如何在url_for内部声明一个变量来传递给在flask中被调用的函数？

from flask import Flask, url_for

app = Flask(__name__)

@app.route('/')
def index():
    # 在url_for内部声明变量，并传递给被调用的函数
    username = 'John'
    profile_url = url_for('profile', username=username, _external=True)
    return f"Hello, {username}! Your profile URL is: {profile_url}"

@app.route('/profile/<username>')
def profile(username):
    return f"This is the profile page of {username}"

if __name__ == '__main__':
    app.run()