Access to Flask Global Variables in Blueprint Apps

Question

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?

Answer 1

This happens because the data are lost when the context (with app.app_context()) ends (doc).

Inside the context, everything is ok :

from flask import Flask, g
app = Flask(__name__)
with app.app_context():
    g.my_db = 'database ok'
    print(g.my_db)

# >>> this prints 'database ok'

But outside, you cannot access the attribute :

from flask import Flask, g
app = Flask(__name__)
with app.app_context():
    g.my_db = 'database ok'

print(g.my_db)

# >>> this throws RuntimeError: Working outside of application context

even if you create a new context:

from flask import Flask, g
app = Flask(__name__)
with app.app_context():
    g.my_db = 'database ok'

with app.app_context():
    print(g.my_db)

>>> this throws AttributeError: '_AppCtxGlobals' object has no attribute 'my_db'

Your best call should be to declare the database object before the context, and then import it. Or maybe you can create it directly inside my_app.py where you need it ?