Python MySQL连接器中的变量在函数中不起作用

import pymysql

# 全局变量
connection = None

def connect_to_mysql():
    # 使用global关键字声明connection为全局变量
    global connection
    
    # 连接MySQL数据库
    connection = pymysql.connect(
        host='localhost',
        user='root',
        password='password',
        database='mydatabase'
    )

def execute_query(query):
    # 使用全局变量connection进行数据库操作
    global connection
    
    # 执行查询操作
    cursor = connection.cursor()
    cursor.execute(query)
    result = cursor.fetchall()
    cursor.close()
    
    return result

# 调用函数
connect_to_mysql()
result = execute_query('SELECT * FROM mytable')
print(result)