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Translation toolkit for Python
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What is lingua?

Lingua is a package with tools to extract translatable texts from your code, and to check existing translations. It replaces the use of the xgettext command from gettext, or pybabel from Babel.

Message extraction

The simplest way to extract all translatable messages is to point the pot-create tool at the root of your source tree.

$ pot-create src

This will create a messages.pot file containing all found messages.

Specifying input files

There are three ways to tell lingua which files you want it to scan:

  1. Specify filenames directly on the command line. For example:

    $ pot-create
  2. Specify a directory on the command line. Lingua will recursively scan that directory for all files it knows how to handle.

    $ pot-create src
  3. Use the --files-from parameter to point to a file with a list of files to scan. Lines starting with # and empty lines will be ignored.

    $ pot-create

You can also use the --directory=PATH parameter to add the given path to the list of directories to check for files. This may sound confusing, but can be useful. For example this command will look for and in the current directory, and if they are not found there in the ../src directory:

$ pot-create --directory=../src


In its default configuration lingua will use its python extractor for .py files, its XML extractor for .pt and .zpt files and its ZCML extractor for .zcml files. If you use different extensions you setup a configuration file which tells lingua how to process files. This file uses a simple ini-style format.

There are two types of configuration that can be set in the configuration file: which extractor to use for a file extension, and the configuration for a single extractor.

File extensions are configured in the extensions section. Each entry in this section maps a file extension to an extractor name. For example to tell lingua to use its XML extractor for files with a .html extension you can use this configuration:

.html = xml

To find out which extractors are available use the -list-extractors option.

$ bin/pot-create --list-extractors
chameleon         Chameleon templates (defaults to Python expressions)
python            Python sources
xml               Chameleon templates (defaults to Python expressions)
zcml              Zope Configuration Markup Language (ZCML)
zope              Zope templates (defaults to TALES expressions)

A section named extractor:<name> can be used to configure a specific extractor. For example to tell the XML extractor that the default language used for expressions is TALES instead of Python:

default-engine = tales

Either place a global configuration file named .config/lingua to your home folder or use the --config option to point lingua to your configuration file.

$ pot-create -c lingua.cfg src

Domain filtering

When working with large systems you may use multiple translation domains in a single source tree. Lingua can support that by filtering messages by domain when scanning sources. To enable domain filtering use the -d option:

$ pot-create -d mydomain src

Lingua will always include messages for which it can not determine the domain. For example, take this Python code:

print(gettext(u'Hello, World'))
print(dgettext('mydomain', u'Bye bye'))

The first hello-message does not specify its domain and will always be included. The second line uses dgettext to explicitly specify the domain. Lingua will use this information when filtering domains.

Including comments

You can add comments to messages to help translators, for example to explain how a text is used, or provide hints on how it should be translated. For chameleon templates this can be done using the i18n:comment attribute:

<label i18n:comment="This is a form label" i18n:translate="">Password</label>

Comments are inherited, so you can put them on a parent element as well.

<form i18n:comment="This is used in the password reset form">
  <label i18n:translate="">Password</label>
  <button i18n:translate="">Change</button>

For Python code you can tell lingua to include comments by using the --add-comments option. This will make Linua include all comments on the line(s) immediately preceeding (there may be no empty line in between) a translation call.

# This text should address the user directly.
return _('Thank you for using our service.')

Alternatively you can also put a comment at the end of the line starting your translation function call.

return _('Thank you for using our service.')  # Address the user directly

If you do not want all comments to be included but only specific ones you can add a keyword to the --add-comments option, for example --add-comments=I18N.

# I18N This text should address the user directly, and use formal addressing.
return _('Thank you for using our service')

Setting message flags in comments

Messages can have flags. These are to indicate what format a message has, and are typically used by validation tools to check if a translation does not break variable references or template syntax. Lingua does a reasonable job to detect strings using C and Python formatting, but sometimes you may need to set flags yourself. This can be done with a [flag, flag] marker in a comment.

