python 中以 “__” 开头和结尾的成员，都被称为类的特殊成员（特殊属性和方法）。
一、__getitem__ 获取属性
在字典和列表中，我们经常会用到 [] 来获取对应元素，因为字典和列表中都内置了 __getitem__ 方法。
"__getitem__" in (dir(list))
>>>
True
"__getitem__" in (dir(dict))
>>>
True
__getitem__ 的作用：
当定义了一个类的时候，当该类的实例化对象通过 [] 来取值的时候，会调用 __getitem__ 方法，也就是说这个方法能够返回与指定键相关联的值
使用 [] 对对象中的属性进行取值、赋值或删除时，会自动触发对应的 __getitem__、__setitem__、__delitem__ 方法
class DataBase:
def __init__(self,id,address):
#初始化方法
self.id=id
self.address=address
self.d={self.id:1,self.address:"192.168.1.1"}
def __getitem__(self,key):
return self.d.get(key,"default")
data=DataBase(1,"192.168.1.1")
print(data['hi']) # output default
print(data[data.id]) # output 1
print(data[data.address]) # output 192.168.1.1
print(data[1]) # output 1
class Person(object):
def __init__(self, info):
self.info = info
def __getitem__(self, key):
print('__getitem__:', key)
return self.info[key]
person_info = {'xiaowang': 1, 'xiaoming': 2, 'xiaohong': 3}
person = Person(person_info)
print(person['xiaoming'])
>>>
__getitem__: xiaoming
2
如果没有定义 __getitem__ 但使用 [] 来取属性的话，则会报错：
class Person(object):
def __init__(self, info):
self.info = info
person_info = {'xiaowang': 1, 'xiaoming': 2, 'xiaohong': 3}
person = Person(person_info)
print(person['xiaoming'])
>>>
TypeError: 'Person' object is not subscriptable
二、__setitem__ 设置属性
__setitem__ 可以设置属性，可以新增，也可以修改现有的
class Person(object):
def __init__(self, info):
self.info = info
def __setitem__(self, key, value):
print('__setitem__:', key, value)
self.info[key] = value
person_info = {'xiaowang': 1, 'xiaoming': 2, 'xiaohong': 3}
person = Person(person_info)
###### 2、修改
person['xiaobai'] = 4
print('person_info after change:', person_info)
>>>
__setitem__: xiaobai 4
person_info after change: {'xiaowang': 1, 'xiaoming': 2, 'xiaohong': 3, 'xiaobai': 4}
class Person(object):
def __init__(self, info):
self.info = info
def __setitem__(self, key, value):
print('__setitem__:', key, value)
self.info[key] = value
person_info = {'xiaowang': 1, 'xiaoming': 2, 'xiaohong': 3}
person = Person(person_info)
###### 2、修改
person['xiaowang'] = 5
print('person_info after change:', person_info)
>>>
__setitem__: xiaowang 5
person_info after change: {'xiaowang': 5, 'xiaoming': 2, 'xiaohong': 3}
三、__delitem__ 删除属性
class Person(object):
def __init__(self, info):
self.info = info
def __delitem__(self, key):
print('__delitem__:', key)
del self.info[key]
def __len__(self):
return len(self.info)
person_info = {'xiaowang': 1, 'xiaoming': 2, 'xiaohong': 3}
person = Person(person_info)
###### 3、删除
del person['xiaoming']
print('person_info after del:', person_info)
>>>
__delitem__: xiaoming
person_info after del: {'xiaowang': 1, 'xiaohong': 3}
四、__len__ 求长度
class Person(object):
def __init__(self, info):
self.info = info
def __len__(self):
return len(self.info)
person_info = {'xiaowang': 1, 'xiaoming': 2, 'xiaohong': 3}
person = Person(person_info)
###### 4、求长度
print('final len of info:', len(person_info))
>>>
final len of info: 3
总体代码：
class Person(object):
def __init__(self, info):
self.info = info
def __getitem__(self, key):
print('__getitem__:', key)
return self.info[key]
def __setitem__(self, key, value):
print('__setitem__:', key, value)
self.info[key] = value
def __delitem__(self, key):
print('__delitem__:', key)
del self.info[key]
def __len__(self):
return len(self.info)
person_info = {'xiaowang': 1, 'xiaoming': 2, 'xiaohong': 3}
person = Person(person_info)
###### 1、获取
print('xiaoming:', person['xiaoming'])
>>>
__getitem__: xiaoming
xiaoming: 2
###### 2、修改
person['xiaobai'] = 4
print('person_info after change:', person_info)
>>>
__setitem__: xiaobai 4
person_info after change: {'xiaowang': 1, 'xiaoming': 2, 'xiaohong': 3, 'xiaobai': 4}
###### 3、删除
del person['xiaoming']
print('person_info after del:', person_info)
>>>
__delitem__: xiaoming
person_info after del: {'xiaowang': 1, 'xiaohong': 3, 'xiaobai': 4}
###### 4、求长度
print('final len of info:', len(person_info))
>>>
final len of info: 3
五、__call__ 将类变成一个可调用的函数
__call__ 的本质其实是将一个类变成一个函数，使得这个类的实例可以像函数一样调用
python 中，凡是可以将 () 直接应用到自身并执行，都称为“可调用对象”，可调用对象包括自定义函数、python 内置函数、以及类实例对象（即有 call 的类）
对于可调用对象，名称() 可以理解为 名称.__call__() 的简写，
class Test(object):
def __init__(self, name):
self.name = name
def __call__(self):
print('here is in __call__!')
print('my name is {}'.format(self.name))
test = Test('xiaowang')
test()
>>>
here is in __call__!
my name is xiaowang
当类中没写 __call__ 的时候：
class Test(object):
def __init__(self, name):
self.name = name
test = Test('xiaowang')
test()
>>>
TypeError: 'Test' object is not callable
报错该对象无法调用
