22.1. gettext
— 多语种国际化服务¶
源代码: Lib/gettext.py
The gettext
module provides internationalization (I18N) and localization
(L10N) services for your Python modules and applications. It supports both the
GNU gettext
message catalog API and a higher level, class-based API that may
be more appropriate for Python files. The interface described below allows you
to write your module and application messages in one natural language, and
provide a catalog of translated messages for running under different natural
languages.
同时还给出一些本地化 Python 模块及应用程序的小技巧。
22.1.1. 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])¶ Bind the domain to the locale directory localedir. More concretely,
gettext
will look for binary.mo
files for the given domain using the path (on Unix):localedir/language/LC_MESSAGES/domain.mo
, where languages is searched for in the environment variablesLANGUAGE
,LC_ALL
,LC_MESSAGES
, andLANG
respectively.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])¶ Bind the domain to codeset, changing the encoding of strings returned by the
gettext()
family of functions. If codeset is omitted, then the current binding is returned.2.4 新版功能.
-
gettext.
textdomain
([domain])¶ Change or query the current global domain. If domain is
None
, then the current global domain is returned, otherwise the global domain is set to domain, which is returned.
-
gettext.
gettext
(message)¶ Return the localized translation of message, based on the current global domain, language, and locale directory. This function is usually aliased as
_()
in the local namespace (see examples below).
-
gettext.
lgettext
(message)¶ Equivalent to
gettext()
, but the translation is returned in the preferred system encoding, if no other encoding was explicitly set withbind_textdomain_codeset()
.2.4 新版功能.
-
gettext.
ldgettext
(domain, message)¶ Equivalent to
dgettext()
, but the translation is returned in the preferred system encoding, if no other encoding was explicitly set withbind_textdomain_codeset()
.2.4 新版功能.
-
gettext.
ngettext
(singular, plural, n)¶ Like
gettext()
, but consider plural forms. If a translation is found, apply the plural formula to n, and return the resulting message (some languages have more than two plural forms). If no translation is found, return singular if n is 1; return plural otherwise.The Plural formula is taken from the catalog header. It is a C or Python expression that has a free variable n; the expression evaluates to the index of the plural in the catalog. See the GNU gettext documentation for the precise syntax to be used in
.po
files and the formulas for a variety of languages.2.3 新版功能.
-
gettext.
lngettext
(singular, plural, n)¶ Equivalent to
ngettext()
, but the translation is returned in the preferred system encoding, if no other encoding was explicitly set withbind_textdomain_codeset()
.2.4 新版功能.
-
gettext.
dngettext
(domain, singular, plural, n)¶ Like
ngettext()
, but look the message up in the specified domain.2.3 新版功能.
-
gettext.
ldngettext
(domain, singular, plural, n)¶ Equivalent to
dngettext()
, but the translation is returned in the preferred system encoding, if no other encoding was explicitly set withbind_textdomain_codeset()
.2.4 新版功能.
Note that GNU gettext also defines a dcgettext()
method, but
this was deemed not useful and so it is currently unimplemented.
Here’s an example of typical usage for this API:
import gettext
gettext.bindtextdomain('myapplication', '/path/to/my/language/directory')
gettext.textdomain('myapplication')
_ = gettext.gettext
# ...
print _('This is a translatable string.')
22.1.2. Class-based API¶
The class-based API of the gettext
module gives you more flexibility and
greater convenience than the GNU gettext API. It is the recommended
way of localizing your Python applications and modules. gettext
defines
a “translations” class which implements the parsing of GNU .mo
format
files, and has methods for returning either standard 8-bit strings or Unicode
strings. Instances of this “translations” class can also install themselves in
the built-in namespace as the function _()
.
-
gettext.
find
(domain[, localedir[, languages[, all]]])¶ This function implements the standard
.mo
file search algorithm. It takes a domain, identical to whattextdomain()
takes. Optional localedir is as inbindtextdomain()
Optional languages is a list of strings, where each string is a language code.If localedir is not given, then the default system locale directory is used. 2 If languages is not given, then the following environment variables are searched:
LANGUAGE
,LC_ALL
,LC_MESSAGES
, andLANG
. The first one returning a non-empty value is used for the languages variable. The environment variables should contain a colon separated list of languages, which will be split on the colon to produce the expected list of language code strings.find()
then expands and normalizes the languages, and then iterates through them, searching for an existing file built of these components:localedir/language/LC_MESSAGES/domain.mo
The first such file name that exists is returned by
find()
. If no such file is found, thenNone
is returned. If all is given, it returns a list of all file names, in the order in which they appear in the languages list or the environment variables.
