如何在flask中全局访问变量

from flask import Flask, g

app = Flask(__name__)

@app.before_request
def before_request():
    g.my_variable = "Hello, World!"

@app.route('/')
def index():
    return g.my_variable

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

from flask import Flask, current_app

app = Flask(__name__)

@app.route('/')
def index():
    my_variable = current_app.config.get('MY_VARIABLE')
    return my_variable

if __name__ == '__main__':
    app.config['MY_VARIABLE'] = "Hello, World!"
    app.run()

from flask import Flask, render_template

app = Flask(__name__)

@app.context_processor
def inject_variables():
    my_variable = "Hello, World!"
    return dict(my_variable=my_variable)

@app.route('/')
def index():
    return render_template('index.html')

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