setattr

例子说明setattr的无限递归错误:

  1. def    __setattr__(self, name, value):     
  2.     self.name = value     
  3.     # 每一次属性赋值时, __setattr__都会被调用，因此不断调用自身导致无限递归了。
  5. 正确写法：
  6. def__setattr__(self, name, value):
  7.     self.__dict__[name] = value  #self[k]=value

new

new实现单例模式

  1. class Student(object):
  3.     instance = None
  5.     def __new__(cls, *args, **kwargs):
  7.         if cls.instance is None:
  9.             cls.instance = super(Student, cls).__new__(cls, *args, **kwargs)
  11.         return cls.instance   #类属性不是实例属性，在实例中共享
  13. s1 = Student()
  14. s2 = Student()
  15. s1.value = 20
  16. print(s1, s2)
  17. print(s1.value, s2.value)
  18. print(s1 is s2)
  20. ############## 打印结果如下 ##############
  21. <__main__.Student object at 0x1033a1c88> <__main__.Student object at 0x1033a1c88>
  22. 20 20
  23. True

with exit

  1.     def __enter__(self):
  2.         return self
  5.     def __exit__(self, exc_type, exc_val, exc_tb):
  6.         pass