译的消
      息。在派生类中被重写。

      3.8 新版功能.

   lgettext(message)

   lngettext(singular, plural, n)

      与 "gettext()" 和 "ngettext()" 等效，但是如果没有用
      "set_output_charset()" 显式设置编码，则返回的翻译将以首选系统编
      码来编码字节串。在派生类中被重写。

      警告:

        应避免在 Python 3 中使用这些方法。请参阅 "lgettext()" 函数的警
        告。

      Deprecated since version 3.8, will be removed in version 3.10.

   info()

      返回 "protected"（受保护的） "_info" 变量，它是一个字典，包含在
      消息编目文件中找到的元数据。

   charset()

      返回消息编目文件的编码。

   output_charset()

      返回由 "lgettext()" 和 "lngettext()" 翻译出的消息的编码。

      Deprecated since version 3.8, will be removed in version 3.10.

   set_output_charset(charset)

      更改翻译出的消息的编码。

      Deprecated since version 3.8, will be removed in version 3.10.

   install(names=None)

      This method installs "gettext()" into the built-in namespace,
      binding it to "_".

      If the *names* parameter is given, it must be a sequence
      containing the names of functions you want to install in the
      builtins namespace in addition to "_()".  Supported names are
      "'gettext'", "'ngettext'", "'pgettext'", "'npgettext'",
      "'lgettext'", and "'lngettext'".

      Note that this is only one way, albeit the most convenient way,
      to make the "_()" function available to your application.
      Because it affects the entire application globally, and
      specifically the built-in namespace, localized modules should
      never install "_()". Instead, they should use this code to make
      "_()" available to their module:

         import gettext
         t = gettext.translation('mymodule', ...)
         _ = t.gettext

      This puts "_()" only in the module's global namespace and so
      only affects calls within this module.

      在 3.8 版更改: Added "'pgettext'" and "'npgettext'".


"GNUTranslations" 类
--------------------

"gettext" 模块提供了一个派生自 "NullTranslations" 的其他类：
"GNUTranslations"。该类重写了 "_parse()"，同时能以大端序和小端序格式读
取 GNU **gettext** 格式的 ".mo" 文件。

"GNUTranslations" parses optional metadata out of the translation
catalog. It is convention with GNU **gettext** to include metadata as
the translation for the empty string. This metadata is in **RFC
822**-style "key: value" pairs, and should contain the "Project-Id-
Version" key.  If the key "Content-Type" is found, then the "charset"
property is used to initialize the "protected" "_charset" instance
variable, defaulting to "None" if not found.  If the charset encoding
is specified, then all message ids and message strings read from the
catalog are converted to Unicode using this encoding, else ASCII is
assumed.

Since message ids are read as Unicode strings too, all "*gettext()"
methods will assume message ids as Unicode strings, not byte strings.

The entire set of key/value pairs are placed into a dictionary and set
as the "protected" "_info" instance variable.

If the ".mo" file's magic number is invalid, the major version number
is unexpected, or if other problems occur while reading the file,
instantiating a "GNUTranslations" class can raise "OSError".

class gettext.GNUTranslations

   The following methods are overridden from the base class
   implementation:

   gettext(message)

      Look up the *message* id in the catalog and return the
      corresponding message string, as a Unicode string.  If there is
      no entry in the catalog for the *message* id, and a fallback has
      been set, the look up is forwarded to the fallback's "gettext()"
      method.  Otherwise, the *message* id is returned.

   ngettext(singular, plural, n)

      Do a plural-forms lookup of a message id.  *singular* is used as
      the message id for purposes of lookup in the catalog, while *n*
      is used to determine which plural form to use.  The returned
      message string is a Unicode string.

      If the message id is not found in the catalog, and a fallback is
      specified, the request is forwarded to the fallback's
      "ngettext()" method.  Otherwise, when *n* is 1 *singular* is
      returned, and *plural* is returned in all other cases.

      例如：

         n = len(os.listdir('.'))
         cat = GNUTranslations(somefile)
         message = cat.ngettext(
             'There is %(num)d file in this directory',
             'There are %(num)d files in this directory',
             n) % {'num': n}

   pgettext(context, message)

      Look up the *context* and *message* id in the catalog and return
      the corresponding message string, as a Unicode string.  If there
      is no entry in the catalog for the *message* id and *context*,
      and a fallback has been set, the look up is forwarded to the
      fallback's "pgettext()" method.  Otherwise, the *message* id is
      returned.

