查找特定类的实例

Question

假设以下代码：

class Person():
    def __init__(self,name):
        self.name = name

class Student(Person): pass

class Teacher(Person): pass

John = Student('John')
Tom = Student('Tom')
Victor = Teacher('Victor')

运行脚本后，类Person通过

了解其子类

In [22]: Person.__subclasses__()
Out[22]: [__main__.Student, __main__.Teacher]

如何找到类Student的所有实例，如：

Student.__instances__()
AttributeError: type object 'Student' has no attribute '__instances__'