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Michal Kolodziejski edited this page Jan 25, 2019 · 7 revisions

From v0.98 on, Indico relies on Babel to manage language dictionaries and provide some basic date formatting and other locale-related functions.

Writing i18n-aware code

The basics

First of all, this video is highly recommended if you are new to i18n.

The basic tool in code i18n is _(). In indico, this function is available from indico.util.i18n. So, the code:

from indico.util.i18n import _
print _('Fetch the cow')

results in:

Fetchez la vache

(imagining that we had a previously loaded a Franglais dictionary, of course)

A dictionary is just a map that associates an "original string" with a "translated string". Every language will have its own dictionary, that is application dependent and has to be loaded beforehand (we will see more below).

Anyone who has done (web) interface programming before can immediately sport some tricky use cases. For example:

print _('Fetch {} cows'.format(number))

would require us to have an infinite dictionary. Of course the fix is easy:

print _('Fetch {} cows').format(number)

It is, then, important, that translators are aware of how format strings work. Babel can handle them easily, and even warn you whenever the translation does not include the correct characters, etc.

Speaking of format strings, there are several ways of specifying them in Python. E.g.:

"I would like a %s and a %s" % ('shrubbery', 'grrrrail')  # old-style format strings
"I would like a %(first)s and a %(second)s" % {first: 'shrubbery', second: 'grrrrail'}  # old-style named fields
"I would like a {} and a {}".format('shrubbery', 'grrrrail')  # new-style format strings (abbreviated)
"I would like a {0} and a {1}".format('shrubbery', 'grrrrail')  # "explicit" new-style format strings
"I would like a {first} and a {second}".format(first='shrubbery', second='grrrrail')  # new-style named fields

While we recently tend to favour new-style format strings (in their abbreviated form), it is possible that old sections of the code still use %s.

IMPORTANT: If the sentence you are translating contains more than one replacement field, then it's essential that translations can set the order of clauses in a sentence. That means that {0} ... {1} is favoured over {} ... {}. Different languages use different grammatical constructs. We shouldn't assume they will all behave the same way.


However, this is not enough. Languages normally establish a difference between singular and plural. Suppose that 'number = 1' (singular). How do we handle that? We could add an if expression and a single translation for "Fetch 1 cow". However, there is an easier/cleaner way:

from indico.util.i18n import ungettext
print ungettext("Fetch {} cow", "Fetch {} cows", number).format(number)

You might be asking why we do not simply do:

print "{0} {1} {2}".format(_("Fetch"), number, ungettext("cow", "cows", number))

The answer is: once again, we should never make any assumptions about the order of words in a sentence. That's why it's better to spend some more characters and translate the whole sentence - like this, translators won't see themselves limited by the assumptions you've done in your code.

Another good example:

print ungettext('Fetch the cow', 'Fetch the cows', number)

Which in Franglais would be Fetchez la vache or Fetchez les vaches (notice that the article changes!). So, make no assumptions about phrase articulation.

Translating variable sentences

... speaking of which, a common case that keeps showing up while developing i18n code is one in which a part of a sentence changes:

Your abstract is set as ACCEPTED
Your abstract is set as REFUSED

which many people fall into the temptation of writing as:

_("Your abstract is set as") + " " + _(state)

Even worse are more general terms such as:

_("Delete") + " " + _(state)

Why should we assume that "delete" has the same translation in every possible context? Not to mention, again, the positioning of the verb in the sentence and possible declensions.

A better (yet not perfect) approach would be:

_("Your abstract is set as {0}").format(_(state))

The ideal would be, of course, to translate the whole sentence. One could mark all possible results as translatable:

N_("Your abstract is set as ACCEPTED")
N_("Your abstract is set as REFUSED")

And then just translate the whole thing:

_("Your abstract is set as {0}".format(_(state)))

This can, however, get pretty messy if there are many possible states. You will always have to try finding a compromise between handling i18n in a fair and language-agnostic way while keeping the amount/complexity of code under control.

You will bump into this case particularly when trying to make text that contains HTML code i18n-aware. String formatting can help, but many times you will find out that there is no optimal solution, only a lesser evil.


It is tempting to use Python stdlib's strftime() each time we want to convert a datetime to a string. However, this function uses the currently set locale by default (process-specific), which is not thread safe. Babel provides a format_datetime function that works more or less the same way and can take a locale as parameter. We put a wrapper around it so that it takes the currently defined (thread-specific) Indico locale, making things easier for everyone.

>>> from indico.util.date_time import format_datetime, now_utc

>>> format_datetime(now_utc())
'28 Jul 2011 12:38:03'

>>> format_datetime(now_utc(), locale="fr_FR")
'28 juil. 2011 12:39:23'

>>> format_datetime(now_utc(), locale="fr_FR", format="long", timezone='Europe/Zurich')
'28 juillet 2011 12:40:05 +0000'

Timezones are also supported:

>>> from pytz import timezone
>>> format_datetime(now_utc(), locale='pt_PT', format='full', timezone=timezone('Europe/Zurich'))
'quinta-feira, 28 de Julho de 2011 14H45m18s Horário Suíça'

Custom formats may be specified using LDML:

>>> format_datetime(now_utc(), locale='es_ES', format='yyyy G')
'2011 d.C.'

