Flask 带烧瓶的巴贝尔-未调用localeselector

我以为localeselector会被自动调用，但事实并非如此。这是一款测试巴贝尔的玩具应用程序：

from flask import Flask
from datetime import (
    datetime
)

from babel import (
    dates
)

from flask_babel import Babel

app = Flask(__name__)
app.config["BABEL_DEFAULT_LOCALE"] = "fr_CA"
babel = Babel(app)


@babel.localeselector
def get_locale():
    locale = "fr_CA"
    return locale

@app.route("/")
def date():
    date_heure = datetime(2020, 9, 10, 20, 22, 22)
    a = dates.format_date(date_heure)
    return a

路由告诉我：“AttributeError:'NoneType'对象没有'date_formats'属性”