# I18N [markdown,c-format]
header =  _(u'# Hello *%s*')

Specifying keywords

When looking for messages a lingua parser uses a default list of keywords to identify translation calls. You can add extra keywords via the --keyword option. If you have your own mygettext function which takes a string to translate as its first parameter you can use this:

$ pot-create --keyword=mygettext

If your function takes more parameters you will need to tell lingua about them. This can be done in several ways:

  • If the translatable text is not the first parameter you can specify the parameter number with <keyword>:<parameter number>. For example if you use i18n_log(level, msg) the keyword specifier would be i18n_log:2
  • If you support plurals you can specify the parameter used for the plural message by specifying the parameter number for both the singular and plural text. For example if your function signature is show_result(single, plural) the keyword specifier is show_result:1,2
  • If you use message contexts you can specify the parameter used for the context by adding a c to the parameter number. For example the keyword specifier for pgettext is pgettext:1c,2.
  • If your function takes the domain as a parameter you can specify which parameter is used for the domain by adding a d to the parameter number. For example the keyword specifier for dgettext is dgettext:1d,2. This is a lingua-specified extension.
  • You can specify the exact number of parameters a function call must have using the t postfix. For example if a function must have four parameters to be a valid call, the specifier could be myfunc:1,4t.


Lingua includes a number of extractors:

  • python: handles Python source code.

  • chameleon: handles Chameleon files, using the Zope i18n syntax

  • zcml: handles Zope Configuration Markup Language (ZCML) files.

  • zope: a variant of the chameleon extractor, which assumes the default

    expression language is TALES instead of Python.

  • xml: old name for the chameleon extractor. This name should not be used anymore and is only supported for backwards compatibility.

Babel extractors

There are several packages with plugins for Babel's message extraction tool. Lingua can use those plugins as well. The plugin names will be prefixed with babel- to distinguish them from lingua extractors.

For example, if you have the PyBabel-json package installed you can instruct lingua to use it for .json files by adding this to your configuration file:

.json = babel-json

Some Babel plugins require you to specify comment tags. This can be set with the comment-tags option.

comment-tags = TRANSLATOR:

Comparison to other tools

Differences compared to GNU gettext:

  • Support for file formats such as Zope Page Templates (popular in Pyramid, Chameleon, Plone and Zope).
  • Better support for detecting format strings used in Python.
  • No direct support for C, C++, Perl, and many other languages. Lingua focuses on languages commonly used in Python projects, although support for other languages can be added via plugins.

Differences compared to Babel:

  • More reliable detection of Python format strings.
  • Lingua includes plural support.
  • Support for only extracting texts for a given translation domain. This is often useful for extensible software where you use multiple translation domains in a single application.

Validating translations

Lingua includes a simple polint tool which performs a few basic checks on PO files. Currently implemented tests are:

  • duplicated message ids (can also be checked with GNU gettext's msgfmt). These should never happen and are usually a result of a bug in the message extraction logic.
  • identical translations used for multiple canonical texts. This can happen for valid reasons, for example when the original text is not spelled consistently.

To check a po file simply run polint with the po file as argument:

$ polint nl.po

    ${val} ist keine Zeichenkette
Used for 2 canonical texts:
1       ${val} is not a string
2       "${val}" is not a string

Writing custom extractors

First we need to create the custom extractor:

from lingua.extractors import Extractor
from lingua.extractors import Message

class MyExtractor(Extractor):
    '''One-line description for --list-extractors'''
    extensions = ['.txt']

    def __call__(self, filename, options):
        return [Message(None, 'msgid', None, [], u'', u'', (filename, 1))]

Hooking up extractors to lingua is done by lingua.extractors entry points in

      my_extractor = mypackage.extractor:MyExtractor

Note - the registered extractor must be a class derived from the Extractor base class.

After installing mypackage lingua will automatically detect the new custom extractor.

Helper Script

There exists a helper shell script for managing translations of packages in docs/examples named Copy it to package root where you want to work on translations, edit the configuration params inside the script and use:

./ lang

for initial catalog creation and:


for updating translation and compiling the catalog.

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