Flask-Babel convert Flask-WTF SelectField

I want to convert Flask-WTF SelectField value with Flask-Babel.

Here is the snippet of my code:

from flask_babel import _, lazy_gettext as _l

class PaymentStatus(enum.Enum):
    REJECTED = 'REJECTED'
    COMPLETED = 'COMPLETED'
    EXPIRED = 'EXPIRED'

    def __str__(self):
        return self.value

payment_status = [(str(y), y) for y in (PaymentStatus)]

def course_list():
    return Course.query.all()

class PaymentForm(FlaskForm):
    course_name = QuerySelectField(_l('Course name'), validators=[required()], query_factory=course_list)
    status_of_payment = SelectField(_l('Payment Status'), choices=payment_status)
    # ...
    # ...

There, I want to localization the SelectField choices value and QuerySelectField query_factory value with Flask-Babel.

Is it possible..?, if so, any example or refer tutorial would be appreciated :)

Solution

The SelectField choices could be handled by lazy_gettext().

Quote from The Flask Mega-Tutorial Part XIII: I18n and L10n

Some string literals are assigned outside of a request, usually when the application is starting up, so at the time these texts are evaluated there is no way to know what language to use.

Flask-Babel provides a lazy evaluation version of _() that is called lazy_gettext().
from flask_babel import lazy_gettext as _l

class LoginForm(FlaskForm):
    username = StringField(_l('Username'), validators=[DataRequired()])
    # ...

For choices

from flask_babel import _, lazy_gettext as _l

class PaymentStatus(enum.Enum):
    REJECTED = _l('REJECTED')
    COMPLETED = _l('COMPLETED')
    EXPIRED = _l('EXPIRED')

    def __str__(self):
        return self.value

QuerySelectField query_factory accepts values queried from the database. These values should not be handled by Flask-Babel/babel. Cause the database stores data outside the Python source code.

Possible solutions:

Add a translation field in the database table and update the translation manually. Or
Using a Third-Party Translation Service on the webpage and handle it by AJAX

BTW, The Flask Mega-Tutorial made by Miguel Grinberg is a very famous Flask tutorial. All these situations are included in it.