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)