我想在主程序中使用在函数中设置的变量，但在声明变量时出错

def my_function():
    global my_variable
    my_variable = "Hello, world!"

my_function()
print(my_variable)  # 输出：Hello, world!

def my_function():
    my_variable = "Hello, world!"
    return my_variable

result = my_function()
print(result)  # 输出：Hello, world!

def my_function():
    my_variable = "Hello, world!"
    return my_variable

def another_function(variable):
    print(variable)

result = my_function()
another_function(result)  # 输出：Hello, world!