python - 限制类变量的修改，新实例除外

Question

这是对以下帖子的跟进问题(理解问题不需要检查链接)

Counter variable for class

我们将 idCounter 设置为 Student 类的类变量，它计算创建的实例数。

这是类:

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，那么维护一个计数器就没有实际意义了:

Student.idCounter = 2000

现在创建新实例:

student5 = Student()

并检查idCounter:

Student.idCounter

2001

idCounter 可以简单地搞砸计数器而无需运行 __init__。

如何创建一个只在 __init__ 运行时递增的计数器(或任何类变量)？并且不能通过从类中调用类变量来独立修改，如上所示。

是否有通用的方法来限制使用语法修改类变量？

ClassName.ClassVariable = new_value

谢谢。

Answer 1

编辑

具有属性但原理相同的改进版本:

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

这仍然可以被故意破坏:

>>> Student._Student__hidden_counter = 100
>>> Student.idCounter
100

但并非偶然。