More information can be found at Babel's docs on Date Formatting.

Important: Using your own format is seldom the right thing to do, since date formats change a lot between locales, even in their short versions (e.g. 2011 d.C. vs 2011 AD, not to mention week day names).


A special case of date/time translation is the translation of names of week days or months. In the past, strings like "Mon", "Tue", ... and "Jan", "Feb", ... were translated strings in PO files. Obviously, it's better to use a built-in function, rather than requesting all translations from human translators.

The use of English

As a matter of fact, many languages import english expressions in common or technical discussions. Also it may happen that users with different interface languages have to discuss matters or problems in Indico. Therefore the suggestion is to use explanatory translations for Indico-software related terms.

print _('%s : status set to %s) % (xy, _("PENDING"))
print _('%s : status set to %s) % (xy, _("REFUSED"))
print _('%s : status set to %s) % (xy, _("ACCEPTED"))

The german translations could be:

Word Translation
PENDING PENDING (Warteschlange)

A similar complication applies to user-defined strings that end up stored in the DB. It's the case of attachment/material folder names (example en-fr) and their default values:

English French
Slides Transparents
Minutes Compte-rendu

Whereas a fr user should be able to submit his/her slides as Transparents, an en reader should see them under Slides.


Punctuation should be integrated in the original strings

print _('Error: <font color="red">%s</font>') % _('Cannot execute!')

rather than added automatically

print _('Error: <font color="red">%s!</font>') % _('Cannot execute')  # WRONG!

Explanation: In this example the spanish equivalent would use the form

'¡No puede ejecutar!'

whereas french typography requests

'Ne peut pas exécuter !'

Numerical values

If you are using numbers, percentages, etc. you should read this.

Common mistakes

It is very hard to write/maintain an application that is 100% internationalised, since this requires a great deal of coordination not only with the translators, but between developers as well. Not only people usually forget to properly internationalise strings, but also when they remember to do so they choose to do it at the wrong place. Here are some examples of some good/bad practices.

For example:

class UserContainer(object):
    _userTypes = {
        # wrong!
        'admin': _('Administrator'),
        'regular': _('Regular user')

    # ...

    def getUserType(self):
        # can you spot the problem?
        return UserContainer._userTypes[self.user_type]

Why is this wrong? Class attributes are initialised only once, at module load time. So, since modules are normally only loaded once per process, a French user that is using this app will theoretically (see paragraph below) get the same translation as a Japanese one. The right way to do so would be delaying translation till the template is rendered: we could have a dictionary of "English" strings and then just call _() from the piece of template (or other i18n-safe code) that calls getUserType.

Using L_() instead of _() would solve the problem as lazy translation would be forced, thus avoiding the immediate translation of the string at module load time.

Another classic mistake is:

from persistent import Persistent

# ...

class User(Persistent):
    def __init__(self, name, position):
        self.position = _(position)

This one should be easier to spot, if you consider that Persistent objects are stored in the database. By translating position, we get it in the language that was in use at the time of object creation. The right way to do it is, once again, to translate "as late as possible".

Managing dictionaries

This section concerns the programmatic management of the internationalisation files within Indico. If you want to use new translations or test your developments, you should read this.

Babel does all the dirty work of extracting internationalised strings for us, and to create the respective messages.pot file. Since it is integrated with setuptools, nothing more than is needed.

For developers

Extracting messages

Extracting messages is the first step in the translation process. This should be done after any modification that includes new text strings that are displayed in the user interface or changes existing ones.

$ indico i18n extract_messages
$ indico i18n extract_messages_js

Notice that there are two catalogs: one for Python/template code (server side), and another one for JavaScript (client side). This allows us to have a lighter, client-side JavaScript dictionary.

Creating a new dictionary

In order to create a new translation dictionary for a new locale, you should use e.g.:

$ indico i18n init_catalog --locale=pt_PT
$ indico i18n init_catalog_js --locale=pt_PT

This will take messages.pot and messages-js.pot and create new empty translation files (messages.po and messages-js.po) under the appropriate locale dir (indico/translations/pt_PT in this case).

Updating existing dictionaries

After every message extraction, you should update the dictionaries that may already exist with the new catalog. That can be done using the following commands:

$ indico i18n update_catalog
$ indico i18n update_catalog_js

After this, it's time to commit the result and push it to the global repo, so that Transifex will automatically fetch it and make it available for all the different translators. If for some reason this automatic mechanism doesn't work, you can always manually upload the *.pot files.


After all translation work is finished, you should replace the *.po files with the ones downloaded from Transifex, and compile the dictionaries:

$ indico i18n compile_catalog

Notice that the JS dictionary gets automatically generated from the *.po files (no need to explicitly compile it).

And now the translations are ready for use by Indico or binary distribution.

Uploading dictionary to Transifex (Dev Team only)

You'll need the transifex client:

$ pip install transifex-client

Don't forget to set up your .transifexrc.

Then, execute the following command:[

$ tx push -s

That should be enough to update the source strings for all languages.

For translators

We have a transifex project for Indico. If you wish to take part in the translation effort, apply for the team corresponding to your language, or propose the creation of a new one if needed.


  • Here's a nice presentation, by Ruchi Varshney, on i18n done right in Python. The examples are Django-based, but the principles are the exact same.
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