Registering a blueprint does not update the url_prefix

[danstewart]

When setting url_prefix in app.register_blueprint it does not update the blueprints url_prefix property.
Not sure if this is a bug or intentional but seems odd.

---

Example:
app.py

from flask import Flask
import bp


def create_app():
    app = Flask(__name__)

    with app.app_context():
        app.register_blueprint(bp.v, url_prefix="/custom-prefix")

    return app

bp.py

from flask import Blueprint

v = Blueprint('the_blueprint', __name__, url_prefix='/default-prefix')


@v.route('/')
def index():
    return f"url_prefix: {v.url_prefix}"

run

$ FLASK_DEBUG=1 FLASK_APP=app flask run --host 0.0.0.0 --port 5001

$ curl 0.0.0.0:5001/custom-prefix/
url_prefix: /default-prefix

---

I would expect bp.url_prefix to be '/custom-prefix'

Environment:

• Python version: 3.8.5
• Flask version: 2.0.0 and 1.1.0

[pgjones]

Duplicate of #4069, fixed by #4074