gettext — Multilingual internationalization services¶Source code: 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.
Some hints on localizing your Python modules and applications are also given.
The gettext module defines the following API, which is very similar to
the GNU gettext API. If you use this API you will affect the
translation of your entire application globally. Often this is what you want if
your application is monolingual, with the choice of language dependent on the
locale of your user. If you are localizing a Python module, or if your
application needs to switch languages on the fly, you probably want to use the
class-based API instead.
gettext.bindtextdomain(domain, localedir=None)¶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 variables LANGUAGE,
LC_ALL, LC_MESSAGES, and LANG respectively.
If localedir is omitted or None, then the current binding for domain is
returned. [1]
gettext.bind_textdomain_codeset(domain, codeset=None)¶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.
gettext.textdomain(domain=None)¶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 with
bind_textdomain_codeset().
gettext.ldgettext(domain, message)¶Equivalent to dgettext(), but the translation is returned in the
preferred system encoding, if no other encoding was explicitly set with
bind_textdomain_codeset().
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.
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 with
bind_textdomain_codeset().
gettext.dngettext(domain, singular, plural, n)¶Like ngettext(), but look the message up in the specified domain.
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 with
bind_textdomain_codeset().
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.'))
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 strings. Instances of this “translations”
class can also install themselves in the built-in namespace as the function
_().
gettext.find(domain, localedir=None, languages=None, all=False)¶This function implements the standard .mo file search algorithm. It
takes a domain, identical to what textdomain() takes. Optional
localedir is as in bindtextdomain() 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, and
LANG. 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, then None 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=None, languages=None, class_=None, fallback=False, codeset=None)¶Return a Translations instance based on the domain, localedir,
and languages, which are first passed to find() 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, otherwise GNUTranslations. The class’s constructor must
take a single file object argument. If provided, codeset will change
the charset used to encode translated strings in the lgettext() and
lngettext() methods.
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 raises OSError if
fallback is false (which is the default), and returns a
NullTranslations instance if fallback is true.
gettext.install(domain, localedir=None, codeset=None, names=None)¶This installs the function _() in Python’s builtins namespace, based on
domain, localedir, and codeset which are passed to the function
translation().
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.
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:
gettext.NullTranslations(fp=None)¶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 calls self._parse(fp) if fp is not
None.
_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.
ngettext(singular, plural, n)¶If a fallback has been set, forward ngettext() to the fallback.
Otherwise, return the translated message. Overridden in derived classes.
lngettext(singular, plural, n)¶If a fallback has been set, forward lngettext() to the fallback.
Otherwise, return the translated message. Overridden in derived classes.
info()¶Return the “protected” _info variable.
charset()¶Return the “protected” _charset variable, which is the encoding of
the message catalog file.
output_charset()¶Return the “protected” _output_charset variable, which defines the
encoding used to return translated messages in lgettext() and
lngettext().
set_output_charset(charset)¶Change the “protected” _output_charset variable, which defines the
encoding used to return translated messages.
install(names=None)¶This method installs self.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' (bound to
self.gettext()), 'ngettext' (bound to self.ngettext()),
'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.
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.
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, else ASCII encoding 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.
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 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.
GNUTranslations.lgettext(message)¶Equivalent to gettext(), but the translation is returned as a
bytestring encoded in the selected output charset, or in the preferred system
encoding if no encoding was explicitly set with set_output_charset().
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 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.
Here is an example:
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}
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.
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.
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:
gettext module so that message strings are properly translatedIn 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.
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.
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 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')
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()
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().
The following people contributed code, feedback, design suggestions, previous implementations, and valuable experience to the creation of this module:
Footnotes
| [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.prefix/share/locale. 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. |