-
gettext.
translation
(domain[, localedir[, languages[, class_[, fallback[, codeset]]]]])¶ Return a
Translations
instance based on the domain, localedir, and languages, which are first passed tofind()
to get a list of the associated.mo
file paths. Instances with identical.mo
file names are cached. The actual class instantiated is either class_ if provided, otherwiseGNUTranslations
. The class’s constructor must take a single file object argument. If provided, codeset will change the charset used to encode translated strings.If multiple files are found, later files are used as fallbacks for earlier ones. To allow setting the fallback,
copy.copy()
is used to clone each translation object from the cache; the actual instance data is still shared with the cache.If no
.mo
file is found, this function raisesIOError
if fallback is false (which is the default), and returns aNullTranslations
instance if fallback is true.在 2.4 版更改: Added the codeset parameter.
-
gettext.
install
(domain[, localedir[, unicode[, codeset[, names]]]])¶ This installs the function
_()
in Python’s builtins namespace, based on domain, localedir, and codeset which are passed to the functiontranslation()
. The unicode flag is passed to the resulting translation object’sinstall()
method.For the names parameter, please see the description of the translation object’s
install()
method.As seen below, you usually mark the strings in your application that are candidates for translation, by wrapping them in a call to the
_()
function, like this:print _('This string will be translated.')
For convenience, you want the
_()
function to be installed in Python’s builtins namespace, so it is easily accessible in all modules of your application.在 2.4 版更改: Added the codeset parameter.
在 2.5 版更改: Added the names parameter.
22.1.2.1. The NullTranslations
class¶
Translation classes are what actually implement the translation of original
source file message strings to translated message strings. The base class used
by all translation classes is NullTranslations
; this provides the basic
interface you can use to write your own specialized translation classes. Here
are the methods of NullTranslations
:
-
class
gettext.
NullTranslations
([fp])¶ Takes an optional file object fp, which is ignored by the base class. Initializes “protected” instance variables _info and _charset which are set by derived classes, as well as _fallback, which is set through
add_fallback()
. It then callsself._parse(fp)
if fp is notNone
.-
_parse
(fp)¶ No-op’d in the base class, this method takes file object fp, and reads the data from the file, initializing its message catalog. If you have an unsupported message catalog file format, you should override this method to parse your format.
-
add_fallback
(fallback)¶ Add fallback as the fallback object for the current translation object. A translation object should consult the fallback if it cannot provide a translation for a given message.
-
gettext
(message)¶ If a fallback has been set, forward
gettext()
to the fallback. Otherwise, return the translated message. Overridden in derived classes.
-
lgettext
(message)¶ If a fallback has been set, forward
lgettext()
to the fallback. Otherwise, return the translated message. Overridden in derived classes.2.4 新版功能.
-
ugettext
(message)¶ If a fallback has been set, forward
ugettext()
to the fallback. Otherwise, return the translated message as a Unicode string. Overridden in derived classes.
-
ngettext
(singular, plural, n)¶ If a fallback has been set, forward
ngettext()
to the fallback. Otherwise, return the translated message. Overridden in derived classes.2.3 新版功能.
-
lngettext
(singular, plural, n)¶ If a fallback has been set, forward
lngettext()
to the fallback. Otherwise, return the translated message. Overridden in derived classes.2.4 新版功能.
-
ungettext
(singular, plural, n)¶ If a fallback has been set, forward
ungettext()
to the fallback. Otherwise, return the translated message as a Unicode string. Overridden in derived classes.2.3 新版功能.
-
info
()¶ Return the “protected”
_info
variable.
-
charset
()¶ Return the “protected”
_charset
variable.
-
output_charset
()¶ Return the “protected”
_output_charset
variable, which defines the encoding used to return translated messages.2.4 新版功能.
-
set_output_charset
(charset)¶ Change the “protected”
_output_charset
variable, which defines the encoding used to return translated messages.2.4 新版功能.
-
install
([unicode[, names]])¶ If the unicode flag is false, this method installs
self.gettext()
into the built-in namespace, binding it to_
. If unicode is true, it bindsself.ugettext()
instead. By default, unicode is false.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'
(bound toself.gettext()
orself.ugettext()
according to the unicode flag),'ngettext'
(bound toself.ngettext()
orself.ungettext()
according to the unicode flag),'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 2.5 版更改: Added the names parameter.
-
22.1.2.2. The GNUTranslations
class¶
The gettext
module provides one additional class derived from
NullTranslations
: GNUTranslations
. This class overrides
_parse()
to enable reading GNU gettext format .mo
files
in both big-endian and little-endian format. It also coerces both message ids
and message strings to Unicode.
GNUTranslations
parses optional meta-data out of the translation
catalog. It is convention with GNU gettext to include meta-data as
the translation for the empty string. This meta-data 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. The ugettext()
method always returns a Unicode, while the
gettext()
returns an encoded 8-bit string. For the message id arguments
of both methods, either Unicode strings or 8-bit strings containing only
US-ASCII characters are acceptable. Note that the Unicode version of the
methods (i.e. ugettext()
and ungettext()
) are the recommended
interface to use for internationalized Python programs.
