在Python中构造类时，如何初始化和声明*可选*变量？

class MyClass:
    def __init__(self, required_var, optional_var="default value"):
        self.required_var = required_var
        self.optional_var = optional_var

# 创建类的实例
obj1 = MyClass("required value")
print(obj1.required_var)  # 输出："required value"
print(obj1.optional_var)  # 输出："default value"

obj2 = MyClass("required value", "custom value")
print(obj2.required_var)  # 输出："required value"
print(obj2.optional_var)  # 输出："custom value"