无法正确修改Python类中的字典

class MyClass:
    def __init__(self):
        self.my_dict = {'key1': 'value1', 'key2': 'value2'}

    def modify_dict(self, key, value):
        self.my_dict[key] = value

# 创建类的实例
my_obj = MyClass()

# 修改字典中的值
my_obj.modify_dict('key1', 'new_value1')

# 打印修改后的字典
print(my_obj.my_dict)

{'key1': 'new_value1', 'key2': 'value2'}