Flask App not calling Babel localeselector

Question

I am adding localisation to my Flask app with Flask-Babel, and I cannot seem to change the language. I followed all the Babel instructions for creating the po/mo files - I don't believe my issue is related to the translation data... its that I don't know how to change languages... which seems like it would/should be obvious.

My debugging shows that babel.localeselector is not being called. My implementation calls refresh(), which I think should call babel.localeselector (somehow... because I don't see how it works, as the refresh() command seems to just delete some keys from the app context... I don't know how that triggers the call to babel.localeselector)

app = Flask(__name__)
babel = Babel(app)

@babel.localeselector
def get_locale():
    # if a user is logged in, use the locale from the user settings
    user = User.get_by_id(session['user_id'])
    if user is not None: return user.lang
    return 'en'

and, when the user logs in, this function is called,

from flask_babel import refresh

def login(user_id):
    # Gets called when user successfully logs in
    refresh()

I have confirmed refresh() is being called. But the language is not changed.

UPDATED: Also tried this,

from flask import current_app
from common.models import User
from flask_babel import refresh

def login(user_id):
    # Gets called when user successfully logs in
    user = User.get_by_id(user_id)
    current_app.config['BABEL_DEFAULT_LOCALE'] = user.lang
    refresh()

Answer 1

Figured out the problem. I was using,

import gettext
_ = gettext.gettext

instead of,

from flask_babel import gettext as _