python编程题(继承和元类)

1.实现不可修改的字典

def is_immutable(self):
    raise TypeError('%r objects are immutable' % self.__class__.__name__)


class ImmutableDict(dict):

    def setdefault(self, key, default=None):
        is_immutable(self)

    def update(self, *args, **kwargs):
        is_immutable(self)

    def pop(self, key, default=None):
        is_immutable(self)

    def popitem(self):
        is_immutable(self)

    def __setitem__(self, key, value):
        is_immutable(self)

    def __delitem__(self, key):
        is_immutable(self)

    def clear(self):
        is_immutable(self)

2.自动注册子类
实现一个名为 Base 的类，且任何继承自 Base 子类将被记录，且可以通过迭代 Base 输出所有的子类名称。

两种思路：
1.直接通过__subclasses__()方法获取到子类,并用__iter__迭代

#coding: utf-8

class IterableBase(type):
    def __iter__(cls):
        return iter(cls.__subclasses__())
    def __str__(cls):
        return cls.__name__

class Base(metaclass=IterableBase):
    pass


if __name__ == '__main__':
    class Lab(Base):
        pass

    class Course(Base):
        pass

    for cls in Base:
        print(cls)

2.通过一个数据结构记录子类，并迭代

class selfreigster(type):
    def __init__(cls, name, bases, dct):
        if not hasattr(cls, 'subclasses'):
            cls.subclasses = []
        else:
            cls.subclasses.append(cls)
        super(selfreigster, cls).__init__(name, bases, dct)

    def __iter__(self):
        return iter(self.subclasses)

    def __str__(self):
        return self.__name__


class Base(object, metaclass=selfreigster):
    """docstring for Base"""

if __name__ == '__main__':
    class Lab(Base):
        pass

    class Course(Base):
        pass

    for cls in Base:
        print(cls)