Python中如何使用描述符？

class MyDescriptor:     def __get__(self, instance, owner):         print("Getting value")         return self._value      def __set__(self, instance, value):         print("Setting value")         if value <p>在這個例子中，MyDescriptor類實現了描述符協議。通過MyClass中的attr屬性，我們可以看到描述符是如何控制屬性的訪問和修改的。</p><p>描述符的強大之處在于它們可以用于各種復雜的場景。例如，你可以使用描述符來實現屬性驗證：</p><pre class="brush:python;toolbar:false;">class PositiveNumber:     def __init__(self, name):         self.name = name      def __get__(self, instance, owner):         return instance.__dict__[self.name]      def __set__(self, instance, value):         if value <p>在這個例子中，PositiveNumber描述符確保BankAccount的balance屬性只能設置為正數。</p><p>描述符還可以用于實現計算屬性。例如，你可以創建一個描述符來計算一個對象的年齡：</p><pre class="brush:python;toolbar:false;">class AgeDescriptor:     def __get__(self, instance, owner):         birth_year = instance.__dict__.get('birth_year')         if birth_year is None:             raise AttributeError("birth_year is not set")         return 2023 - birth_year      def __set__(self, instance, value):         raise AttributeError("Cannot set age directly")  class Person:     age = AgeDescriptor()     def __init__(self, birth_year):         self.birth_year = birth_year  person = Person(1990) print(person.age)  # 輸出: 33 try:     person.age = 30  # 會引發 AttributeError except AttributeError as e:     print(e)  # 輸出: Cannot set age directly