      3.8 新版功能.

   npgettext(context, singular, plural, n)

      Do a plural-forms lookup of a message id.  *singular* is used as
      the message id for purposes of lookup in the catalog, while *n*
      is used to determine which plural form to use.

      If the message id for *context* is not found in the catalog, and
      a fallback is specified, the request is forwarded to the
      fallback's "npgettext()" method.  Otherwise, when *n* is 1
      *singular* is returned, and *plural* is returned in all other
      cases.

      3.8 新版功能.

   lgettext(message)

   lngettext(singular, plural, n)

      Equivalent to "gettext()" and "ngettext()", but the translation
      is returned as a byte string encoded in the preferred system
      encoding if no encoding  was explicitly set with
      "set_output_charset()".

      警告:

        应避免在 Python 3 中使用这些方法。请参阅 "lgettext()" 函数的警
        告。

      Deprecated since version 3.8, will be removed in version 3.10.


Solaris message catalog support
-------------------------------

The Solaris operating system defines its own binary ".mo" file format,
but since no documentation can be found on this format, it is not
supported at this time.


The Catalog constructor
-----------------------

GNOME uses a version of the "gettext" module by James Henstridge, but
this version has a slightly different API.  Its documented usage was:

   import gettext
   cat = gettext.Catalog(domain, localedir)
   _ = cat.gettext
   print(_('hello world'))

For compatibility with this older module, the function "Catalog()" is
an alias for the "translation()" function described above.

One difference between this module and Henstridge's: his catalog
objects supported access through a mapping API, but this appears to be
unused and so is not currently supported.


Internationalizing your programs and modules
============================================

Internationalization (I18N) refers to the operation by which a program
is made aware of multiple languages.  Localization (L10N) refers to
the adaptation of your program, once internationalized, to the local
language and cultural habits. In order to provide multilingual
messages for your Python programs, you need to take the following
steps:

1. prepare your program or module by specially marking translatable
   strings

2. run a suite of tools over your marked files to generate raw
   messages catalogs

3. create language-specific translations of the message catalogs

4. use the "gettext" module so that message strings are properly
   translated

In order to prepare your code for I18N, you need to look at all the
strings in your files.  Any string that needs to be translated should
be marked by wrapping it in "_('...')" --- that is, a call to the
function "_()".  For example:

   filename = 'mylog.txt'
   message = _('writing a log message')
   with open(filename, 'w') as fp:
       fp.write(message)

In this example, the string "'writing a log message'" is marked as a
candidate for translation, while the strings "'mylog.txt'" and "'w'"
are not.

There are a few tools to extract the strings meant for translation.
The original GNU **gettext** only supported C or C++ source code but
its extended version **xgettext** scans code written in a number of
languages, including Python, to find strings marked as translatable.
Babel is a Python internationalization library that includes a
"pybabel" script to extract and compile message catalogs.  François
Pinard's program called **xpot** does a similar job and is available
as part of his po-utils package.

(Python also includes pure-Python versions of these programs, called
**pygettext.py** and **msgfmt.py**; some Python distributions will
install them for you.  **pygettext.py** is similar to **xgettext**,
but only understands Python source code and cannot handle other
programming languages such as C or C++. **pygettext.py** supports a
command-line interface similar to **xgettext**; for details on its
use, run "pygettext.py --help".  **msgfmt.py** is binary compatible
with GNU **msgfmt**.  With these two programs, you may not need the
GNU **gettext** package to internationalize your Python applications.)

**xgettext**, **pygettext**, and similar tools generate ".po" files
that are message catalogs.  They are structured human-readable files
that contain every marked string in the source code, along with a
placeholder for the translated versions of these strings.

Copies of these ".po" files are then handed over to the individual
human translators who write translations for every supported natural
language.  They send back the completed language-specific versions as
a "<language-name>.po" file that's compiled into a machine-readable
".mo" binary catalog file using the **msgfmt** program.  The ".mo"
files are used by the "gettext" module for the actual translation
processing at run-time.

