对象的属性访问

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>>> # hasattr()用于判断对象中是否拥有某属性，getattr()用于获取对象中某属性的值，setattr()用于设置对象中某属性的值，delattr()用于删除对象中的某属性。与之相对应的有4个魔法方法，分别是__getattribute__()、__getattr__()、__setattr__()和__delattr__()
>>> # 其中，__getattribute__()、__setattr__()和__delattr__()分别与getattr()、setattr()和delattr()相对应，而__getattr__()则是当用户试图获取一个不存在的属性时才会被触发的魔法方法，与getattr()貌合神离，不要搞错了。此外，无论访问的属性是否存在，__getattribute__()都会优先被触发；__setattr__()和__delattr__()看似普通不过的赋值或删除操作却可能会造成无限递归的“死亡螺旋”，只有使用super()或引入__dict__后再进行相应操作，曲线救国，方能安全实现。

>>> # hasattr()用于判断对象中是否拥有某属性
>>> class C:
...         def __init__(self, name, age):
...                 self.name = name
...                 self.age = age
...
>>> c = C("小甲鱼", 18)
>>> hasattr(c, "name")
True

>>> # getattr()用于获取对象中某属性的值
>>> getattr(c, "name")
'小甲鱼'
>>> getattr(c, "age")
18

>>> # setattr()用于设置对象中某属性的值
>>> setattr(c, "_C__age", 19)
>>> getattr(c, "age")
18
>>> c.__dict__
{'name': '小甲鱼', 'age': 18, '_C__age': 19}
>>> getattr(c, "_C__age")
19

>>> # delattr()用于删除对象中的某属性
>>> delattr(c, "_C__age")
>>> getattr(c, "_C__age")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'C' object has no attribute '_C__age'
>>> c.__dict__
{'name': '小甲鱼', 'age': 18}

>>> #__getattribute__()：当获取属性时触发，对应getattr()
>>> class C:
...         def __init__(self, name, age):
...                 self.name = name
...                 self.__age = age
...         def __getattribute__(self, item):
...                 print("拿来吧你")
...                 return super().__getattribute__(item)
...
>>> c = C('小甲鱼', 18)
>>> print(getattr(c, 'name'))
拿来吧你
小甲鱼
>>> print(c._C__age)
拿来吧你
18
>>> try:
...         c.Fishc
... except AttributeError as e:
...         print(e)
...
拿来吧你
'C' object has no attribute 'Fishc'

>>> #__getattr__()：当试图获取一个不存在的属性时触发
>>> class C:
...         def __init__(self, name, age):
...                 self.name = name
...                 self.__age = age
...         def __getattribute__(self, item):
...                 print("拿来吧你")
...                 return super().__getattribute__(item)
...         def __getattr__(self, item):
...                 if item == 'fishc':
...                         print('I love fishc')
...                 else:
...                         raise AtttributeError(item)
...
>>> c = C('小甲鱼', 18)
>>> try:
...         c.x
... except AttributeError as e:
...         print(e)
...
拿来吧你
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "<stdin>", line 12, in __getattr__
NameError: name 'AtttributeError' is not defined

>>> #__setattr__()当设置属性时触发，对应setattr()
>>> # class D:
>>> #     def __setattr__(self, name, value):
>>> #         self.name = value
>>> #
>>> # d = D()
>>> #  d.name = '小甲鱼'#报错RecursionError: maximum recursion depth exceeded
>>> #捕获到赋值操作时，执行self.name = value，相当于又给自己调用了一次赋值操作，则会继续执行self.name = value，无限递归导致死循环
>>> class D:
...         def __setattr__(self, name, value):
...                 self.__dict__[name] = value
...
>>> d = D()
>>> d.name = '小甲鱼'
>>> print(d.name)
小甲鱼

>>> #__delattr__()：当一个属性被删除时触发，对应delattr()
>>> class D:
...         def __setattr__(self, name, value):
...                 self.__dict__[name] = value
...         def __delattr__(self, item):
...                 del self.__dict__[item]
...
>>> d= D()
>>> d.name = '小甲鱼'
>>> print(d.__dict__)
{'name': '小甲鱼'}
>>> del d.name
>>> print(d.__dict__)
{}
>>>

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