这是以下帖子的后续问题(不需要检查链接就可以理解问题)
Counter variable for class
我们将idCounter设置为类student的类变量,它计算创建的实例数。
这是班级:

class Student:
# A student ID counter
    idCounter = 0
def __init__(self):
    self.gpa = 0
    self.record = {}
    # Each time I create a new student, the idCounter increment
    Student.idCounter += 1
    self.name = 'Student {0}'.format(Student.idCounter)

现在,我们实例化几个实例,然后检查idCounter的值:
student1 = Student()
student2 = Student()
student3 = Student()
student4 = Student()

Student.idCounter
4

但是,如果您可以这样做,则维护计数器将变得毫无意义:
Student.idCounter = 2000

现在创建新实例:
student5 = Student()

检查一下:
Student.idCounter

2001

idCounter可以简单地将计数器调高,而无需运行idCounter
如何创建只在__init__运行时递增的计数器(或任何类变量)?不能通过调用类中的类变量独立修改,如上图所示。
有没有一种通用的方法来限制使用语法修改类变量?
ClassName.ClassVariable = new_value

谢谢您。

最佳答案

编辑
改进版具有property但原则相同:

class Meta(type):

    def __init__(cls, *args, **kwargs):
        cls.__value = 0
        super().__init__(*args, **kwargs)

    @property
    def idCounter(cls):
        return cls.__value

class Student(metaclass=Meta):

    def __init__(self):
        self.__class__._Meta__value += 1

现在:
>>> s1 = Student()
>>> Student.idCounter
1
>>> s2 = Student()
>>> Student.idCounter
2
>>> Student.idCounter = 100
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-64-a525899df18d> in <module>()
----> 1 Student.idCounter = 100

AttributeError: can't set attribute

旧版本
使用描述符和元类:
class Counter:
    def __init__(self):
        self.value = 0
    def __get__(self, instance, cls):
        return getattr(instance, '_{}__hidden_counter'.format(instance.__name__ ))
    def __set__(self, instance, value):
        raise NotImplementedError

class Meta(type):
    idCounter = Counter()

class Student(metaclass=Meta):
    __hidden_counter = 0

    def __init__(self):
        Student.__hidden_counter += 1

似乎实现了这一点:
>>> s1 = Student()
>>> Student.idCounter
1
>>> s2 = Student()
>>> Student.idCounter
2
>>> Student.idCounter = 200
---------------------------------------------------------------------------
NotImplementedError                       Traceback (most recent call last)
<ipython-input-51-dc2483b583f6> in <module>()
----> 1 Student.idCounter = 200

<ipython-input-46-b21e03bf3cb3> in __set__(self, instance, value)
      5         return getattr(instance, '_{}__hidden_counter'.format(instance.__name__ ))
      6     def __set__(self, instance, value):
----> 7         raise NotImplementedError
      8
      9 class Meta(type):

NotImplementedError:
>>> Student.idCounter
2

这仍然可以故意打破:
>>> Student._Student__hidden_counter = 100
>>> Student.idCounter
100

但不是偶然的。

09-11 17:18