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, or if other problems occur
while reading the file, instantiating a GNUTranslations
class can raise
IOError
.
The following methods are overridden from the base class implementation:
-
GNUTranslations.
gettext
(message)¶ Look up the message id in the catalog and return the corresponding message string, as an 8-bit string encoded with the catalog’s charset encoding, if known. 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.
-
GNUTranslations.
lgettext
(message)¶ Equivalent to
gettext()
, but the translation is returned in the preferred system encoding, if no other encoding was explicitly set withset_output_charset()
.2.4 新版功能.
-
GNUTranslations.
ugettext
(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
ugettext()
method. Otherwise, the message id is returned.
-
GNUTranslations.
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 an 8-bit string encoded with the catalog’s charset encoding, if known.
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.2.3 新版功能.
-
GNUTranslations.
lngettext
(singular, plural, n)¶ Equivalent to
gettext()
, but the translation is returned in the preferred system encoding, if no other encoding was explicitly set withset_output_charset()
.2.4 新版功能.
-
GNUTranslations.
ungettext
(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
ungettext()
method. Otherwise, when n is 1 singular is returned, and plural is returned in all other cases.Here is an example:
n = len(os.listdir('.')) cat = GNUTranslations(somefile) message = cat.ungettext( 'There is %(num)d file in this directory', 'There are %(num)d files in this directory', n) % {'num': n}
2.3 新版功能.
22.1.2.3. 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.
22.1.2.4. 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.
22.1.3. 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:
prepare your program or module by specially marking translatable strings
run a suite of tools over your marked files to generate raw messages catalogs
create language specific translations of the message catalogs
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')
fp = open(filename, 'w')
fp.write(message)
fp.close()
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.
The Python distribution comes with two tools which help you generate the message
catalogs once you’ve prepared your source code. These may or may not be
available from a binary distribution, but they can be found in a source
distribution, in the Tools/i18n
directory.
The pygettext 3 program scans all your Python source code looking
for the strings you previously marked as translatable. It is similar to the GNU
gettext program except that it understands all the intricacies of
Python source code, but knows nothing about C or C++ source code. You don’t
need GNU gettext
unless you’re also going to be translating C code (such as
C extension modules).
pygettext generates textual Uniforum-style human readable message
catalog .pot
files, essentially structured human readable files which
contain every marked string in the source code, along with a placeholder for the
translation strings. pygettext is a command line script that supports
a similar command line interface as xgettext; for details on its use,
run:
pygettext.py --help
Copies of these .pot
files are then handed over to the individual human
translators who write language-specific versions for every supported natural
language. They send you back the filled in language-specific versions as a
.po
file. Using the msgfmt.py 4 program (in the
Tools/i18n
directory), you take the .po
files from your
translators and generate the machine-readable .mo
binary catalog files.
The .mo
files are what the gettext
module uses for the actual
translation processing during 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.
22.1.3.1. 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.lgettext
If your translators were providing you with Unicode strings in their .po
files, you’d instead do:
import gettext
t = gettext.translation('spam', '/usr/share/locale')
_ = t.ugettext
22.1.3.2. 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 or the unicode flag, you can pass
these into the install()
function:
import gettext
gettext.install('myapplication', '/usr/share/locale', unicode=1)
22.1.3.3. 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()
22.1.3.4. 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 pygettext program, since it is not a string.
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_()
,
5 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_()
. pygettext and xpot both support
this through the use of command line switches.
22.1.3.5. gettext()
vs. lgettext()
¶
In Python 2.4 the lgettext()
family of functions were introduced. The
intention of these functions is to provide an alternative which is more
compliant with the current implementation of GNU gettext. Unlike
gettext()
, which returns strings encoded with the same codeset used in the
translation file, lgettext()
will return strings encoded with the
preferred system encoding, as returned by locale.getpreferredencoding()
.
Also notice that Python 2.4 introduces new functions to explicitly choose the
codeset used in translated strings. If a codeset is explicitly set, even
lgettext()
will return translated strings in the requested codeset, as
would be expected in the GNU gettext implementation.
22.1.4. 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
备注
- 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
. Thegettext
module does not try to support these system dependent defaults; instead its default issys.prefix/share/locale
. For this reason, it is always best to callbindtextdomain()
with an explicit absolute path at the start of your application.- 2
See the footnote for
bindtextdomain()
above.- 3
François Pinard has written a program called xpot which does a similar job. It is available as part of his po-utils package.
- 4
msgfmt.py is binary compatible with GNU msgfmt except that it provides a simpler, all-Python implementation. With this and pygettext.py, you generally won’t need to install the GNU gettext package to internationalize your Python applications.
- 5
The choice of
N_()
here is totally arbitrary; it could have just as easily beenMarkThisStringForTranslation()
.