[bug] FlaskDynaconf doesn't handle __contains__

Describe the bug

Checking key in app.config returns False if the key is only set on app.config after the extension is initialized. However, accessing app.config[key] still works. If the key was set before initializing the extension, or through an env var, it correctly reports True.

This is important for extensions like Flask-SQLAlchemy. It checks if the key SQLALCHEMY_DATABASE_URI is set and shows a warning otherwise, incorrectly in this case.

To Reproduce

from dynaconf import FlaskDynaconf
from flask import Flask

dynaconf = FlaskDynaconf()
app = Flask(__name__)
dynaconf.init_app(app)
app.config["SQLALCHEMY_DATABASE_URI"] = "sqlite:///app.db"

print("SQLALCHEMY_DATABASE_URI" in app.config)  # False, should be True
print(app.config["SQLALCHEMY_DATABASE_URI"])  # sqlite:///app.db

Expected behavior

The in operator reports the correct result when a key is set after calling init_app().

Environment (please complete the following information):

• OS: Linux/Arch
• Dynaconf 3.1.2
• Flask 1.1.2

[rochacbruno]

passing CI I will do the hotfix release.

[damian-t]

Any update on this (https://github.com/rochacbruno/dynaconf/pull/502)? I would love to see this in the next release