How you use the "gettext" module in your code depends on whether you
are internationalizing a single module or your entire application. The
next two sections will discuss each case.


Localizing your module
----------------------

If you are localizing your module, you must take care not to make
global changes, e.g. to the built-in namespace. You should not use the
GNU **gettext** API but instead the class-based API.

Let's say your module is called "spam" and the module's various
natural language translation ".mo" files reside in "/usr/share/locale"
in GNU **gettext** format.  Here's what you would put at the top of
your module:

   import gettext
   t = gettext.translation('spam', '/usr/share/locale')
   _ = t.gettext


Localizing your application
---------------------------

If you are localizing your application, you can install the "_()"
function globally into the built-in namespace, usually in the main
driver file of your application.  This will let all your application-
specific files just use "_('...')" without having to explicitly
install it in each file.

In the simple case then, you need only add the following bit of code
to the main driver file of your application:

   import gettext
   gettext.install('myapplication')

If you need to set the locale directory, you can pass it into the
"install()" function:

   import gettext
   gettext.install('myapplication', '/usr/share/locale')


Changing languages on the fly
-----------------------------

If your program needs to support many languages at the same time, you
may want to create multiple translation instances and then switch
between them explicitly, like so:

   import gettext

   lang1 = gettext.translation('myapplication', languages=['en'])
   lang2 = gettext.translation('myapplication', languages=['fr'])
   lang3 = gettext.translation('myapplication', languages=['de'])

   # start by using language1
   lang1.install()

   # ... time goes by, user selects language 2
   lang2.install()

   # ... more time goes by, user selects language 3
   lang3.install()


Deferred translations
---------------------

In most coding situations, strings are translated where they are
coded. Occasionally however, you need to mark strings for translation,
but defer actual translation until later.  A classic example is:

   animals = ['mollusk',
              'albatross',
              'rat',
              'penguin',
              'python', ]
   # ...
   for a in animals:
       print(a)

Here, you want to mark the strings in the "animals" list as being
translatable, but you don't actually want to translate them until they
are printed.

Here is one way you can handle this situation:

   def _(message): return message

   animals = [_('mollusk'),
              _('albatross'),
              _('rat'),
              _('penguin'),
              _('python'), ]

   del _

   # ...
   for a in animals:
       print(_(a))

This works because the dummy definition of "_()" simply returns the
string unchanged.  And this dummy definition will temporarily override
any definition of "_()" in the built-in namespace (until the "del"
command). Take care, though if you have a previous definition of "_()"
in the local namespace.

Note that the second use of "_()" will not identify "a" as being
translatable to the **gettext** program, because the parameter is not
a string literal.

Another way to handle this is with the following example:

   def N_(message): return message

   animals = [N_('mollusk'),
              N_('albatross'),
              N_('rat'),
              N_('penguin'),
              N_('python'), ]

   # ...
   for a in animals:
       print(_(a))

In this case, you are marking translatable strings with the function
"N_()", which won't conflict with any definition of "_()". However,
you will need to teach your message extraction program to look for
translatable strings marked with "N_()". **xgettext**, **pygettext**,
"pybabel extract", and **xpot** all support this through the use of
the "-k" command-line switch. The choice of "N_()" here is totally
arbitrary; it could have just as easily been
"MarkThisStringForTranslation()".


致谢
====

以下人员为创建此模块贡献了代码、反馈、设计建议、早期实现和宝贵的经验：

* Peter Funk

* James Henstridge

* Juan David Ibáñez Palomar

* Marc-André Lemburg

* Martin von Löwis

* François Pinard

* Barry Warsaw

* Gustavo Niemeyer

-[ 脚注 ]-

[1] The default locale directory is system dependent; for example, on
    RedHat Linux it is "/usr/share/locale", but on Solaris it is
    "/usr/lib/locale". The "gettext" module does not try to support
    these system dependent defaults; instead its default is
    "*sys.base_prefix*/share/locale" (see "sys.base_prefix"). For this
    reason, it is always best to call "bindtextdomain()" with an
    explicit absolute path at the start of your application.

[2] See the footnote for "bindtextdomain()" above.
