[Python] Access to Flask Global Variables in Blueprint Apps

my source code has this structure:

main.py:

from flask import Flask, g
app = Flask(__name__)
with app.app_context():
g.my_db = PostgreSQL()
app.register_blueprint(my_app, url_prefix="/my_app")


my_app.py:

from flask import Blueprint, g
my_app = Blueprint("my_app", __name__)
@my_app.route("/")
def index():
return g.my_db.fetch_all() <<< ERROR


but it shows this error:

AttributeError: '_AppCtxGlobals' object has no attribute 'my_db'

Even when I try to use g outside of app context, it shows this error:

RuntimeError: Working outside of application context.

So how to set and access to global variables in Flask?

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