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appengine-i18n-sample-python

A simple example app showing how to build an internationalized app with App Engine. The main purpose of this example is to provide the basic how-to.

What to internationalize

There are lots of things to internationalize with your web applications.

  1. Strings in Python code
  2. Strings in HTML template
  3. Strings in Javascript
  4. Common strings
    • Country Names, Language Names, etc.
  5. Formatting
    • Date/Time formatting
    • Number formatting
    • Currency
  6. Timezone conversion

This example only covers first 3 basic scenarios above. In order to cover other aspects, I recommend using Babel and [pytz] (http://pypi.python.org/pypi/gaepytz). Also, you may want to use webapp2_extras.i18n module.

Wait, so why not webapp2_extras.i18n?

webapp2_extras.i18n doesn't cover how to internationalize strings in Javascript code. Additionally it depends on babel and pytz, which means you need to deploy babel and pytz alongside with your code. I'd like to show a reasonably minimum example for string internationalization in Python code, jinja2 templates, as well as Javascript.

How to run this example

First of all, please install babel in your local Python environment.

Wait, you just said I don't need babel, are you crazy?

As I said before, you don't need to deploy babel with this application, but you need to locally use pybabel script which is provided by babel distribution in order to extract the strings, manage and compile the translations file.

Extract strings in Python code and Jinja2 templates to translate

Move into this project directory and invoke the following command:

$ env PYTHONPATH=/google_appengine_sdk/lib/jinja2 \
    pybabel extract -o locales/messages.pot -F main.mapping .

This command creates a locales/messages.pot file in the locales directory which contains all the string found in your Python code and Jija2 tempaltes.

Since the babel configration file main.mapping contains a reference to jinja2.ext.babel_extract helper function which is provided by jinja2 distribution bundled with the App Engine SDK, you need to add a PYTHONPATH environment variable pointing to the jinja2 directory in the SDK.

Manage and compile translations.

Create an initial translation source by the following command:

$ pybabel init -l ja -d locales -i locales/messages.pot

Open locales/ja/LC_MESSAGES/messages.po with any text editor and translate the strings, then compile the file by the following command:

$ pybabel compile -d locales

If any of the strings changes, you can extract the strings again, and update the translations by the following command:

$ pybabel update -l ja -d locales -i locales/messages.pot

Note: If you run pybabel init against an existant translations file, you will lose your translations.

Extract strings in Javascript code and compile translations

$ pybabel extract -o locales/jsmessages.pot -F js.mapping .
$ pybabel init -l ja -d locales -i locales/jsmessages.pot -D jsmessages

Open locales/ja/LC_MESSAGES/jsmessages.po and translate it.

$ pybabel compile -d locales -D jsmessages

How it works

As you can see it in the appengine_config.py file, our main.application is wrapped by the i18n_utils.I18nMiddleware WSGI middleware. When a request comes in, this middleware parses the HTTP_ACCEPT_LANGUAGE HTTP header, loads available translation files(messages.mo) from the application directory, and install the gettext and ngettext functions to the __builtin__ namespace in the Python runtime.

For strings in Jinja2 templates, there is the i18n_utils.BaseHandler class from which you can extend in order to have a handy property named jinja2_env that lazily initializes Jinja2 environment for you with the jinja2.ext.i18n extention, and similar to the I18nMiddleware, installs gettext and ngettext functions to the global namespace of the Jinja2 environment.

What about Javascript?

The BaseHandler class also installs the get_i18n_js_tag() instance method to the Jinja2 global namespace. When you use this function in your Jinja2 template (like in the index.jinja2 file), you will get a set of Javascript functions; gettext, ngettext, and format on the string type. The format function can be used with ngettexted strings for number formatting. See this example:

window.alert(ngettext(
    'You need to provide at least {0} item.',
    'You need to provide at least {0} items.',
    n).format(n);